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78th EAGE Conference and Exhibition 2016
- Conference date: May 30, 2016 - June 2, 2016
- Location: Online
- Published: 30 May 2016
1 - 100 of 1034 results
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A High-resolution Weighted Semblance for Dealing with AVO Phenomenon
Authors S. Ebrahimi, A. Roshandel Kahoo, Y. Chen, M.J. Porsani and W. ChenVelocity analysis employs coherency measurement along a hyperbolic or non-hyperbolic trajectories time window to build velocity spectra. Accuracy and resolution are strictly related to the method of coherency measurements. Semblance has poor resolution velocity which affects its ability to distinguish distinct peaks. The problem of traditional semblance are two folds: low resolution and inability of handling AVO phenomenon. Although the AB semblance method can peak velocities in area with AVO anomaly, it has a lower resolution than conventional semblance. In this paper, we proposed a weighted AB semblance method that can handle the two problems simultaneously. We introduced new weighting functions to the AB semblance in order to enhance the resolution of velocity spectra in time and velocity axis. The first weighting function is defined based on the ratio between the first and second singular values of the time window in order to improve the resolution of velocity spectra in velocity axis. The second weighting function is based on the position of seismic wavelet in time window and can enhance the resolution of velocity spectra in time axis. We use both synthetic and field data examples to show the superior performance of the proposed approach over traditional approaches.
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Study of the Casing Effect on Borehole-to-surface Onshore CSEM
Authors E. Vilamajó, V. Puzyrev, P. Queralt, A. Marcuello and J. LedoSummaryMonitoring the resistivity changes occurred in reservoirs during hydrocarbon production or CO2 sequestration is essential to determine fluid distribution and to optimize resources and efforts. Several modeling or theoretical studies have demonstrated the capability of the CSEM method to monitor onshore reservoirs. However there exist few published studies reporting experimental results from real CSEM experiments. Realistic modeling studies are needed to understand real data and to identify and characterize different effects affecting them. In this paper we investigate the sensitivity of the borehole-to-surface configuration using a deep VED. In order to study the influence of the source position and the effect of the steel casing on the data, we present numerical examples and a comparison with real results. The effect of the casing has been studied from two different approaches: a realistic modeling of the casing or replacing the casing by dipoles simulating the current induced along the casing. Both approximations can qualitatively reproduce the experimental data. In the first case, the vertical separation of the VED and the bottom of the casing is the key factor to be determined. In the second case, a precise determination of the current induced in the bottom of the casing is required.
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3D Inversion of Controlled-source Electromagnetic Data in the Presence of Steel-cased Wells
Authors K. Tietze, C. Patzer, O. Ritter, P. Veeken and B. VerboomSummaryControlled-source electromagnetic (CSEM) methods are investigated for their applicability to monitor fluids in a German oilfield, where injected brines have much lower electrical resistivity than oil. In spring 2014, a borehole CSEM survey (4 transmitters, 25 MT-stations) was conducted across the oil-field including a new horizontal-vertical source using the steel-casing of a 1.3km deep abandoned oil-well for current injection. The survey was repeated in autumn 2015, expanded by measurement of the vertical electric field with a newly developed sensor in a 200m deep observation borehole.
Steel-cased wells in the oilfield influence the propagation of electromagnetic fields in the subsurface and cannot be neglected in CSEM modelling. Since horizontal dimensions of well casings are very small compared to their vertical extent, discretising boreholes as conductivity anomalies of the subsurface becomes computationally prohibitive. To tackle the modelling problem, we extended an integral equation method and describe the influence of steel-casings by equivalent source currents which generate a secondary primary field. The new approach allows to include steel-cased wells into our modelling codes, including interaction between multiple wells. We demonstrate the effect of steel-casings on CSEM responses and sensitivity of 3D inversion. Finally, the new inversion method is applied to the field data.
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Field Distortion Due to Surface Pipes in Surface to Borehole Electromagnetic
By N. CuevasSummarySurface to borehole (STB) and borehole to surface (BTS) EM measurements are expected to provide useful resolution of the oil/water contact at reservoir depth. It has been shown however that the effect of the vertical steel casing above the reservoir can yield a distortion of the fields due to current channeling and leakage away from the casing. This paper extends the analysis of casing effect to investigate the channeling arising when the exciting source induces currents in pipes that extend horizontally in front of dipolar antennas. The problem considers a halfspace system (air-sediments or seawater-sediments), a pipe buried in the lower medium extending along the Y axis and a horizontal dipole source embedded in the upper medium. The problem is treated in a semi-analytical formulation, providing a general framework to study the current channeling effect in the pipe and in turn to compute the secondary vertical electric and magnetic field arising downhole due to an exciting dipole source oriented in the X and Y direction.
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Calculating the Effect of Multiple Steel Cased Deviated Wells on Electromagnetic Surveys
Authors C. Kohnke, F. Lavoué, R. Streich and A. SwidinskySummaryElectromagnetic methods are sensitive to conductivity contrasts in the subsurface. This makes the methods useful for monitoring oil and gas reservoirs, which will experience changes in electrical conductivity over their lifetime. In a modern production environment, such a survey would be completed in the presence of multiple deviated cased wells. Common well casing materials, such as steel, are very good conductors and can create a strong secondary electromagnetic field that contaminates the data at the receivers with unwanted signal. To resolve this problem, it is necessary to be able to accurately model the effect of the well casings on collected data.
In this paper, we make use of earlier casing modeling work using the Method of Moments and extend the approach to model the electromagnetic response of a group of deviated wells in a layered geology subject to a marine CSEM survey and produce a tool fit for practical applications. An illustrative example is used to show the electric fields calculated by a group of deviated wells in a layered marine geologic setting. Results show that most of the electromagnetic fields channeled by the horizontal wells into a thin resistor are unable to escape into the surrounding media.
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A Comparative Analysis of SP Data Inversion by Spectral, Tomographic and Global Optimization Approaches
Authors R. Di Maio, E. Piegari and P. RaniSummarySelf-Potential (SP) fields are natural fields that originate from various forcing mechanisms related to electrical, hydraulic, chemical and thermal gradients. Due to the complexity of the source mechanisms, inversion of SP data is not easy and motivates the development of suitable techniques depending on application field, which ranges from engineering and geotechnical investigations to geothermal and mineral explorations. In this work, quantitative interpretations of self-potential data are given when SP anomaly sources can be modelled by simple polarized bodies whose parameters have to be determined. In particular, a comparative analysis is performed for the solutions of three different methods based on high-resolution spectral analysis, tomographic approach and global optimization, respectively. The efficiency of each technique has been tested by finding depth, polarization angle and shape factor of the anomaly source on synthetic data generated by simple geometrical structures (like sphere, horizontal and vertical cylinder and inclined sheet) and on field examples. The study shows limits and potentialities of the investigated methods and suggests hybrid algorithms as suitable tools for an accurate and full characterization of the anomaly source.
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A New Generation of Vertical CSEM Receiver
Authors S.L. Helwig, W. Wood, B. Gloux and T. HoltenSummaryThe combination of a vertical dipole source and vertical dipole receiver has several advantages for measurements of subsurface resistivity in a marine environment. To fully exploit the capabilities of this methodology the receiver system needs to meet strict requirements with regards to verticality and noise levels. Previously existing receiver systems have met theses technical requirements, but at the cost of operational efficiency as the mode of operation for launch and recovery of has limited the efficiency during data acquisition. In this paper we present a new and operationally efficient receiver system capable of recording high quality vertical E-field data. To meet specifications development on the electronics and the mechanical structure of the unit has been critical.
We will detail key components of the system and present data examples from offshore testing of the system.
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MT Noise Suppression for Marine CSEM Data
Authors K.R. Hansen, V. Markhus and R. MittetSummaryWe present a simple and effective method for suppression of MT noise in marine CSEM data. The method can be applied to any CSEM data set where both electric and magnetic fields are measured, and does not require deployment of reference receivers. By applying the method to field data from the Barents Sea, we obtained a significant reduction of MT noise.
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A 2.5D Comparison between Two CSEM Methods
Authors Ø. Frafjord, K. Eide, A.M. El Kaffas, S.L. Helwig and T. HoltenSummarySeveral different CSEM technologies are currently commercially available for offshore exploration, with different characteristics and advantages. Few direct and realistic comparisons between the different methodologies have been made.
In this paper we use a 2.5D finite element code capable of handling both frequency domain and time domain to investigate potential for depth penetration for two methods, horizontal source frequency domain and vertical source time domain. Synthetic data with realistic noise levels has been generated for specific simple models and then inverted. Our results show that vertical time domain solution has potential to resolve deeper targets for the assumed conditions.
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Fast Pseudo-spectral Method for Wave Propagations
More LessSummaryNumerical simulations of elastic wave propagations are the critical components for seismic imaging and inversion. Finite-difference schemes yield high efficiency but fail to ensure the accuracy of the high wavenumber components. The pseudo-spectral algorithm is accurate up to the Nyquist frequency, and it is efficient because of the high level optimization of the fast Fourier transform (FFT) algorithm. The calculation of derivatives in elastic wave propagation could employ the success of FFT optimization. A conventional spectral method consists of three procedures: a forward real to complex (R2C) FFT, a multiplication of the derivative term, such as a for first derivatives, and a complex to real (C2R) inverse FFT. For any even number, the R2C FFT is actually utilizing a half-sized C2C FFT, and then a further spectral manipulation is employed to obtain the spectrum. We propose an efficient scheme to calculate the derivatives for elastic wave propagation in which we apply a forward C2C FFT, and then multiply it with a set of coefficients, and finally a C2C inverse FFT is applied to complete the calculations. With such scheme, a 30% of efficiency has been achieved. In the end, we demonstrate the accuracy and efficiency on a seismic imaging project.
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Viscoelastic Forward and Adjoint Modeling with OpenCL on Heterogeneous Clusters
Authors G. Fabien-Ouellet, E. Gloaguen and B. GirouxSummaryEfficient seismic modeling is more and more needed because of the advent of full waveform inversion (FWI). For real case FWI, an efficient usage of the available computer resources is paramount. With the diversity of processor architectures found today, this is not a trivial task. In this study, we investigate the use of OpenCL to take advantage of large heterogeneous clusters in the context of FWI. The main objective is to present a scalable, multi-device code for the resolution of the viscoelastic wave equation that can compute the gradient of the objective function by the adjoint state method. We present several algorithmic aspects of our program in details, with an emphasis on its different levels of parallelism. The performance of the program is shown with several tests performed on large clusters with nodes containing three types of processors: Intel CPUs, NVidia GPUs and Intel Xeon PHI. We obtain a speed-up of more than 80 when using GPUs compared to a single threaded implementation and a linear scaling when computations are divided on separate nodes. Our results show that OpenCL allows a better usage of the computing resources available using a single source code for a multitude of devices.
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A CPU/GPU Heterogeneous Hybrid Parallel Algorithm of Prestack Time Migration in Local Angle-domain
More LessSummaryPrestack time migration (PSTM) in local angle-domain (LAD) prompts more reasonable utilizations of recorded seismic data but consumes massive amounts of memory and long computing time. To fully use the computing resource and to improve the computing efficiency, a central processing unit (CPU) and graphic processing unit (GPU) heterogeneous hybrid parallel algorithm of LAD migration is developed on GPU cluster. This algorithm realizes distributed computing on different platforms successfully and contains both GPU algorithm and CPU algorithm of LAD migration. It not only uses GPUs but also utilizes CPU cores to share the computing tasks. First, a new seismic data division method is proposed to reasonably divide large-scale data among CPU cores and GPUs. Second, a new Message Passing Interface (MPI) and Compute Unified Device Architecture (CUDA) pattern is presented to call CPU cores and GPUs at the same time. Third, the CPU/GPU heterogeneous hybrid parallel algorithm is developed and is applied into the actual data. Results demonstrate that our algorithm has the same migration profiles with the CPU algorithm and high accelerating performance. The computing time of our algorithm is the shortest, which is 235 times shorter than 20 4-core CPUs and 1.7 times shorter than that of 4GPUs.
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Exploring the Use of SPIKE-based Solvers on Large Electromagnetic Modeling
Authors S. Rodriguez Bernabeu, V. Puzyrev, M. Hanzich and S. FernándezSummaryFrequency-domain seismic and electromagnetic modeling requires solving the linear systems resulting from the discretization of the corresponding time-harmonic equations. Geophysical inversion is typically performed using several discrete frequencies and multiple (up to tens of thousands) source/receiver combinations. Limitations of classical direct and iterative sparse linear solvers have caused the development of the so-called hybrid methods that can be viewed as an intermediate approach between the direct and iterative methods. We present an efficient parallel solver based on the SPIKE algorithm. Several examples in frequency domain electromagnetic modeling illustrate the computational efficiency of the developed method in terms of memory demand and floating-point operations. Multiple sources can be efficiently handled by employing sparse direct solvers in the factorization of diagonal blocks of the system matrix. Based on the divide and conquer idea, this kind of algorithms exposes different parallelism levels, being suitable to take advantage of multiple accelerator devices. The SPIKE solver partially overcomes the fill-in problem of direct solvers, allowing to solve much larger domains on the same system.
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GPU Accelerations on the 3D Elastic RTM Method
More LessSummaryAs one of the important migration methods, reverse time migration (RTM) generally accounts for a large part of the computing time. In recent decades, the desire for covering larger region and acquiring better resolution has greatly increased the algorithmic complexity of RTM. Therefore, computing platforms and optimizing methods that can better meet such challenges in seismic applications become great demands. This work focuses on accelerating the 10th-order stencil kernels from an elastic RTM algorithm by using the Nvidia GPUs. We first modify the backward process in the matrix format by adding extra layers, to generate a straightforward stencil kernel. A set of optimizing techniques including memory and computing approaches is then performed to design the RTM stencil on the K40 GPU. By further using the the streaming mechanism, we manage to obtain an communication-computation overlapping among multiple GPUs. The best performance employing four K40 GPU cards is 28 times better over an OpenMP version based on a socket with two E5-2697 CPUs.
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Intel Xeon Optimizations for Elastic Wave Propagators
Authors A. Farres and M. HanzichSummaryIn order to cope with the huge amount of computational resources need by an elastic, anisotropic, wave propagation engine, optimizations must be done taking into account the architecture where it runs. We will show strategies evaluated and applied to an elastic propagator based on a Fully Staggered Grid, running on the Intel R Xeon family processors. The evaluated set of optimizations ranges from memory to compute optimizations. Our results show that it is possible to obtain a total speed-up of 4 on this architecture.
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High Performance GPGPU Structure-preserving Smoothing for Seismic Amplitude Data by Anisotropic Diffusion
Authors G.M. Faustino, P.C. Pampanelli, J.M.V. Duarte Junior, E.A. Perez, E.R. Silva, P. Frederick and P.M.C. SilvaSummaryNoise attenuation plays an important role in seismic data processing and interpretation. In recent years, the anisotropic diffusion filter has received much attention since it has superior performance in edge-preserving while smoothing noise from noisy signals. This work presents a high-performance General Purpose Graphics Processing Unit (GPGPU) structure-preserving smoothing for seismic amplitude data by anisotropic diffusion. The obtained results show that the proposed method runs in iterative time, and is able to remove noise and preserves structural features efficiently. We also compare the computational performance of CPU and GPU implementations and show that GPU is about 14 times faster.
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The Parallel Forward Modeling of the Wave Equation Based on AVX Instruction Set
More LessSummaryThis paper describes the parallel simulation of the memory/computing-intensive and large-scale three-dimensional acoustic wave equation with CPU stencil optimization. Taking the 8-core shared storage platform as an example, we obtain a one-time speed-up ratio of 6.7× compared with the serial program by using a coarse-grained OpenMP parallel scheme. Our method vectorizes the data on the template buffer with Single Instruction-Multiple Data techniques to further exploit the computing potential of the CPUs. We apply an 8-channel parallel vector to simulate seismic wave fields with the 256-bit AVX instruction set. This increases the computing bandwidth, thereby eliminating a significant volume of computing instructions and ultimately obtaining a secondary speed-up ratio of 3-7×. Finally, we analyze the factors affecting the secondary speed-up effect of AVX through complicated three-dimensional forward modeling experiments using the Salt model. The results indicate that the memory, cache, and register can better cooperate with each other when vectorization is conducted along the shortest direction of the model data cube, and that the speed-up effect can be enhanced by optimizing the AVX algorithm under such a principle.
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Time-lapse Observations from PRM at Snorre
Authors M. Thompson, M. Andersen, S.M. Skogland, C. Courtial and V.B. BiranSummaryThe PRM operation at Snorre, by autumn 2015, had acquired four seismic surveys. It will be demonstrated, though examples, that through PRM it is possible to monitor production effects, with only some months interval between surveys, and that it is further possible to differentiate between a water and gas during a WAG cycle. Additionally it will be shown that the fast turnaround of data allows for speedy feedback on newly completed well operations. The requirements for frequent acquisition and faster turnaround have been achieved by PRM.
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Grane Permanent Reservoir Monitoring - Meeting Expectations!
Authors R.M. Elde, S.S. Roy, C.F. Andersen and T. AndersenSummaryBy August 2014 a full Permanent Reservoir Monitoring system was installed on the seafloor at the Grane field.Thirteen months later, in September 2015, three full field surveys have been acquired.
The requirements for high data quality, frequent acquisition and expected turnaround for acquisition and processing have been retrieved. Preliminary processed data have been delivered few weeks after last shot was acquired. Production effects are observed and confirm the usability and ability of frequently acquire high quality 4D seismic data.
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Seabed Subsidence Monitoring at Snorre with PRM Inclinometer Measurements
Authors M. Houbiers, R. Macault, T. Røste and M. ThompsonSummaryGeophysical reservoir monitoring (GRM) is important for optimizing field management. Along with traditional GRM methods, monitoring of seabed subsidence might provide additional insight in what is going on in the reservoir and the overburden over time, both from a production optimization as well as from a health, safety, and environmental point of view. We propose a method for monitoring seabed subsidence using inclinometer measurements acquired with a PRM system. The method is applied to passive PRM data acquired at Snorre between May and November 2015, resulting in a qualitative map of seabed subsidence. The method requires that the PRM sensors only move due to subsidence. However, the Snorre PRM system is installed in 2013-2014 and natural backfill of the trenches is expected to be ongoing for some time after the sensors are deployed in the seabed. Yet we believe that the method can be used for monitoring seabed subsidence when the sensors have stabilized, and inclination measurements are acquired frequently and regularly over time.
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A New Method for Field-wide Real-time Subsidence Monitoring with Sub-centimeter Accuracy
Authors H. Ruiz, R. Agersborg, B. Fagerås, L.T. Hille, M. Lien, J. E. Lindgård and M. VatshelleSummarySeafloor subsidence is an observable effect of reservoir compaction, and hence provides important information for the management of offshore reservoirs. In some extreme cases, seafloor subsidence can compromise the safety of the installations and even cause well failure.
This abstract proposes a new, patented system for real-time monitoring of subsidence over large areas with sub-cm accuracy. The system consists of two main elements. The first is a grid of pressure sensors permanently deployed on the seafloor. The sensors can be integrated in a full-scale permanent reservoir monitoring system or a smaller caprock integrity monitoring system.
The second element is a periodical surveying that provides the calibration of the seafloor sensors, by means of the comparison of the real-time seafloor measurements with the subsidence measured between the baseline and the repeat surveys.
This abstract describes first the method used for the periodical surveying, traditionally used for measuring both 4D gravity and subsidence. Then, the challenges related with the integration of pressure sensors in a permanent monitoring system at the seafloor are introduced, and the need for an in-situ correction for the drift of the sensors is motivated. The new method for solving this problem is described in the last section.
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CO2 Reservoir Monitoring Using a Permanent Electrode Array - The Ketzin Case Study
Authors C. M. Schmidt-Hattenberger, D. Rippe, T. Labitzke, P. Bergmann and F.M. WagnerSummaryAt the Ketzin pilot site (Germany), a permanent downhole electrode array is accompanying the geological CO2 storage operation from June 2007 until December 2016. Electrical resistivity tomography (ERT) results provide images about relevant operational stages of the storage reservoir. The presented geoelectrical downhole system displays a very promising long-term behavior, and its technical effort and cost has been amortized by its continuous application during the complete injection history. The experiences drawn from the Ketzin site yield strong arguments for the application of the ERT method as part of a multi-disciplinary monitoring concept.
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The CO2CRC Otway Project deployment of a Distributed Acoustic Sensing Network Coupled with Permanent Rotary Sources
Authors B.M. Freifeld, R. Pevzner, S. Dou, J. Correa, T.M. Daley, M. Robertson, K. Tertyshnikov, T. Wood, J. Ajo-Franklin, M. Urosevic and B. GurevichSummaryWe have deployed a novel permanent monitoring system at the Australian CO2CRC Otway Site that includes a surface and borehole distributed acoustic sensing (DAS) network with orbital vibrator (rotary) surface seismic sources. DAS is an emerging technology for performing seismic acquisition based on optical interferometric techniques, which allows for data collection with a wide spatial aperture and high temporal resolution using commercially available telecommunications fibres. DAS sensitivity currently lags behind conventional discrete geophone and hydrophone sensor technologies. Our implementation of surface rotary seismic sources is based on open-loop controlled asynchronous motors. This avoids the complexity of feedback loops for phase control, instead using deconvolution of the source function as measured by a shallow source-monitor sensor. Initial data analysis shows that the amount of energy available from long source sweeps overcomes limitations in DAS sensitivity. The combination of relatively inexpensive but powerful permanent surface sources with permanent DAS deployment in an areal array provides a new paradigm for time-lapse seismic monitoring. The methodology we describe has broad applicability for long-term reservoir surveillance, with time-lapse change sensitive to many subsurface properties.
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Correlating Frequent InSAR Deformations with Reservoir Pressure for Areal Conformance in Thermal EOR at Peace River
Authors C. Didraga and J.L. LopezSummaryWe present a preliminary analysis of the InSAR data acquired over the Pad 31 thermal EOR development area, located in the Peace River Oil Sands, Alberta, Canada.
While InSAR is often viewed as a technology for reservoir containment monitoring, we demonstrate that it may be very effectively used for conformance monitoring if data are acquired frequently.
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How Permanent DTS Installation Could Improve Well and Reservoir Knowledge
Authors AL Leone and G.G. GALLISummaryThe paper describes, by means of real and blind example, how the continuous thermal profiling provided by permanent DTS is useful for many extra application than the pure fluid allocation purposes. For an unconventional reservoir a permanent and continuous monitoring tool represents the key point to provide a correct management of the reservoir and hence to maximize the recovery factor of the field. The proposed case is based on a horizontal producer drilled in gas shale located in North America. In order to evaluate which are the possible issues that could affect the overall monitoring system, DTS analysis already started before the production phase, directly during the installation. This was the occasion to test and bring the technology to the next step: a proper analysis of the data taken during installation provides additional informations about wellbore conditions whereas the warmback dataset, that followed frack job, increased knowledge about the dynamic attitude of the fractures just created, providing a qualitative forecast about the expected production profile. The final result is that the continuous update in temperature profiling given by DTS could improve significantly well efficiency knowledge and the reservoir performances evaluation even when no further investigations are feasible in the wellbore.
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4D Using Non-repeated OBS Acquisition Systems on the Njord Field
Authors M.S. Guttormsen, S. Ng, Ø.H. Solbu, H. Westerdahl, J. Oukili and T. HøySummarySeismic monitoring has been challenging on the Njord field. The rather weak 4D responses related to production have been difficult to detect due to the noise level in the streamer 4D seismic data and due to dominant overpressure effect after injection. The streamer seismic data was replaced by Ocean Bottom Seismic (OBS) in 2010 and the first repeat was performed in 2014. The 4D noise level is expected to improve using Ocean Bottom Seismic (OBS) due to repeated receiver positions and better coverage closer to installations. However the sensor technologies and the seismic source were not repeated and we show how we accommodated for this. In addition, we will show how the lack of shallow overburden illumination through the OBS acquisition was compensated for using streamer seismic data and imaging with multiples.
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Angle-dependent Water Column Statics Correction through Sparse TauP Inversion
Authors R. Huang, P. Wang, K. Nimsaila and M. VuSummaryWater column statics caused by tidal variation and water velocity change during seismic surveys is one major source of noise in marine 4D projects. Correction of this statics effect is a key step in any marine 4D processing. Applying water column statics correction requires a good knowledge of the distance or surface take-off angle when waves travel through the water column, which conventional methods such as ray tracing are not able to obtain accurately when the subsurface velocity is complex.
We propose a new method to apply water column statics correction through progressive sparse TauP inversion. This method does not need prior inputs of subsurface velocity and reflector dips, as required for ray-tracing methods, and benefits from the progressive sparse TauP inversion engine that can properly handle spatially aliased marine seismic data and mitigate energy leakage in the TauP domain.
We demonstrate the effectiveness of this new method using synthetic ocean bottom seismometer (OBS) data derived from a SEAM velocity model and using real OBS data from 4D surveys over the Atlantis field in the Green Canyon area of the Gulf of Mexico (GOM).
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Simultaneous Time-lapse Imaging via Joint Migration and Inversion
Authors S. Qu and D.J. VerschuurSummaryWe propose a simultaneous Joint Migration and Inversion (SJMI) method for time-lapse migration/ inversion, which combines a joint time-lapse data processing strategy with the Joint Migration and Inversion (JMI) method, and also extend it to include an L1-norm sparsity constraint on the reflectivity model-difference in a suitable transform domain and a total-variation (TV) edge-preserving constraint on the velocity model-difference. We tested the proposed method with two synthetic examples, from which it is shown that our method is effective, even when the datasets contain strong noise, are generated by different acquisitions, and also contain a strongly scattering overburden.
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Time-lapse Repeatability Evaluation of a Multimeasurement Towed-streamer System - A North Sea Case Study
Authors C. Ocampo, P.A. Watterson, C. Cunnell, L. Hodgson and D. DaviesSummaryWe present a case study comprising a time-lapse seismic (4D) repeatability test in the North Sea using a broadband multimeasurement towed-streamer acquisition system. Repeat acquisition with unchanged parameters and minimal time-lapse enabled various wavefield separation techniques to be evaluated, additionally providing a rigorous test for 3D capabilities. Wavefield separation approaches comprised both dual-sensor (PZ) and full multimeasurement (PZY) approaches. The results show consistently higher spatial resolution in the multimeasurement prestack depth migrated volumes, particularly in the shallow section, but also slightly elevated noise levels, especially in the higher-frequency bands. The 4D difference plots are consistent with respect to the elevated noise, but higher 4D signal leakage is observed on the dual-sensor (PZ) data, also within the main signal bandwidth at the reservoir level. This demonstrates a subtle but consistent uplift in the full multimeasurement data set, extending from shallow to reservoir level, suggesting that processing steps including multiple attenuation and migration benefit from dense spatial sampling, even at depth.
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4D Feasibility Case Study in a Mature Oilfield
More LessSummaryFeasibility study is an essential component for 4D seismic projects. It includes rock physics modelling, seismic forward modelling, 4D difference calculation and feasibility study. Among them, rock physics modelling is very important because it links the attributes of reservoir fluid-flow simulation model with elastic properties such as P-wave and S-wave properties, and the measured data of cores in the laboratory are required. However, these data are lack of for some oilfields. But well logging data are available. We propose a method of rock physics modelling using well logging data, based on which 4D feasibility is carried out. It is applied to a mature oilfield where a seismic survey was shot more than ten years ago, which is different from what we usually do, to an undeveloped oilfield. The subsequent 4D case study proves the proposed method is practical, and it is the first 4D project offshore in China.
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A Regularization Algorithm Optimized for Time-lapse Processing
Authors A. Khalil, H. Hoeber, B. Deschizeaux, M. Ibram and D. DaviesSummaryIn time-lapse processing, independent regularization of each vintage is the typical approach. This disregards any geometrical limitations imposed by different surveys. Here we recast the regularization process as a minimization problem with model-space constraints. These constraints couple geometrical relations between surveys to improve repeatability. We also demonstrate how to solve the minimization problem using a practical and pragmatic approach. Results from a North Sea dataset show overall reduction of 4D noise, especially around less repeatable parts of the surveys including the undershoot zone.
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A Novel Approach for Cost Effective PRM Seismic Operations at Snorre
Authors M. Thompson, A.S. Pedersen, M. Andersen and S.M. SkoglandSummaryThe Snorre Permanent Reservoir Monitoring system has a receiver area encompassing approximately 200 km2, and aims to monitor, seismically, the reservoir twice a year; through autumn and spring seismic campaigns. In order to acquire two seismic surveys during a single seismic season in the North Sea, while still maintaining enough separation in time to allow for monitoring of the production effects in the reservoir requires consideration of a novel technique to ensure that both surveys are effectively acquired. Since the seismic sensors are stable from one survey to another, and in the case of no production effects outside the receiver area, shots from outside the receiver area can be borrowed from earlier surveys and used to pad out the repeated surveys during the processing of the seismic. This enables a managed reduction in the annual source effort, while ensuring good time-lapse data quality, and contributes to acquisition of two monitoring surveys per year.
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Measurement and Dynamic Wavefield Correction for Time-dependent Water-velocity Changes
Authors R. Zietal and R.R. HaackeSummaryChanges in water velocity produce significant 4D noise in time-lapse images. To be addressed accurately, the water-velocity problem requires two major ingredients: 1) water velocity must be estimated accurately at all acquisition times and for all shot/receiver locations, 2) time-variable corrections to the data must be dynamic to treat the full wavefield accurately. We present a new approach to parameterization of water-velocity changes which minimizes sensitivity to water depth, allowing data redundancy to be exploited to increase robustness and precision of water velocity estimation. Dynamic correction of the wavefield is then achieved by designing 3D time-variable phase-shift operators that extrapolate data through the water column with a time-variable water velocity and re-extrapolate back to the acquisition datum with a stationary (reference) velocity. This is applied in a tau-px-py least-squares modeling process. Application of the method to deep-water OBN data shows significant improvements in data repeatability, decreasing 4D noise and increasing focus and clarity of the 4D signal.
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High Resolution Model Building and Broadband Imaging in Deep Water Offshore Angola
Authors A.A. Shmelev, A. Cooke, O. Zdraveva and J. PenwardenSummaryWith increased interest in pre-salt hydrocarbon exploration in Kwanza Basin offshore Angola, reliable and accurate information from surface seismic data is critical to successful imaging. This area, however, presents many challenges to the geophysicist, being characterized by complex salt geometry, high velocity carbonate layers, a faulted tertiary section and limited well control. In such an environment the desired seismic would have long offsets, rich azimuthal coverage and be broadband by design. Using the latest data-processing techniques, including deghosting and high-resolution model building, a combination of reflection tomography and full-waveform inversion (FWI) we add significant value to existing, conventionally acquired, narrow-azimuth datasets, ultimately leading to improved imaging and interpretation confidence in pre-salt structures.
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Full Waveform Inversion and Ambiguities Related to Strong Anisotropy in Exploration Areas – Case Study Barents Sea
Authors Ø. Korsmo, S. Marinets, S. Naumann and G. RønholtSummaryIn this case study from the Barents Sea, we have used refraction based Full Waveform Inversion (FWI) in an extreme anisotropy regime without the support from well information. We reveal our observations for decoupling the vertical and horizontal velocity, which enables us to achieve good data matching as well as flat gathers, focused images and a geological consistent model. Our frequency cascaded FWI flow results in a high resolution velocity model to the depth of interest, following the faulted crest in great detail, as well as low velocity zones correlating with the bright spots in the seismic image.
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TI Anisotropy Calibration with Sonic and Walkaway VSP
Authors R. Guerra, E. Wielemaker, F. Miranda, M. Ferla, F. Pampuri, S. Gemelli and V. MattonelliSummaryVelocity anisotropy calibration using Walkaway VSPs or sonic data has been shown to deliver net quality improvements in seismic imaging. Nevertheless, common industry workflows still do not take full advantage of all available data. In this study, advanced borehole sonic and Walkaway VSP data acquired in a single presalt vertical well were combined to estimate continuous depth profiles of Thomsen parameters. These parameters were successfully propagated away from the well in the surface seismic velocity model, in order to minimize the Walkaway transit time residuals.
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RTM Imaging Conditions and Image Enhancement via Optical Stacking
Authors I.F. Jones, M. Kobylarski and J. BrittanSummaryThe final stage of a migration process is usually the imaging condition, which brings together elements of the upcoming and downgoing wavefields for each shot gather in order to form an image contribution. This procedure suffers limitations due to the approximations made in representing the physics of the system, but in addition to that, the final summation of all shot contributions necessarily assumes that the subsurface parameter model was perfect, such that all image contributions align perfectly for summation (within a Fresnel zone), as well as having recorded data that are noise free and adequately sampled. In this work, we assess the effect of unresolvable velocity errors on the final image, and present a case study example of a technique for compensating for these errors via techniques borrowed from astronomical image processing applied to each of each of the many thousands of elemental traces that contribute to the final image.
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Integrating Geophysical and Geological Models for De-risking Hydrocarbon Exploration – a Rio del Rey Basin Case Study
Authors P.G. Wilson, J. Wanstall, M.P. Jameson, M. Nuzzo, P. Nguema and S. TamfuSummaryThis paper presents a case study outlining key technical developments and the integration of geophysical and geological models that have advanced our understanding of the hydrocarbon plumbing in the Agbada Formation in the Rio del Rey Basin, offshore Cameroon. Geophysical techniques including extended elastic impedance and seismic fluid classification were developed using elastic log data and modern 3D seismic data. These techniques were shown through the drilling of the Oak wells to aid the prediction of presence and hydrocarbon phase in exploration targets. Gas composition analysis in the wells combined with seismic interpretation has resulted in an understanding of how the different oil and gas reservoirs have been sourced. Integrating the geophysical and geological models means there is a coherent approach to de-risk the presence and phase of hydrocarbons in the prospects.
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A Successful Geophysical Prediction of Fractured Porous Basement Reservoir - Rolvsnes Oil Discovery 2015, Utsira High
Authors J.E. Lie, E.H. Nilsen, E. Grandal, K. Grue and R. SørlieSummaryThe Southern Utsira High in the Norwegian North Sea was explored on and off for more than 40 years before Lundin Norway made the Edvard Grieg breakthrough discovery in 2007 (186 million bbls of recoverable oil). This discovery opened up for the giant Johan Sverdrup 16/2-6 discovery in 2010 (reserve range of 1,7-3,0 billion bbl of oil). Triassic to Cretaceous clastic sandstones are here the main reservoirs.
The most recent oil discovery on the Southern Utsira High was made by the 16/1-25 S, Rolvsnes, well. This well targeted and found oil in fractured porous granitic basement. In this paper, Lundin Norway will present this basement discovery and the geophysical methods used to distinguish between tight and fractured/porous basement which allowed for a successful placement of the Rolvsnes well.
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De-risking Drill Decisions - A Case Study on the Benefit of Re-processing Conventionally Acquired Seismic data
Authors E. Knight, J. Raffle, S. Davies, H. Sherazi-Selby, E. Evans, M. Johnson and I.F. JonesSummaryAcquiring and processing a new vintage of seismic data can often fall outside the time frame of ongoing field development, In this case, careful and detailed reprocessing of vintage seismic data can be a practical and timely way of de-risking any imminent drilling decisions.
Here we consider one such case study over the Thistle field, in the Northern Sector of the North Sea, demonstrating how contemporary de-ghosting of conventional marine streamer data, combined with refined demultiple techniques and iterative non-parametric tomographic preSDM model building facilitate more reliable well-track planning.
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Infill Opportunities after 45 Years of Production from the L10 Central Gas Field, Dutch Sector of the Souther North Sea
Authors G. Daniau, J. Guyomard, H. de Haan and T. BenedictusSummaryIn 1969 the Placid Oil company made the first offshore gas discovery in the L10 block of the Dutch sector of the Southern North Sea. The asset now operated by ENGIE is producing gas from the Permian Rotliegend sandstone reservoir, located at a depth of about 4000m. After 40 years of production, ENGIE decided to acquire a new Pre Stack Depth migrated seismic. The aim was to benefit from a large continuous seismic dataset and apply the latest processing technology in order to unlock remaining potential of this asset. It was the starting point of a complete data review going from structural geology (field compartmentalization and fault seal analysis), to sedimentology (core description, well correlation), petrophysics (homogenize decades of well logs interpretations) and production data (formation pretest and material balance analysis). This work has resulted in de-risking several prospects and identifying new infill opportunities. The example of the L10 Central field review will illustrate this successful story.
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Reservoir Architecture Modelling for Geothermal Energy Production - Case Study of the Delft Sandstone Member, West Netherlands
More LessSummaryDeep-geothermal energy projects operate with a geothermal doublet comprising a hot-water produc-tion well and a cooled-down water injection well. Both wells target the same aquifer to maintain pressure support in the reservoir. The injection well ends at shallower depth than the production well to allow for gravity-driven cold-water front propagation. These conditions determine the doublet layout. A detailed knowledge of reservoir architecture and connectivity in the aquifer is required to assess the pressure communication between the wells, and to reduce the economic risk of these projects. A case study for a planned geothermal doublet research project in the Delft Sandstone Member (Valanginian, West Netherlands Basin) shows that the target comprises stacked sandstone interbedded with mudstone and lignite. Core, cutting, well-log and seismic analyses show that the sandstone formed by meandering rivers in a SE-NW elongated rift basin. The reservoir is subdivided in three units on the basis of variations in net-to-gross, log signature, lithofacies interpretation and stacking density. Unit 3 the best porosity and permeability values, is sand-prone and characterized by multi-storey and laterally-amalgamated meandering river sandstone bodies with minor mudstone floodplain intervals. Optimal placement of geothermal doublet is in the NW-SE trend of the fluvial pathway.
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Comparison of the Petrography and Petrophysical Parameters of Fontainebleau Sandstone - Measurements and Literature
Authors F.S Al Saadi, K.H. Wolf and C.K. KruijsdijkSummaryCharacterizing and understanding porous media is essential prior to standardized core-flow experiments, to investigate oil mobilization on a single-mineral porous medium with a limited permeability/porosity band width, and a homogenous pore- and grain-framework. Literature shows that one almost pure quartz horizon in the Fontainebleau sandstone meet these preconditions. Fresh samples were gathered from the “Gres de Fontainebleu and Cie” Quarry. Knowing the depositional environment and burial history, we measured and quantified spatial attributes of the matrix by CTS image analysis, associated stereological measurements, statistical 2D/3D reconstructions and petrophysical laboratory measurements. Permeability, porosity, capillarity, specific surface, pore framework and pore coordination number distribution were measured and compared with literature. This new database provides a comprehensive review on the Fontainebleau sandstone from micro-scale to meter scale for a porosity bandwidth of 0.05 - 0.11 and permeability bandwidth of 10 - 400 mD. The combined measurements, petrophysical and spatial properties support, are used for prediction, modeling and interpretation of comparative core-flow experiments meant for oil mobilization by chemical injections (surfactant & solvent), i.e. chemo-physical interaction of rock/fluids and multi-phase fluid/fluid adsorptions. Similarly, mapping the pore framework helps modelling the mobilization and transport of the oil from mm-scale to m-scale.
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Reservoir Quality within the Johan Castberg (Formerly Skrugard) Field in the South-Western Barents Sea
More LessSummaryThe Johan Castberg Field is located on the western margin of Loppa High in the south-western Barents Sea and comprises a reservoir in Lower-Middle Jurassic sandstones of Stø and Nordmela formations containing both oil and gas. Cored intervals, 15 samples (well 7220/5-1) and wells log data (7219/8-1, 7219/9-1, 7220/8-1 and 7220/7-1) have been used for petrophysical and petrographical study of sandstone reservoirs. Reservoir properties are preserved significantly due to uplifting and erosion of the entire region. Sandstone diagenesis is a function of burial rate, mineralogical composition and texture, climate, and hydrodynamic and geothermal gradients.
X-ray diffraction (XRD), Optical Microscopy, Scanning Electron Microscopy (SEM) and Core logging have been performed to investigate the depositional environment, clay mineralogy, role of sediments composition, facies distribution, and provenance of the reservoir sandstones. Petrographical study has been carried out to find the diagenetic clay and microquartz coatings, quartz cementation and its distribution in the sandstone reservoirs.
Lower-Middle Jurassic sandstones are moderate to well sorted, fine to medium grained and are mineralogical mature. Sandstones are deposited in prograding coastal regime whereas shale interval indicates regional transgressive pulses during deposition. The porosity and IGV values of sandstones range 6–26% and 25–34% respectively. The porosity is still well preserved and reservoir quality of Lower-Middle Jurassic sandstones in well 7220/5-1 is very good.
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Characterization of Microscopic Pore Structure and its Effect on Macroscopic Physical Parameters in Tight Gas Reservoirs
Authors L.C. Zhang, S.F. Lu and D.S. XiaoSummaryA combination of mercury intrusion capillary pressure (MICP) and N adsorption experiments were performed on tight rock samples from the Shahezi formation in Xujiaweizi Default Depression in order to detect the applicability of the two techniques for characterizing the complete pore size distribution and gaining insight into the microscopic pore structure and its effect on macroscopic physical parameters. N adsorption and MICP measurements were used to analyze the pore size distribution of mesopores and macropores respectively. The result shows that tight gas reservoirs are characterized by complicated microscopic pore structure with a broad pore size distribution from 2nm to 200μm. The dominant pores range from 10nm to 2μm, which mainly consist of slit-shaped pores and ink-bottle shaped pores. Porosity, permeability and free fluid saturation decrease with the increase of the volume proportion of small pores (diameter< 50nm), and increase with the increase of the volume proportion of large pores (diameter>200nm). Furthermore, the relationship between the volume proportion of different pores and permeability and free fluid saturation is better than that with the porosity.
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New Insights on the Characterisation of the Pyroclastic-rich Bajo Barreal Fluvial Reservoir (Argentina)
SummaryThe Cretaceous fluvio-lacustrine Bajo Barreal Formation, known for its large hydrocarbon accumulations, consists of a 1) Lower Member made of floodplain mudstones with isolated channels with an upward increase of sandstone content, and an 2)Upper Member composed by grey and purple mudstones with thicker channel sand bodies. Both stratigraphical units are characterised by the presence of acid volcanoclastics and tuffaceous material mixed with siliciclastic sediments. Previous studies on the Bajo Barreal Formation suggest an active sedimentary input from contemporaneous volcanoclastic material derived from both direct fall-outs or rain-off processes. This material can make up to 15% of host rocks forming a so-called pseudomatrix produced by the disintegration of tuff and d pyroclastic material. In this contribution we present the preliminary results on the reservoir petrography and mineralogy focusing on the characterisation of its volcanic component and related neo-formed mineralization especially considering the clay and zeolite pore infill. The identification of key petrographically distinct stratigraphic units and the detailed quantification of their mineralogical composition (i.e. clay and zeolite content) has an important impact on characterisation of pore-filling material, porosity calculation from density-based wire-line log and the stratigraphic distribution of reservoir properties and hence definition of flow units.
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Relationship between Rock Typing and Petrography - A Case Study on the Fahliyan Formation in the Persian Gulf
Authors M. Goodarzi, M. Jamalian, D. Amirsardari, M. Jamalian and J. ShoghiSummaryThe Lower Cretaceous Fahliyan Formation in the Persian Gulf was the subject of this study. The main target of the study is investigation of relationship between different rock types and sedimentology study. In order to define rock types based on porosity-permeability data, hydraulic flow units were investigated based on Flow Zone Indicators (FZI). As a result, six hydraulic flow units were recognized in the Fahliyan Formation of the studied area. Moreover, productive intervals of the reservoir were identified using Normalized Cumulative Reservoir Quality Index (NCRQI).
Assessment of porosity and permeability data indicates that microfacies pertaining to shoal and channel facies belts with grain-supported texture have the best reservoir quality. This is due to abundance of interparticle porosity. These facies belts correspond with HFU-4, 5, and 6. According to Winland plot, the mentioned HFUs are comparable with macroport and megaport classes. Microfacies of tidal flat facies belt also have good reservoir quality due to fenestra porosity. Microporosity is dominant in most facies of the studied samples. The samples with microporosity pore type (such as most of the microfacies belong to shallow open marine and lagoon facies belts) do not have enough permeability to make a good reservoir zone.
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Reservoir Characterization of Giant Gas Condensate Bearing Kangan and Dalan Formations
By S. DowlatiSummaryKangan and Dalan formations are a permo-triasic basin in the Persian Gulf Region which contains four lean-condensate gas bearing reservoir units named as K1, K2, K3, and K4 from top to bottom. These formations, outstretched in a vast area of Persian Gulf region, embedded gigantic reserves of gas condensate mainly deposited in Iran, Qatar, Saudi Arabia, Bahrain, and Abu Dhabi. Kangan/Dalan formations are synonymously called Khuff formation in southern countries of the Region, while in this paper Iranian part of the formations is studied.The purpose of this paper is to characterize reservoir properties of these four layers based on their well logs, special core analysis, rock typing methods, and PVT lab data. So a comprehensive approach applied to correlate porosity, permeability, capillary pressure, mineralogy, and rock typing of these layers and divide them into sub-layers to develop reservoir geological model.
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Geomechanically Calibrated Rock Physics Modelling for 4D Seismic Response Prediction
Authors M. Paydayesh and A. ShamsaSummaryDisturbing reservoir equilibrium by production or injection causes changes in saturation, pressure, and temperature, hence, substantial alterations on the stress state of a reservoir. In addition to vertical p-wave and s-wave velocity changes, the anisotropy parameters vary during production activities due to both pore pressure and saturation change. The anisotropic nature of stress-induced velocity change must be considered in 4D seismic analysis. Therefore, coupling fluid substitution with geomechanical modelling as demonstrated need to be considered. This work addresses the integrated modelling of 4D seismic response and how geomechanical stress direction calculation can be embedded in the petro-elastic modelling and fluid substitution.
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Structural Parameters Effect on the Sleipner CO2 Underground Storage Simulation History Match
More LessSummarySeismic history matching of the Sleipner CO2 injection simulation is a challenging and ongoing research area. At this paper, capacity of a number of seismic attributes in improving the structural interpretation of the baseline seismic data is demonstrated. In addition, effect of two structural uncertainties, namely, reservoir boundary and structural relief, on the Sleipner simulation history match is addressed. The simulation results illustrate potential of the alternative reservoir boundary on the Sleipner seismic history match improvement.
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Integrated Structural Reconstruction and History Matching Using Ensemble Filter and Low-frequency Electromagnetic Data
Authors C. Etienam, R. Villegas and M. BabaeiSummaryIn this work, we describe a new integrated structural reconstruction approach based on the level set technique and ensemble Kalman filter-based history matching1 using water saturation distribution from low-frequency electromagnetic techniques (Maxwell’s equations) 2 and initial stochastic realisations of permeability distribution. In this work, we discuss the use of the ensemble Kalman filter combined with the Level Set method3 to solve the severely ill-posed problem of parameter and shape reconstruction in history matching of 3D reservoirs. The developed algorithm utilizes Sequential Gaussian Simulation for the creation of the initial geostatistical guesses for permeability which will then be utilized in the ensemble Kalman filter for honoring dynamic production data. The production data here includes water and oil production rates and water saturation distribution from low frequency EM reconstruction. This integrated methodology allows to obtain a better match of the produtcion data honouring the water monitoring obtained from low frequency EM data and also considering the initial model uncertatinty based on geostatistical techniques. The methodology is applied to a 3D syntethic oil reservoir model, the simulator (Schlumberger’s ECLIPSE E100) has been employed for the forward modelling and our in house Fortran and matlab codes has been employed for the History Matching and Electromagnetic reconstruction.
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Geostatistical History Matching with Ensemble Updating
Authors M.J. Quintão, A. Soares and L. AzevedoSummaryIn this work we propose a new history matching methodology that couples within the same framework the benefits of using geostatistical sequential simulation and the principles of ensemble Kalman filters, denominated as history matching based on ensemble updating. The main idea is to use simultaneously the relationship between petrophysical properties and the dynamical results in this process, to update the static properties at each iteration and to define the areas of influence of each well. This relation is established through the experimental non-stationary covariances, simply computed in the several forms of the Ensemble Kalman Filters (EnKF) for history matching of hydrocarbon reservoirs. The proposed iterative history matching procedure was applied to a 2D synthetic reservoir, built to mimic a classical 5-spot configuration with 4 producers located near the corners of the model and an injector well in the center of the grid. In order to assess the performance of the proposed approach, comparison tests were carried out in order to distinguish the advantages of the two main enhancements proposed: the use of areas of influence and the ensemble updating.
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Different approaches for prediction of reservoir volume in slope channel complexes (Miocene, offshore Nile Delta, Egypt)
Authors S. Lang, G. von Halem, A.H. Hassan Soliman and U. LorangSummaryTurbidite channel complexes are important reservoirs. Due to their heterogeneous architecture it is difficult to quantify the gross reservoir volume. Three methods were tested at a prospect case study. Seismic data quality allowed the detailed mapping of four sets of amalgamated channels inside the channel complex. The reservoir volume was calculated by multiplying the horizon area by thickness. Afterwards, it was tried to obtain the reservoir volume by applying an amplitude cutoff to the seismic and multiplying the cumulative amplitude area with a thickness. In a third approach, an acoustic inversion has been carried out and calibrated at a nearby well. Sand bodies have been extracted as geobodies based on an impedance cutoff. All three gross reservoir volumes were divided by the channel complex volume in order to compare the resulting net/gross ratios and they vary from 5%–20%. These net/gross ratios are lower than the net/ gross encountered in a well in a nearby channel complex, which is in-line with an observed higher sand content on seismic at that well location. Therefore, estimating gross rock volume from seismic could reduce the uncertainty compared to using offset well data only.
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Stratigraphic Compartmentalization of the Alvheim Field
Authors A. Hjellbakk, A. Bang, A. Kotwicki, K. Langaas, M.A. Leonthin and K.L. StuartSummaryThe reservoir of the Alvheim Field is the upper part of the deep-marine deposited Palaeocene Heimdal Formation. It is located in the Northern part of the Norwegian North Sea with a small portion extending into the UK sector. Despite the reservoir’s high net to gross ratio of more than 85%, a 1.5–5 meter thick continuous mudstones exists and is in places causing reservoir compartmentalization, even resulting in multiple hydrocarbon columns. A tripartite subdivision of the reservoir was originally made based on log correlation, biostratigraphy and seismic interpretation. This reservoir subdivision was used as input to the reservoir models, which together with a new 4D seismic data set built a good basis for the 2015 infill-well planning. During the infill drilling of pilot wells with single, dual and trilateral producers, new and important information became available, including data from deep resistivity tools which were run in all horizontal branches. The well results were interesting and partly surprising, leading to a significant adjustment of the stratigraphic model. The incorporation of the persistent claystone units into static and dynamic 4D supported models is essential to get a realistic simulation of the reservoir behavior, and to locate undrained targets for future infill drilling.
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Mid-late Miocene Sea Level Falls, Gas Hydrates Decay, Submarine Sliding, and Tsunamites in the Black Sea Basin
Authors A.A. Kitchka, A.P. Tyshchenko and V.I. LysenkoSummaryFull range of structural settings and related features of extensional, translation and compressive domains of the submarine mass transport complexes (MTCs) has been recognized by thematic interpreting of the regional 2D seismic data covering Ukrainian deepwater area in the Black Sea. At the same the time the synchronous to deepwater MTCs the Sarmatian tempestites or even tsunamites known under local name heraclites as well as seismites are described and studied in the SW Crimea in the rock cliffs of the Heraclean Plateau south of Sevastopol. This study links the rapid pulses of the Black sea level fall in the Middle to Late Miocene epoch accompanied by erosion of the shelf break areas and formation of submarine incision valleys, dissociation of gas hydrates, slope failures and large scale rock downslides triggering tsunamis (that imprinted the shore sequences outcropping around the basin) as precursors and characteristic features of the Messinian crisis s.l. manifestation in the Paratethys sea belt further evolved during the Meotian to culminate in mid-Pontian time.
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Deep Water Depositional Architecture, Evolution and Reservoir Potential in the Rakhine Basin, Offshore Myanmar
More LessSummaryFrom 2004 to 2006, 3 biogenic gas fields (Shwe, Shwe Phyu, and Mya) were discovered in Rakhine Basin. Since then, the Rakhine Basin has been the focus of deep water exploration. In our research, industrial and high-resolution 2D and 3D seismic and well data were analyzed to study deepwater architectural elements, depositional evolution and hydrocarbon reservoir potential of the Rakhine.
Six different types of architectural elements are recognized in the Rakhine basin: submarine canyons, confined slope channel complexes, aggradational channel-levee complexes, frontal splays, isolated slope channels, and mass-transport complexes.
Deepwater depositional system showed continuous progradation, and the extent and scope strengthened and widened gradually from the Late Eocene to Pleistocene. The Oligocene mainly comprised confined slope channel complexes. The Miocene was dominated mainly by confined channel complexes and levee-overbank sediments. The Pliocene mainly consisted of confined slope channel complexes, frontal splays. The Pleistocene mainly developed aggradational channel-levee complexes, frontal splays, and mass transport complexes.
Controlled by structural and depositional evolution, the Pliocene confined slope channel complexes and frontal splays have the best reservoir potential in the study area. In gentle fold belt, there are numerous frontal splays and various types of slope channels that remain untested.
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Sequence Stratigraphy of the Middle Jurassic Dhruma Formation in the Oman Mountains (Sultanate of Oman)
Authors M. Schlaich and T. AignerSummaryThis outcrop analog study focuses on a genetic understanding of the reservoir/seal potential and architecture of the Jurassic Dhruma Formation in Oman. As a joint project of the University of Tübingen (Germany) and Petroleum Development Oman (PDO) outcrop logging in nine sections in the Oman Mountains was carried out, supplemented by thin section analyses and spectral gamma-ray measurements. Observed facies types range from peritidal sandstones and microbial laminites to low-energy lagoonal deposits and high-energy, peloid and ooid dominated, shoal associated deposits. Cores and cuttings of the Dhruma, Tuwaiq Mountain and Hanifa/Jubaila Formations were logged over central and northern Oman. A sequence stratigraphic framework and 2D correlations were established based on litho-, chemo- and biostratigraphy. They reveal a clear decrease in thickness of the Dhruma Formation from west to east, which is due to (1) a small unconformity at the bottom and (2) a large Tithonian unconformity, which cuts down into Jurassic strata, eroding the whole Hanifa/Jubaila and Tuwaiq Mountain Formations as well as the top of the Dhruma Formation in outcrops. A 3D geo-cellular facies model was created with Petrel for the Dhruma, Tuwaiq Mountain and Hanifa/Jubaila Formations in central and northern Oman which has implications for the regional prospectivity.
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Sequence Stratigraphic Analysis of the Lower Cretaceous of South Pakistan
By S. MahmoudSummarySedimentological, chronostratigraphic, and sequence stratigraphic interpretations were integrated in order to predict four types of systems tracts, which were used as the building blocks of each interpreted sequence, which, in turn, were used in prediction the chronostratigraphic framework and depositional history of the Lower Indus Basin within the Jati Block. The study highlighted the importance of forced regressive sands of the Falling Stage systems Tract, as high-quality reservoirs encased in shelf mudstones, forming favorable targets for hydrocarbons due to the effective lateral and vertical seals and the expected maturity of intra-formational source rocks. Several lines of evidence support the frequent presence of these forced regressive sands as indicated from seismic attributes, well log sedimentology and sequence stratigraphy, well to well correlation, and from the established sequence models.
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Hydrocarbon Occurrences and Deformation Styles in the Zagros - A Focus on the Kurdistan and Lorestan HC Provinces
Authors R. Di Cuia, P. Pace, R. Bitonte and A. RivaSummaryThe Zagros Thrust Belt is a prolific hydrocarbon province. Intense exploration has been mainly focused on the productive Fars Zone and Dezful Embayement regions of Iran in the central sector of the Zagros thrust belt. During the last years, the NE-SW-trending northwestern sector of the Zagros, extending over the Kurdistan region of northeast Iraq and the Lorestan province of Iran has increasingly received attention related to a renewed interest for hydrocarbon exploration. In this study, compressional features over the Kurdistan and Lorestan regions were characterized from regional to local-scale by integrating various data and methodologies in order to contribute in a better understanding of the compressional deformation styles, mechanisms and evolution of the trap-forming structures and their impact on hydrocarbon occurrences.
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Detachment Folding in the Polar Urals Foreland -Structural Geology and Hidden Petroleum Potential
Authors K. Sobornov, V. Danilov, P. Prijmak and N. NikinovSummaryRegional interpretation of vintage and recently acquired geological and geophysical data provided new insight into structure and petroleum habitat of the Polar Urals foldbelt. The study shows that the deformations observed in the Polar Urals foreland are mainly related to the thrusting and detachment faulting. The principal detachment in the sedimentary cover is provided by the Upper Ordovician evaporates which is pinching out if the area of the Chernyshov swell. The updated structural model permits the identification of the new large exploration opportunities. Bulk of the new gas reserves is predicted in the folded zone in front of the Urals involving large thrust-related closures. Significant oil discoveries are expected in the transition zone between the Chernyshov swell and the Kosyu Rogov foredeep which is a focus of regional hydrocarbon migration. New seismic data provide evidence for the considerable development of the hypogenic karst and dolomitization of the Serpukhovian sulphates and carbonates capable increasing porosity of the reservoirs.
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Sparse Ocean Bottom Node on the Alwyn Field - From Acquisition to Joint PP-PS Imaging
Authors J. Brunelliere, S. Sioni, A. Mitra, X. Lu and A. KaragulSummaryThis case study presents the first ocean bottom node (OBN) survey acquisition and processing over the Alwyn field. The objectives of this sparse acquisition were the enhancement of lateral and vertical resolution of Triassic reservoir. Vintage seismic data consists of three conventional towed streamer surveys acquired in 1981, 1995, and 2000, suffering from limited illumination and none are repeat surveys. This new OBN survey, full azimuth, long offset seismic provided both P and PS-waves datasets for imaging and pre-stack AVO joint inversion.
The objective of this paper is to present the acquisition and processing of both P and PS-wave. Early P-wave output, obtained three months after start of processing, showed the potential of OBN data. This volume confirmed the step change in imaging over existing vintage data. PS-wave intermediate images could only be compared with P-wave due to lack of legacy PS data; structural imaging using PS data remained a challenge, however results are expected to bring value at the joint pre-stack PP-PS inversion stage.
This project demonstrated the ability of sparse OBN data to provide improved P-wave imaging at reservoir levels. Parallel P & PS-wave processing allowed a reduced turnaround time for an early PP & PS joint AVO inversion.
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Sparse Nodes and Shallow Water - PS Imaging Challenges on the Alwyn North Field
Authors J. Holden, D. Fritz, O. Bukola, J. McLeman, R. Refaat, C. Page, J. Brunelliere, S. Sioni, A. Mitra and X. LuSummaryImaging PS-wave data acquired in the shallow water at Alwyn North with ROV-deployed ocean-bottom nodes presented particular challenges due to the sparsity of the receivers. Having ensured vector fidelity of all recorded wavefields, the processing flow made simultaneous use of the PP and PS wavefields at several junctures, including construction of the imaging velocity-depth model, requiring all wavefields to be processed in parallel and in a consistent manner. Shear-wave splitting corrections and PS demultiple were addressed to improve PS data resolution and achieve data consistency. Constructing an anisotropic velocity model based on residual move-out analysis alone was not feasible as the sparse receiver sampling resulted in poor near-offset coverage. To mitigate this full waveform inversion was used to update the P-leg velocity, and surface wave inversion for the S-leg in the crucial near-seabed interval most affected by the slowest shear velocities. Joint PP-PS non-linear tomography was used to refine this velocity model. PS-wave controlled-beam pre-stack depth migration was used extensively to assess the pre-processing as well as to validate updates to the velocity model. The resultant PS-wave data, imaged with a depth model consistent with that used for the P-wave imaging, were thus suitable for joint PP-PS elastic inversion.
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Benefit of OBS PP and PS Data for Structural Interpretation on Snøhvit and Albatross Fields
Authors B. Osdal, H. M. Zadeh, M. S. Guttormsen, H. A. Aronsen, D. C. Cannavo and G. O. ØxnevadSummaryIn this case study from Snøhvit and Albatross we show how Ocean Bottom Seismic (OBS) PP and PS data is used to improve the structural interpretation of reservoirs below overburden shallow gas anomalies. Comprehensive processing, velocity model building and data integration were crucial to achieve good results. The OBS PS has a good quality below the shallow gas anomaly on both Albatross and Snøhvit. On Snøhvit, OBS PP data shows better quality compare to streamer data. This clearly reduces uncertainties related to structural interpretation compare to streamer data alone.
However, on Albatross the quality of both OBS PP and streamer data are poor, and high quality OBS PS data has given largest impact on structural interpretation. Updated structural interpretation greatly improved our geological understanding and may change the reservoir management of the field.
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Full-azimuth Ocean Bottom Seismic for Imaging Beneath Complex Overburden at Johan Sverdrup
Authors B. King, S. Winterstø, J. Nilssen, D. Underwood, D. Brager, S. Mitchell and J. AvilesSummaryStructural interpretation of reservoir events at the Johan Sverdrup field in the North Sea can be challenging in areas with complex overburden. Re-mobilised sands (injectites) with very high velocity relative to surrounding sediments cause strongly localized pull-up of deeper reflections. In addition, shallow channels less than 200m below sea bed filled with low velocity sediments cause pull-down effects. Both pull-up and pull-down effects can leave imprints in the interpretation of reservoir events.
We will show how Ocean Bottom Seismic (OBS) data with long offset and rich azimuth can help resolve such problems, by either under-shooting the injectities completely, or by providing high quality data to enable better processing and velocity model building, including FWI.
The survey was acquired at a reduced cost by using a system with multi-component nodes connected by passive cables. We will show that good data quality was achieved using this system, and we will discuss the additional processing steps required.
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Efficient Wave Mode Separation in Anisotropic Media - Part I-Separation Operators
More LessSummaryIn this paper we present efficient wave mode separation operators in anisotropic media. These operators are constructed by local rotation of wave vector to direction where qP-wave is polarized. The deviation angle between wave normal and qP-wave’s polarization direction is spatially estimated using poynting vector. The proposed operators provide comparable results with those generated by classical space domain convolution operators in anisotropic media. In the meantime, the added computational cost by applying new operators is small when compared to divergence and curl operators as used in isotropic case. Synthetic results validate the effectiveness of the present operators.
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Efficient Wave Mode Separation in Anisotropic Media - Part II-Phase and Amplitude Corrections
More LessSummaryIn this paper we present two improved wave mode separation methods in anisotropic media. The proposed methods aim to efficiently implement wave mode separation while preserving properties of decomposed physical quantities from original elastic wavefields. The first approach is using vector decomposition which output decomposed wave modes with same amplitude, phase as the input wavfields. The second method is introducing amplitude and phase corrections to divergence-like and curl-like operators, then wavefields energy can be conserved in wave mode separation. Synthetic results verify the effectiveness of the proposed methods.
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A New Scalar Imaging Condition for Vector-based Elastic Reverse Time Migration
More LessSummaryScalar images (i.e. PP-, PS-, SP- and SS-wave images) of elastic reverse time migration (RTM) can generated by applying an imaging condition to pure wave modes. Decoupled P- and S-wave equation provides an alternative way to separate P- and S-wavefields in elastic RTM. However, the output separated wavefields are both vectorial. To obtain the scalar images, we propose a new imaging condition in which the scalar product of two vector wavefields is exploited to produce scalar images in elastic RTM. For this scalar-product-based source-normalized elastic crosscorrelation condition needs, no additional prior information is required, except for the P- and S-wave velocities. And additional polarity reversal correction methods are not required to correct polarity changes in converted-wave images for the output separated wavefields have consistent amplitude polarity. Numerical example on Marmosui2 is used to test the effectiveness and robustness of our new imaging condition.
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Elastic Reverse Time Migration Based on Decoupled Wave Equation and Inner Product Imaging Condition
More LessSummaryIn conventional elastic reverse time migration, Helmholtz Decomposition is applied for wavefield separation, which leads to polarity reversal in converted wave imaging. In this paper, we introduce a vector wavefields separation method with amplitude preserved based on decoupled elastic wave equation and illuminate the specific physical meaning of the vector P- and S-wave. In addition, the inner product imaging condition is utilized to obtain image results form the vector P- and S-wavefield. Analysis of imaging kernel indicates that the presented method can effectively circumvent polarity reversal in ground seismic exploration. Migration results have demonstrated the feasibility and effectiveness of this imaging method.
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A Single-sided Representation for Virtual Sources and Virtual Receivers
Authors K. Wapenaar, J.W. Thorbecke, J.R. van der Neut, S. Singh, E.C. Slob and R. SniederSummaryVirtual sources can be created in several ways. In seismic interferometry, a virtual source is created by crosscorrelating responses at different receivers, which are illuminated from all directions. Seismic interferometry can be mathematically described by the homogeneous Green’s function representation, which is a closed boundary integral.
Virtual sources can also be created with the Marchenko method. For the Marchenko method it is sufficient that the position of the virtual source is illuminated from one side. We derive a single-sided homogeneous Green’s function representation, which is an open boundary integral along reflection measurements at the surface. Applying this representation, we obtain virtual sources and virtual receivers in the subsurface from real sources and receivers at the surface (note that in our earlier work on the Marchenko method the response to the virtual source was only obtained for receivers at the surface). The retrieved virtual data show the entire evolution of the response to a virtual source in the subsurface, including primary and multiple scattering at unknown interfaces.
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Full-wavefield Redatuming of Perturbed Fields with the Marchenko Method
Authors I. Vasconcelos and J. van der NeutSummaryWavefield extrapolation, or redatuming, is a critical step for imaging. It is particularly challenging in areas such as subsalt or under complex overburdens. The framework of Marchenko redatuming allows for the retrieval of up- and downgoing fields at chosen locations in the subsurface that contain primary arrivals and internal multiples, while requiring relatively little knowledge of the subsurface model. In this paper, we present a new form of the Marchenko system for perturbed fields. Based on this system, we present a new iterative scheme that explicitly reconstructs only the unknown perturbations to the Marchenko focusing functions, and by consequence only the perturbed/scattered up- and downgoing Green’s functions. This new scheme departs from previous versions of the method in that it requires additional inputs, which include an extra initial focusing operator and perturbations to the surface reflection data. We validate our method with numerical tests, showing that it is particularly well-suited to properly handle complex models with large/sharp constrasts such as salt boundaries. We foresee this new approach to be of use not only in general imaging applications, but also for time-lapse studies as it can directly redatum time-lapse changes.
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Constructing Only the Primary Reflections in Seismic Data - Without Multiple Removal
Authors G.A. Meles, K. Wapenaar, A. Curtis and C. da Costa FilhoSummaryState of the art methods to image the Earth’s subsurface using active-source seismic reflection data involve reverse-time migration (RTM). This, and other standard seismic processing methods such as velocity analysis, provide best results only when all waves in the data set are primaries (waves reflected only once). A variety of methods are therefore deployed as pre-processing to predict multiples (waves reflected several times); however, accurate removal of those predicted multiples from recorded data using adaptive subtraction techniques proves challenging, even in cases where they can be predicted with reasonable accuracy. We describe a new, alternative strategy: we construct a parallel data set consisting of only primaries, which is calculated directly from recorded data. This obviates the need for both multiple prediction and removal methods. Primaries are constructed using convolutional interferometry to combine first arriving events of up-going and direct-wave down-going Green’s functions to virtual receivers in the subsurface. The required up-going wavefields to virtual receivers are constructed by Marchenko redatuming. Crucially, this is possible without detailed models of the Earth’s subsurface velocity structure. The method is shown both to be particularly robust against errors in the reference velocity model used, and to improve migrated images substantially.
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Coupled Seismo-Electromagnetic Interferometry for 2D Homogeneous SH-TE Scenarios
Authors N. Grobbe, E.C. Slob and C.P.A. WapenaarSummaryWe here explore the application of interferometric principles to the coupled seismo-electromagnetic system. We consider 2D homogeneous space scenarios, and focus on one of the two seismo-electromagnetic propagation modes, the SH-TE mode, where horizontally polarized shear-waves are coupled to the transverse electric mode.
We start by presenting the theory for retrieving seismo-electromagnetic Green’s function responses via interferometry by cross-correlation.
Using explicit homogeneous space solutions, we numerically investigate the interferometric retrieval of an electric field response and a magnetic field response due to a seismic source. We first study the theoretically desirable circular source configuration, providing illumination from all sides, followed by a more realistic line source configuration, exploiting the interferometric far-field approximation. These two numerical examples prove for the selected source-receiver type combinations, that we can indeed retrieve correct dynamic seismo-electromagnetic 2D SH-TE responses using seismic boundary sources only. This is a promising result for the next step: applying 3D seismo-electromagnetic interferometry in the field.
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Anisotropic Seismic Noise Gradiometry by Elliptically-anisotropic Wave Equation Inversion - An Example Ekofisk
Authors S.A.L. de Ridder and A. CurtisSummaryWe propose an anisotropic wavefield gradiometry technique to extract azimuthally anisotropic phase velocities from seismic noise that is dominated by a single surface wave mode. The method relies on a two-dimensional elliptical-anisotropic wave equation. This wave equation equates the spatial derivatives of the wavefield amplitudes with the temporal derivatives through the elements of a two-by-two matrix characterizing the medium parameters. The derivatives are evaluated using finite differences, and the system is inverted with a smoothness constraint. We test the procedure on ambient seismic noise recorded in a large and dense array installed over Ekofisk field. Because the station spacing is much larger cross-line then inline, the approximation error of the spatial finite difference results in an apparent anisotropy. From an experiment with synthetic isotropic plane waves, we define a Jacobian to correct the finite difference stencils. With the corrected finite difference stencils, we extracted anisotropic phase velocities at Ekofisk from as little as 10 minutes of seismic noise recordings. The azimuthal anisotropy forms a circular geometry around the production induced subsidence bowl. The methodology is a promising technique for studying changes in the subsurface geomechanical stress-state resulting from time-dependent phenomena operating at a short time-scales.
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Use of Ambient Noise to Enhance Low Frequencies Seismic Migration Images
Authors B. De Cacqueray, J. Cotton, F. Duret, C. Berron and E. ForguesSummaryIn this study, we propose an example of body wave retrieval using ambient noise correlation. The correlated data are migrated using a Kirchhoff pre-stack time migration (PSTM) and then with the migration obtained with conventional active data. It allows to considerably broaden the final image spectrum toward the low frequencies.
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Shallow Rayleigh-wave Tomography Using Traffic Noise From Long Beach, California, USA
Authors J.P. Chang, S.A.L. de Ridder and B.L. BiondiSummaryUsing recordings from a dense seismic array in Long Beach, California, USA, we demonstrate the effectiveness of using traffic noise for shallow subsurface imaging. Using the ambient-noise cross-correlation technique at frequencies greater than 3 Hz, we extract fundamental- and first-order-mode Rayleigh waves generated by Interstate 405 and local roads. We use group travel times associated with the fundamental mode in a weighted straight-ray tomography procedure to produce group velocity maps at 3.0 Hz and 3.5 Hz. The velocity trends in our results correspond to shallow depths and coincide well with lithologies outlined in a geologic map of the survey area. The most prominent features resolved in our velocity maps are the low velocities in the north corresponding to less-consolidated materials, high velocities in the south corresponding to more-consolidated materials, a low-velocity zone corresponding to artificial fill in Alamitos Bay, and a low-velocity linear feature in the Newport-Inglewood Fault Zone.
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Locating Scatterers Ahead of a Tunnel Boring Machine Using Noise Correlation
Authors U. Harmankaya, A. Kaslilar, K. Wapenaar and D. DraganovSummaryThe investigation and detection of faults, fractures, karst zones, cavities, etc., is important to reduce hazardous risks, in particular during excavation with tunnel boring machines. To locate such scatterers, we propose a method based on seismic interferometry that uses the noise signals generated by a TBM. Using finite-difference modelling of TBM noise in a homogeneous half-space, we model noise recordings. We then correlate the noise to obtain non-physical scattered arrivals for various scatterer locations. We use both non-physical scattered P- and S-wave arrival times to successfully estimate the location of scatterers.
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3D Modeling of the Regional Basement Structure Off- and Onshore Congo Based on Inversion of Satellite-derived Gravity Da
More LessSummaryThe regional geology of the African equatorial west coast is complex and not well understood since the seismic coverage of deeper crustal features is sparse. The intricate structural settings are also reflected in potential field anomalies, and in particular in gravity data. The presented study primarily aims at estimating basement depths from satellite-derived gravity anomalies in a 400 km by 300 km wide area off-and onshore the territories of Congo, Cabinda, and the Democratic Republic of Congo. Where available, well and seismic data are used to constrain the inversion. The final 3D density model indicates a lateral division in 3 zones (oceanic crust, transition zone, continental crust) and maximum sediment thicknesses of 2.5 / 7.5 km on- / offshore. Although non-uniqueness and ambiguity of potential field data still pose challenges, it is concluded that public-domain gravity data can add useful information for regional modeling.
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Applying Magnetic Magnitude Transforms to Aid Structural Mapping in Areas Where the RTP Calculation Is Unreliable
Authors S. Cheyney, C.M. Green, S.J. Campbell and D. de LermaSummaryA common use for magnetic data is to locate the edges of subsurface bodies. Numerous transformations have been developed in order to obtain more easily interpretable maps, however many initially require the data to be reduced to the pole (RTP) if they are to be used for edge detection. The RTP operation is unstable at low magnetic latitudes, which affects significant areas of equatorial regions currently of interest to the oil and gas industry. In addition, any remanent magnetisation will often lead to anomalous results.
We demonstrate the advantages and disadvantages of using magnitude transformations, focusing on the scenarios where they are likely to be the most useful: at low latitudes and in the presence of remanent magnetisation. Based on synthetic modelling, magnitude transforms equivalent to 1st or 2nd order derivatives of the magnetic field appear most suitable, however when applied to real data the effects of enhancing the noise level limits the usefulness. The Total Amplitude, which is of the same order as the observed TMI data, has higher centricity than the TMI allowing lineament interpretation, without biasing the result to shorter-wavelength features and accentuating the noise level in the data.
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Noise Rejection in Processing of Magnetic Data - 2D Fourier Transformation Treated as an Inverse Problem
Authors H. Szegedi, A. Kiss, M. Dobroka and Á. GyulaiSummaryIn this paper, a new, robust and resistant, inversion based 2D Fourier transformation is presented where the spectrum is discretized by series expansion (S-IRLS-FT). The series expansion coefficients as model parameters are given by the solution of the inverse problem. Since it is advantageous to use squared-integrable, full, orthogonal and normed basis functions, Hermite-functions are chosen as basis functions of the inversion based Fourier transformation. Taking advantage of the beneficial properties of Hermite polynomials, that they are the eigenfunctions of the inverse Fourier transformation, the elements of the Jacobian matrix can be calculated fast and easily, without integration. The procedure can be robustified using Iteratively Reweighted Least Squares (IRLS) method with Steiner weights. The advantage of the Steiner weights is that the scale parameters ( 2) can be determined from the statistic of the measured data set in an inner iteration process. Thus, a very effective robust and resistant inversion procedure can be defined. Its applicability using magnetic data calculated above a square and “L”-shape object is proved.
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Approximate CRUST1.0 Model-based and Space-domain Calculated Gravitational Effect of the Earth Crust
Authors J. Mikuška, I. Marušiak, P. Zahorec, J. Papčo and R. PaštekaSummaryRecent deep seismic studies support the existing isostatic systems only to a limited extent and, in some areas, considerable disagreements have been pointed at. This implies that we should attempt to replace the classic isostatic corrections by quantities calculated from a-priori information which should be as independent of the gravity data themselves as possible. We believe that CRUST1.0 model provides applicable material for such an attempt. In our contribution we demonstrate that to calculate gravitational effects of the CRUST1.0 model layers taking the model densities into consideration is in fact a viable option. We have used two different methods of calculation, more or less independent of each other, both of them working in space domain. We believe that space-domain calculations are suitable for employing the results in gravimetrical practice since they not only are synoptic, i.e. offering a general view of the whole, but also enable us to analyze their elementary and differential properties.
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Applications of the Fractional Order Analytic Signal amplitude and Local Wavenumber
More LessSummaryAnalytic expressions for the fractional order anomaly, analytic signal amplitude, and local wavenumber of different magnetic models are derived. These expressions can be integrated with existing source-distance methods and used as part of the semi-automatic interpretation of magnetic anomalies. The order of the fractional order derivatives can be chosen based on the noise levels of the data.
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3D Inversion of Gravity Gradient Data Based on Cokriging to Identify the Dip Angle of a Dipping Dike Model
More LessSummaryWhen we invert gravity gradient data to recover 3D density distributions by cokriging, the dip angle of a dike directly influence the recovered results. Where there is a lack of the prior information about the dip angle, to obtain the reasonable results we present a method to identify the angle. Firstly, keeping the parameters same in cokriging inversion but we introduce some different dip angles by angle scan approach, then according to the prior information, only zero and positive density contrast are chosen. The standard deviations of the residuals between the observed and predicted data change with the angle. And the estimated dip angle of this method is the scan angle corresponding to the minimum standard deviation. In this paper, we choose four typical dipping dike model with the dip angle of 0o, 45o, 90o, 135o. Tzz data was inverted to test the validity of the mothed. All these four dip angles can be estimated correctly. This result make cokriging can be applied in more cases in which even the dip angle is unavailable.
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Identification the Basin Structure Using FHD and SVD - The Case Study in North East Java Basin
Authors F.H. Hisyam, Q.D.T.F. Fiandani and Z.F. Zuhrotul FirdausSummaryResearch has been done in North East Java Basin using gravity method. The data which be processed on this research are obtained from Bureau Gravimétrique International (International Gravimetric Bureau). Initial data processing yields Complete Bouguer Anomaly map which is filtered using Upward Continuation to separate regional and residual anomaly. The residual anomaly data was processed using derivative analysis include First Horizontal Derivative (FHD) and Second Vertical Derivative (SVD). The angle which used in FHD is 135o. Result from this analysis show six fault structures which has NE - SW orientation. The structures are normal fault which associated with half graben. The high area (ridge zone) is shown with high value Residual Bouguer Anomaly (RBA) ranged 6 until 11 mGal. The low area (trough zone) is shown with low value RBA ranged -16 until -8 mGal. These results appropriate with geological data of the research area.
Keyword: FHD, SVD, Geology Structure, North East Java Basin
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Robust Time-domain Migration Velocity Analysis Methods for Initial-model Building in a Full Waveform Tomography Workflow
Authors H.B. Santos, J. Schleicher, A. Novais, A. Kurzmann and T. BohlenSummaryFull-waveform tomography (FWT) is notorious for its strong dependence on the initial model. We present a workflow for the construction of initial velocity-models for FWT methods consisting of automatic time-migration velocity analysis by means of double multi-stack migration, followed by time-to-depth conversion by image-ray wavefront propagation. Evaluation of the converted velocity model as an initial velocity model in an acoustic FWT process indicates the potential of using a combination of these methods to achieve a fully automatic tool for initial-model building in a FWT workflow. Our tests on a modified version of the Marmousi-2 model have shown that correct background velocity information can be successfully extracted from automatic time-domain migration velocity analysis even in media where time-migration cannot provide satisfactory seismic images.
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Hybrid Super Memory Gradient Method Full Waveform Inversion
Authors Y Hu, L.G. Han, v Zhang, L. Bai and T.Z. ZhangSummaryIn this paper, we discussed the Hybrid Super Memory Gradient (HSPM) method for FWI, whereby the Super Memory Gradient (SPM) method and Unconstrained Super Memory Gradient (USPM) method are used for FWI iterations. SPM method uses many gradients information to correct current descent direction, so we can obtain more accurate descent direction. USPM method without Wolfe linear search which can save a lot of computational time. Firstly, at low frequencies, we use USPM method , and then use SPM method to inverse high frequencies information. The numerical examples show that HSPM delivers similar results as those expected with the SPM method, and improved computational efficiency over Conjugate Gradient method.
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Frequency Domain FWI with Angle Domain Wavenumber Filter Using Local Slant Stacking
More LessSummaryThe scale length of velocity perturbations is closely related to the scattering angels of the wave. In the full waveform inversion, waves with different scattering angles can retrieve model parameters of different scale length. We introduce an angle-domain wavenumber filter for frequency domain full waveform inversion. Both source- and receiver-side waves are decomposed into local plane waves using slant-stack method. At each grid point, the scattering angles between the incident and scattering directions are used as the constraint to determine the wavenumber components to be retrieved. Large scattering angles are related to large-scale model perturbations, so by filtering out small scattering angles in the inversion, we can get the large-scale background model. Numerical examples show that when the initial model has large velocity errors and the low-frequency information is missing in the data, the angle-domain filter can largely improve the convergence in the initial iteration stage.
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Multi-scale Frequency-domain Elastic Full Waveform Inversion Based on Dual-level Parallelism
More LessSummaryThe complexity of the elastic wavefield increases the nonlinearity of inversion. To some extent, multi-scale inversion decreases the nonlinearity of inversion and prevents it from falling into local extremes. A multi-scale strategy based on the simultaneous use of frequency groups and layer stripping method based on damped wave field improves the stability of inversion. A dual-level parallel algorithm is then used to decrease the computational cost and improve practicability. The seismic wave modeling of a single frequency and inversion in a frequency group are computed in parallel by multiple nodes based on MUMPS and MPI. Numerical tests using an overthrust model show that the proposed inversion algorithm can effectively improve the stability and accuracy of inversion by selecting the appropriate inversion frequency and damping factor in low-frequency seismic data.
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An Optimized Correlation-based Full Waveform Inversion
Authors Y. Choi and T. AlkhalifahSummaryThe conventional correlation-based objective function for FWI has a limitation: the desired (accurate) solution is not exactly located at the global minimum (or maximum) since the auto-correlation of the observed wavefield has some energy at non-zero time-lags. We propose two optimized correlation-based objective functions incorporating the auto-correlation term. Including this term in the new objective functions as a reference for the cross-correlation to fit locates the desired solution at the global minimum (or maximum). We derive the gradient expressions of the new objective functions using the adjoint-state method, where only two modelling steps are needed to calculate the gradients. The numerical examples show that FWI based on the new objective functions generate good convergent results, whereas the conventional correlation-based FWI does not.
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Wavelet Filter Based Low-frequency Data Reconstruction for Time Domain Full Waveform Inversion
Authors P. Zhang, L.G. Han, F.J. Zhang and Y. ZhouSummaryThe conventional full waveform inversion (FWI) often uses local optimization algorithm to update velocity model. So the initial model we provide should be good enough to avoid local minimum. Abundant low-frequency information can compensate for the inaccuracy of the initial model and help to avoid cycle-skipping. In this paper, we proposed a wavelet filter based low-frequency data reconstruction method. We extracted the subsurface impulse responses using Fast Iterative Shrinkage-Thresholding Algorithm (FISTA), and convolved the broad-band impulse responses with band-limited low-frequency source wavelets to obtain low-frequency data. The convolution process is equivalent to filtering using low-frequency source wavelets. The accuracy analysis demonstrated that the reconstructed data can meet the requirement of FWI. We proposed a new strategy for multiscale time domain full waveform inversion (TDFWI), which using a series of low-frequency reconstructed data as observed data. One distinct advantage is that the wavelet is accurately known for the reconstructed data, which reduces the uncertainty of FWI. This strategy avoids the effect of source wavelets uncertainty on inversion results, and avoids the simultaneously pre-process of data and wavelets. Numerical example shows that our strategy can avoid cycle-skipping effectively and can converge on a bad initial model.
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Hybrid Frequency Domain Full Waveform Inversion Using Born Sensitivity Kernels
Authors R. Djebbi and T. AlkhalifahSummaryFull Waveform inversion (FWI) using the scattering integral (SI) approach is an explicit formulation of the inversion optimization problem. The inversion equations are straightforward and the dependence on the data residuals and model parameters is clear. However, the biggest limitation with this approach is the huge computational cost for exploration seismology applications. To deal with this issue, we propose a hybrid implementation of the frequency domain FWI using the Born sensitivity kernels. Specifically, we use the sensitivity kernels computed from dynamic ray-tracing to build the gradient. We compute also the truncated Gauss-Newton update direction using the kernels without extra wavefield modeling steps. Considering that in FWI long-to-intermediate wavelengths are updated during the first iterations, using a transmission experiment
we obtain accurate inverted models. The inversion managed to develop the anomaly embedded in the homogeneous background medium. The truncated Gauss-Newton updates helped in the fast convergence. With this approach we managed to reduce the computational cost and the memory requirements. For more complex models, the hybrid inversion method help improving the initial model with little cost compared to conventional SI inversion.
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Multiscale Adaptive Full Waveform Inversion Based on the Wavelet Transform
Authors L. Bai, L.G. Han, F.J. Zhang, P. Zhang and Y. HuSummaryThe conventional full waveform inversion (FWI) is often suffered from the cycle skipping problem. In order to solve this problem, we introduced the wavelet transform to the FWI, and combined with the least squares filter in the wavelet domain. It can effectively reduce the influence of the cycle skipping problem in the inversion procedure, and improve the stability of the FWI. The least square filter has the higher accuracy in the wavelet domain than in the time domain. By using this feature we can narrow the phase difference between the predicted data and observed data, and construct a new objective function to make the inversion procedure steadily converge to the global minimum. Meanwhile, due to the multiscale characteristic of the wavelet transform, the data can be divided into different frequency bands. We could run the FWI in a multiscale way. The results from the synthetic example demonstrates that the multiscale adaptive FWI based on the wavelet transform is much less dependent on the initial model and low-frequency data. The method can also effectively reduce the cycle skipping problem and more robust than the conventional FWI.
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The Comparison between Full-stack Data and Pure P-wave Data on Deeply Buried Ordovician Paleokarst Reservoir Prediction
More LessSummaryThe present burial depth of the paleokarst reservoirs in the Tarim Basin is greater than 5000m, which premises rigorous demands for the data quality for accurate predictions. Although routinely used for reservoir prediction, the conventional full-stack data are often contaminated by the AV O (amplitude versus offset) effects no matter how much signal to noise ratio degree can be enhanced via stacking. This contamination usually changes with different geological deposits, and could reach an inappropriate and unacceptable level for the deeply buried carbonate reservoir in the Tarim Basin. In this paper, the pure P-wave data theoretically inverted through AVO effects removal are employed to improve the reservoir prediction, which are illustrated by the comparisons with that of the conventional full-stack data for the four classical AVO models and the field data in the ZG8 area, Tarim Basin. The dominant frequency has been improved for 8 Hz from 15 to 23 Hz in the target Yingshan formation, Ordovician through AVO effects removal, and the reflection events of the pure P-wave data are obviously more continuous comparing with that of the full-stack data. The fake potential reservoir caused by AVO effects in the northeast area has been removed, while the integral delineation of paleokarst reservoirs have been significantly improved with a higher fitness with the oil-testing results, which are more beneficial for later on exploration. The pure P-wave data inversion is in essence an important complement to current processing strategy.
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Flow Detection Using Well Seismic Data
Authors J.L.O. Mari and G. PorelSummaryThe aquifer studied, 20 to 130 m in depth, consists of tight karstic carbonates. 3D seismic survey revealed three high-porosity, presumably-water-productive, layers. Ambient seismic noise and VSP data have been recorded in 11 wells in order to evaluate the potential of the well seismic method to detect karstic bodies and flows, the results being compared with full waveform acoustic data, BHTV logs and flow measurements.
The paper describes the methodology developed to detect flow using both ambient noise measurement and conversion of P-wave into Stoneley waves, observed on VSP data recorded with hydrophone sensor. An attribute, named VSP flow index, has been calculated to detect both karstic levels and flows. Concerning the karstic level detection, the results obtained on 11 wells are compared with full waveform acoustic data and confirmed by BHTV logging data and at a large scale by seismic data (3D seismic). The results obtained are consistent. The VSP data have also been compared with PLT data. An attempt to obtain a VSP flow detector has been done. The conclusions of the study show that ambient noise analysis and conversion of P- body waves into Stoneley waves can be fruitfully used to detect productive levels and flow circulation.
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Well Controlling Multi-trace Seismic Deconvolution Technology
More LessSummaryConventional deconvolution methods in reducing band-limited wavelet interference often cause lateral instability of the estimated reflectivity or impedance. In addition, conventional deconvolution processing ignores the spatial relationship between traces. Spatial regularization technique can solve the problems to some extent, but there is still lack of recovery of low frequency and high frequency and ill posed problems. In this paper, based on spatial regularization, we induced logging data and proposed wells controlling multi-trace deconvolution method. A synthetic data example and a field data example are adopted to illustrate the method effectively. After processing, the seismic data resolution is improved significantly and its spatial continuity keeps well. The result has good match with low-pass filtering of logging data. The method can provide high resolution seismic data for thin reservoir prediction.
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Multi-scale Fracture Prediction Using P-wave Data - A Case Study
More LessSummaryFracture prediction is very difficult when characterizing unconventional hydrocarbon reservoirs, especially for reservoirs with different fracture scale sizes. Here, we present an integrated workflow combined with poststack and prestack seismic attributes to distinguish between fractures of different scale, which is used in a case study from a basin in China. We classify fracture scales into three categories: macro-scale, meso-scale and micro-scale. In this case study, we used poststack seismic attributes to predict the macro-scale fractures, and utilized prestack seismic inversion to determine the meso-scale fractures. Micro-scale fracture prediction is out of the range of this study. We combined the prediction results with the superimposed images of faults, verified the corresponding relation between meso-scale fractures and faults, and finally comprehensively analyzed the fracture development of the target horizon. The integrated prediction result indicates that the structure of the target horizon is mainly controlled by several major faults and meso-scale fractures are distributed along the fault zones.
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Karst Carbonate Reservoir Identification Using Frequency-dependent AVO Inversion in Tarim Basin, China
More LessSummaryThe dominate storage spaces of karst carbonate reservoir in Hadexun area, Tarim basin are dissolved pores and cavities. The influence of multi-stage structure movement makes the fluid discrimination difficult. Therefore, how to further recognise the fluid type in such reservoir is the key point. Unlike conventional post-stack methods have difficulties in fluid identification, AVOF inversion algorithm utilize P-wave dispersion property reflected by fluid to discriminate its type. Thus greatly improve the accuracy of oil exploration. In this paper, the AVOF inversion method is applied on 3D field seismic data in Hadexun area to solve the problem of fluid discrimination in karst reservoir. Field data application demonstrates that reservoirs filled up with water and hydrocarbon can be effectively separated by inverted P-wave dispersion. To be more important, by analyzing the inversion result and fault distribution, we successfully divided the faults into two groups, which are water-contained and hydrocarbon filled. In sum, the encouraging result of AVOF inversion provided robust information in karst reservoir characterization and fluid distribution, which directly offer key data for well optimization and deployment.
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Seismic Facies Characterization of the Realgrunnen Subgroup in the Wider Hoop Area, Barents Sea
Authors W. Athmer, M. Etchebes, E. Stueland, S.C. Robertson, H.G. Borgos, B.A. Tjostheim, L. Sonneland and J.R. GranliSummaryA seismic facies characterization of the Late Triassic Fruholmen Formation and the Jurassic Stø Formation —both hydrocarbon-bearing reservoir units within the Realgrunnen Subgroup in the Barents Sea—was performed to better understand reservoir facies distributions and variations in the wider Hoop area. Depositional features such as fluvial channels and associated elements were highlighted using (1) RMS amplitude maps draped onto stratal slices, and (2) colour-blended images obtained from partial reconstructions of the seismic volume reflection spectrum (VRS). Depositional thickness variations were investigated using isochore maps, and information obtained from regional well data was considered for the analysis. The study indicates a fluvial-dominated depositional system for the mid- to upper-Fruholmen Formation, and a shallow-marine environment for the overlying Stø Formation. Differences in facies thickness distributions are attributed to morphological changes that constrain the depositional environment and are related to paleo-topography, and fault activity before and during deposition of the reservoir section.
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High Precise Time-frequency Analysis Technology in Hydrocarbon Detection of the Ultra-deep Carbonate Rocks in Tarimbasin
By Q.N. DangSummaryWe present results from a case study being performed in the Ordovician ultra-deep(6500m-8100m) carbonate rocks of TB area in Tarim basin that utilizes the high precise WVD (Wigner Ville distribution) time-frequency technology to detect hydrocarbon of Ordovician carbonate rocks. Through suppressing the cross-term of WVD based on the technique of two weight window function, we obtain the high quality frequency spectrum imaging which preserves low frequency. This technology can not only characterize the boundary of the “string” of carbonate cave-fracture, but also detect the hydrocarbon in the “string”. Two sets of hydrocarbon detection criterion have been built about karst reservoir in buried hill and in interlayer. The results of hydrocarbon detection are compared with several drilled wells to confirm the validity of this technology.
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Influence of Seismic and Velocity Uncertainties on Reservoir Volume
Authors D.T. Odinsson, N. Cardozo and L. SchulteSummaryUncertainty estimation is a well-known concept in geology that can lead to re-evaluation of important development decisions if properly assessed. This study evaluates structural uncertainty through the set-up of “scenarios”. For each parameter of the structural model a low case, base case and high case are defined. The combination of these cases results in numerous structural models that deliver a distribution of the bulk volume. A generally acknowledged way of handling the large number of models coming from the different combinations of the model parameters is experimental design.
This study shows that uncertainties in seismic picks and consequently in reservoir thickness have a large impact on the gross-volume. The reservoir structural geometry controls the influence of fault uncertainty on the reservoir volume. The well velocities used for domain conversion are quite accurate but sparsely sampled and therefore also subject to uncertainty. In addition, the geologic complexity may have a dramatic influence on the uncertainty of the depth conversion. The assessment of seismic and velocity uncertainties was applied to the structural model to estimate the reservoir volume uncertainty. The results confirm that structural uncertainty is a major contributor to the hydrocarbon volume uncertainty.
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Altering the Relaxation Modulus of Crude Oil Using Pulsated Magnetic Field
By D. PandeySummaryThe aim of this technical paper is to present the idea through an experiment that magnetic field can be used as an alternative tool to change the relaxation modulus of crude oil and thus improving its transportation efficiency. During the experiment, it was found that the contact angle between crude oil and sensor probe was influenced by magnetic treatment which resulted in the prevention of wax sticking on the sensor probe. From the experiment it was confirmed that the field can also be used to prevent the wax accumulation.
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