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81st EAGE Conference and Exhibition 2019
- Conference date: June 3-6, 2019
- Location: London, UK
- Published: 03 June 2019
41 - 60 of 1010 results
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Automatic Arrival Time Uncertainty Assessment for Downhole Microseismic Monitoring Data
Authors I. Abakumov and S.A. ShapiroSummaryArrival time uncertainties intrinsically define the accuracy of all arrival time-based measurements, e.g. the precision of wave arrivals in microseismic registration. They are used to weigh the data in inversion algorithms and to define the resolution of reconstructed velocity models. Although a wide range of methods for arrival time uncertainty estimation has been proposed in the literature, the physically most prominent ones are based on the probabilistic formulation. We review two probabilistic approaches for assessment of the lower boundary of picking error which are well-known in the radio signal processing - the Cramer-Rao Bound and the Ziv-Zakai Bound. These classic bounds require explicit knowledge of the spectral variance of the signal that is often hard to determine in microseismic experiments. Hence, we present reformulations of the bounds suitable for the case of downhole microseismic monitoring. The resulting easy-to-use analytical expressions require the signal and noise parameters that can be directly obtained from microseismic data and, hence, can be efficiently applied to the data in a semiautomatic fashion. We further demonstrate that the proposed analytical expressions provide realistic values of the arrival time uncertainties, and that the obtained uncertainties can be used to improve further processing steps, e.g., microseismic source localization.
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Microseismic Event Location Using Time Reversed Imaging Based on Decoupled Wavefields in VTI Media
More LessSummaryLocating microseismic events in a reservoir monitoring system is of great significance due to its capacity of delineating the induced fractures. Time reversed imaging (TRI) method, which refocuses the back-propagated seismic energy to its real origin, has been demonstrated as a reliable location technique especially in noisy data processing. However, the time reversed images are often contaminated with strong imaging artifacts due to the cross talk of coupled wave modes coinciding in time and space especially in anisotropic media, leading to unreliable location estimations. To minimize the interference of strong imaging artifacts, we present a TRI technique based on decoupled wavefields. Spatial filters constructed based on Christoffel equation are applied to separate qP and SV wave modes. The auto and cross-correlations of decoupled qP and SV potentials are then applied as the image conditions to form the final location images. The synthetic example has shown that the spatial filters can fully separate the wave modes in 2D VTI medium compared to Helmholtz decomposition, leading to an improved location images with a much sharper focus.
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A Comparative Study of Ray-Based and Waveform Local Earthquake Tomography: an Application in Albania Thrust Belts
Authors A. Soni and R. PlessixSummaryIn fold and thrust belt settings, local earthquake events often occur and can be recorded when we lay down a patch of seismometers. We can then invert those seismic events to retrieve velocity information as it is done in global seismology. A passive data has been recorded over an exploration block in Albania where we can detect local earthquake events. After identifying them, we have applied a ray-based traveltime inversion and an acoustic waveform inversion. Because of the large velocity variations and the illumination issues, ray-based traveltime inversion seems to give bias results. The waveform inversion appears more robust and geological possible, as expected because some of the scattering effects that may happen within the Fresnel zone are accounted for. This comparative study suggests than waveform inversion applied on passive data can help in such a complex geological setting.
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A Field Data Application of Regularized Elastic Passive Equivalent Source Inversion With Full Waveform Inversion
Authors H. Wang, Q. Guo, T. Alkhalifah and Z. WuSummaryUsing full waveform inversion (FWI) to locate passive events allows for an automatic process. Passive seismic data are often acquired on solid surfaces including the bottom of the sea, in which multi-component measurement under the elastic assumption is important. We develop a regularized elastic FWI of passive seismic events to invert for the source image, source function and the velocity model, simultaneously, without any a prior information about the source. We reformulate the elastic problem by representing the source images by $P$-wave and $S$-wave perturbation coefficients. The unknown source ignition time is mitigated by convolving reference traces with the observed and modeled data. A total variation regularization is applied to improve the robustness of the velocity inversion considering the limited sources and illumination angles of microseismic experiments. We also applied a focusing function to the source to overcome the possible limited aperture coverage of the acquisition, especially in well recording. The adjoint-state method is used to derive the gradient for the source image, source function and velocity. The resulting inversion framework is capable of handling limited aperture data and limited sources. Application to synthetic and real data with limited recording aperture along a well demonstrates the effectiveness of the approach.
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Cross-Correlation Migration of Microseismic Source Location with Multiplication Imaging Condition
More LessSummaryLocating microseismic sources are critical in hydraulic fracturing monitoring for unconventional oil/gas exploration. Waveform-based methods can reliably and automatically image microseismic source locations, but they usually need to scan the source excitation time. The cross-correlation migration (CCM) avoids excitation time scanning and reduces the total scanning dimensions from 4D (3D in space and 1D in time) to 3D. The conventional CCM sums all the contributions obtained from the virtual trace gathers, which leads to low resolution source location image. In this study, we propose to use the multiplication imaging condition (MIC) to replace the summation imaging condition which is used in CCM, and name it as CCM-MIC. Instead of summing all contributions obtained from the virtual trace gathers, the CCM-MIC multiply them to form the final image. By using CCM-MIC, it is more meaningful to select less receivers with good azimuthal coverage, rather than to use a large amount of indiscriminate receivers, which is required by CCM. Our approach is also beneficial for crustal scales seismology applications
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Joint Using PP-Wave and PS-Wave Elastic Impedance to Construct Reservoir Sensitive Elastic Parameters
More LessSummaryDue to the different seismic amplitude information carried by PP-wave and PS-wave, the sensitive of PP-wave and PS-wave to reservoir are different. The joint using of PP-wave and PS-wave information can reduce the multi-solution of reservoir prediction. The construction of conventional elastic parameters is mainly based on PP-wave seismic information. This study investigates elastic parameter construction by using PP-wave and PS-wave elastic impedance jointly. Based on the previous research on PS-wave elastic impedance, a new form of PS-wave elastic impedance equation which can keep consistency with the PP-wave elastic impedance equation is derived. According to the construction formula of conventional elastic parameters, we use PP-wave and new PS-wave elastic impedance instead of P-wave and S-wave impedance to construct a new kind of elastic parameter with the concept of angle elastic parameter. Compared with the conventional elastic parameters, the angle elastic parameters have incidence angle and PS-wave information and can be used as sensitive indicators of reservoir. Both the model and real data show that the angle elastic parameters have high sensitivity to the beneficial reservoir.
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The Analysis of the Influence of Vs/Vp on the Elastic Impedance Inversion
More LessSummaryElastic impedance (EI) inversion as a popular pre-stack seismic inversion method has been studied in the past years. However, the robustness of estimating elastic parameters of P-wave velocity, S-wave velocity, and density in conventional EI inversion is still controversial, especially the accuracy of the S-wave velocity. In this paper, we show that the obvious inversion error of the Vs is associated with the accuracy of the ratio of VS to VP (VS/VP) when the three parameters are inverted. The deviation of the VS/VP in the linearized Zoeppritz equation provided by Aki and Richards results in the mismatch between the forward and backward modeling. A new kind of inversion formula containing VS/VP as the inversion parameter is proposed, by which VS can be estimated in terms of the new inverted parameter. Due to the nonlinear relationship between the new inverted term and the VS, the Newton downhill method is used to solve the nonlinear problem. The synthetic data inversion examples demonstrate the performance of the new formula.
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Azimuthal Amplitude Difference AVAZ Inversion for Orthotropic Parameters
More LessSummaryAnisotropic azimuthal amplitude variation with offset (AVO) inversion works as an important tool to estimate elastic parameters and anisotropic parameters of underground medium from pre-stack and azimuthal seismic data in the geosciences. Shale formation with parallel vertical or near-vertical fractures is often described with orthogonal anisotropic medium. However, the problems, which more model parameters, higher condition number of forward solver and relatively stronger dependence on the initial model for traditional orthotropic azimuthal AVO inversion, render the poor stability of the estimated parameters. In this paper, we carry on the study on the approaches with wide azimuthal pre-stack seismic data to establish the azimuthal amplitude difference AVO forward solver of orthotropic media, and implement the azimuthal amplitude difference AVO inversion for the orthotropic parameters under Bayesian framework with smooth background constraint to reduce the condition number of forward solver and improve the robustness and stability of inversion. Besides, Cauchy and Gaussian probability distributions are respectively utilized as priori information of the model parameters and the likelihood function for enhancing the inversion precision. Finally, field data examples are finally used to verify the feasibility and stability of the proposed approach in the estimation of the anisotropic parameters in orthotropic medium.
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DE-MCMC Based Stochastic Seismic Inversion Incorporating Gaussian Mixture Model
More LessSummaryStochastic seismic inversion works as an important technology to estimate elastic parameters of subsurface media to guide the lithology prediction and fluid discrimination. In this study, an improved stochastic simulation is proposed to invert seismic impedance and lithofacies simultaneously. The Gaussian mixture priori probability density (PDF) is initially utilized to describe the distribution of model parameters influenced by subsurface lithofacies. Furthermore, a novel expression of multi-dimensional posteriori PDF conditioned with time and frequency joint-domain seismic data is derived. And, the differential evolution Markov Chain Monte Carlo (DE-MCMC) algorithm is utilized to implement the optimizations of multi-dimensional posterior PDF in our approach, which runs multiple Markov chains in parallel and estimates the multiple solutions of model parameters with the theory of population evolutionary. The lithofacies is clearly discriminated according to the weights of different Gaussian components in the posterior PDF and the model parameters are sampled from the selected Gaussian components to realize the simultaneous prediction of these two parameters. Finally, the feasibility and robustness of DE-MCMC model are illustrated by several synthetic examples and field datasets. The estimated P-wave impedance and lithofacies classification results of maximum conditional probability density (Cpd) coincide with the well logging curves and interpreted lithofacies.
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One Novel Elastic Impedance Inversion for Brittleness Sensitive Parameter in Sichuan Basin
More LessSummaryAt present, accurate prediction of high brittleness regions is an important issue in the development of shale gas field. Elastic impedance inversion works as an effective technology to estimate elastic parameters of subsurface media to guide the identification of reservoir characteristics. In this study, an improved elastic impedance inversion is proposed to establish a workflow for brittleness prediction on shale gas field. Based on previous research results, an expanded elastic impedance equation is derived from reflection approximation and established relationship between brittleness sensitive parameter Eρ with elastic impedance. After that we advance a workflow for target parameter extraction based on Bayesian theory. Then the novel elastic impedance equation is accepted the accuracy test and verified the reliability. Finally, the workflow is utilized to implement the applicability on shale gas field data of Sichuan Basin, which runs well even on the influence of the SNR=3. and the Eρ section also shows that it is coinciding with the well logging curves in the destination layer.
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Prestack Inversion Based on Bayesian Theory in Ray Parameter Domain and its Application
Authors D.J. Hou, D.H. Zhou, D.Y. Guan, J. Ren and X.J. ZhangSummaryWe can obtain elastic parameters through prestack amplitude versus offset (AVO) inversion, which is used to characterize the reservoir, so it is significant to obtain high precision elastic parameters in order to get highly reliable reservoir prediction result. In this paper, we develop a AVO inversion method based on Bayesian theory in ray parameter domain, whose output is density, P-wave impedance and Vp/Vs. These elastic parameters have high precision, which are valuable input for reservoir characterization because they are related to lithology and fluid content of the reservoir. In ray parameter domain inversion, the ray path of seismic wave propagation is considered polyline, which is more consistent with the actual situation, thus extracted amplitudes of P gathers used in inversion are more accurate. In addition, the reflection coefficient formula in ray parameter domain has higher precision when the incident angle is large. The inversion based on Bayesian theory can improve the stability of the inversion. Test on the actual data shows that the result of ray parameter domain inversion with a Bayesian scheme is more accurate.
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Integrated Pre-Stack Seismic Interpretation Through Volumetric Flattening
Authors K. Wang, D. Hammock, N. Mclean, K. Osypov, T. Van Wagoner and E. XuSummarySeismic stratigraphy and geomorphology interpretation (SSGI) and AVO interpretation are conducted as separate workflows in current common practices. It is a compelling argument to integrate SSGI and AVO interpretation since they interpret essentially the same seismic dataset despite different methodologies used. The main obstacle for the integration to be efficiently executed is the complexity of handling pre-stack data, which arises due to complex AVO responses from real rocks and non-perfect pre-stack gather formed by image migration. Here we propose a new method that effectively bridges volumetric flattening with pre-stack data. With sparse interpretation picked on pre-stack gathers, this method automatically morphs flattened cubes to adapt to angle/offset variations. It honours true AVO signals rather than apparent moveout in the gather. Application to field data showed the benefit of this method in integrated SSGI and AVO interpretation with much improved quality and highly efficient turnaround time.
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North Sea Field Application of 4D Bayesian Inversion to Pressure and Saturation Changes
Authors G. Corte, C. MacBeth and H. AminiSummaryIn this work we present a workflow for 4D seismic inversion to pressure and saturation changes that uses a full petro-elastic and convolution forward seismic modelling operator and a Bayesian formulation for combining information from a history matched simulation model and seismic data. The forward modelling operator is ideal for capturing the non-linearities due to gas saturation and pressure changes. The inversion method is applied to maps of Schiehallion field in the North Sea, to invert seismic 4D AVO attributes into changes in pressure, water and gas saturations. The results can be interpreted as an updated view of reservoir dynamics, bringing together information from seismic data and the current history matched reservoir simulation results. In the presented application to the Schiehallion field, the inversion results offered important insights into overlooked pressurized compartments, updated water fronts and the global distribution of the released gas.
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Improved Reservoir Delineation in Complex Geological Settings Using CLSSA: a Case Study from Offshore Nova Scotia
Authors A. Pant, D. Ghosal and C. PuryearSummaryThe study undertakes the demonstration of Constrained Least Squares Spectral Analysis (CLSSA) as tool for delineating and characterizing reservoir zones while establishing it as an evolving frontier Fourier approach to spectral decomposition via a case study example with a brief review of the model behind CLSSA. It displays the efficiency of CLSSA in imaging and characterizing reservoir zones, thin layers, structures such as faults and complex stratigraphic features such as reefs. The results are also contrasted against the conventional Short Time Fourier Transform (STFT) which consequently appears to be incompetent against CLSSA. This investigation also illuminates details about the hydrocarbon prospect of the Penobscot Bay region, offshore Nova Scotia and shows that the well missed the actual target carbonate reef structure which appears distinctly at an anomalously low frequency (ALF) of 5Hz in the CLSSA section while STFT fails to resolve it. Further, it also signifies the robustness of CLSSA to clearly resolve and illuminate features of interest in complex siliciclastic and carbonate settings. It thus, offers high resolution and localization holds immense prospect to quantitatively assess tuning thickness of beds.
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Shale-Gas Reservoir Characterization Based on Quantitative Seismic Interpretation by Using Anisotropic Seismic Attributes
More LessSummaryThis paper presents a case study of brittleness and organic-matter contents (OMC) quantitative seismic interpretation based on an anisotropic AVO inversion scheme and an improved statistical quantitative interpretation method. The target is a lower Silurian-age shale-gas reservoir formation with strong anisotropy in southern Sichuan Basin. In this case, the anisotropic inversion can be more robust than conventional isotropic inversion. The capability of anisotropic and isotropic attributes in brittleness and OMC interpretation are quantitatively compared by well log analysis. Besides, lithology recognition is combined with brittleness and OMC interpretation to reduce the non-uniqueness of the results. The results show that the predicted brittleness and OMC match well with the observations from well-log.
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Targeting Remaining Barrels in Brown Field Environments: Assessment of Statfjord Formation Using OBN Seismic in Alwyn
Authors M. Edwards, A. Mitra, A. Parsa, O. Onyia and J. MilneSummaryStock-tank oil-in-place (STOIIP) estimates are calculated for two development opportunities in the Alwyn North field in the Northern North Sea. They are from the lower unit within the Statfjord Formation (the Statfjord D), which have been discovered to be light oil-bearing by two wells, N50 and N54Z. Both wells are currently producing gas from the upper Statfjord Unit A. This study presents a methodology workflow with the final aim of calculating reliable STOIIP estimates for the Lower Statfjord.
Using newly acquired and processed (2016) OBN seismic, the existing interpretation of the Top Statfjord horizon is refined with the use of seismic attributes. Extracted wavelets are used to generate synthetic seismograms at three well locations to identify the Statfjord D reflector in the seismic data. An iso-thickness of 100 m is added to the refined Top Statfjord horizon to create a pseudo Statfjord D horizon, which is further corrected for use in volumetric calculations. STOIIP volumes are estimated probabilistically using Monte Carlo stochastic simulation to incorporate uncertainty ranges and distributions for each input parameter. Sensitivity analysis to find out which parameter most influences these estimates revealed that errors in the velocity model contribute the most to the STOIIP uncertainty range.
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Application of the Three-Dimensional Stress Body-Based Weak Strike-Slip Fault Identification Technique in JZ Structure, Bohai Basin
More LessSummaryStructural traps related to weak strike-slip faults have been proven to be potential hydrocarbon-enriched zones in the exploration practice. However, they are difficult to be effectively identified in the seismic profile by conventional techniques. Previous studies suggest the good response of stress field to weak strike-slip faults. Accordingly, this study proposes a three-dimensional stress body-based weak strike-slip fault identification technique. First, the relationship between formation deformation, lithologic variation, and stress field is established. Specifically, the structural curvature body is used to represent the formation deformation, which is obtained by second-order derivation of the wave number domain, while Poisson's ratio and Young's modulus data volumes are employed to reflect the spatial lithologic variation, which are calculated by the multivariate linear fitting method. Finally, the three-dimensional stress body is determined according to the relational equation, and subsequently the weak strike-slip faults are effectively identified.
This study takes the No.1 Fault in the JZ Structure, Bohai Bay Basin as an example. Through seismic interpretation using the proposed technique, it is proved that study area is horsetail structure and a series of structural traps are pinpointed. In general, this technique can well identify weak strike-slip faults and has important significance for seismic interpretation.
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Application of Geostatistical Seismic AVA Inversion for Shale Reservoir Characterization and Brittleness Prediction with Machine Learning
Authors M. Cyz, L. Azevedo and M. MalinowskiSummaryIn this study we present an application of geostatistical AVA seismic inversion method for characterization of a unconventional Lower Paleozoic shale reservoir in Northern Poland. The target formations are of a small thickness (up tp 25 meters) and deeply buried (ca. 3 km) what makes their delineation and characterization especially difficult. An application of the iterative geostatistical AVA inversion method allowed for obtaining the high-resolution density, P-wave and S-wave velocity models together with the assessment of the uncertainty on the predictions. The obtained elastic property models were compared with the results of the deterministic simultaneous Amplitude-versus-Offset inversion proving that the application of a such sophisticated (geostatistical) inversion technique is a must while dealing with the thin and highly variable layers.
The inverted elastic models where further used to improve the prediction of a spatial distribution of the brittleness index with a machine learning (PSVM) algorithm by integrating well-log data and seismic rock property volumes.
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Generating Regional Acoustic Impedance Volume from Large Merge Data Set of the Dutch North Sea
Authors M. Jaya and G. Van NoortSummaryThis study presents a workflow for generating regional acoustic impedance volume from merged seismic data in the Dutch North Sea. The workflow includes seismic data merge, regional well-seismic calibration, adaptive wavelet extraction and post-stack model-based seismic inversion. 3D surveys of various vintages located around blocks P and Q were collected from public database. 103 wells located within the merged blocks are investigated for the well-seismic calibration. 15 out of these 103 wells with which statistically high correlation values are obtained, are selected for well-based wavelet extraction during adaptive and iterative well-seismic matching. The result of seismic inversion is analyzed using “blind” well. High correlation between the synthetic and real seismic traces at “blind” well location is achieved within stipulated inversion interval. The general trend of AI values resulting from the seismic inversion follows reasonably well to those of well data. AI contrasts observed at different stratigraphic formations such as Middle Holland Claystone Member and Rijn Member are properly captured by inversion and matches fairly good with the distribution observed in the regional well log. The use of regional acoustic impedance volume and map allows evaluation of prospects and fields in the regional context.
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Performance of Different Seismic Sources for DAS VSP Acquisitions: a Case Study at Curtin Well Site
Authors K. Tertyshnikov and R. PevznerSummaryDistributed acoustic sensing (DAS) technology lately has undertaken significant developments and been recognised as a novel versatile approach for borehole seismic exploration. We conducted a series of acquisitions at the Curtin well site to evaluate the performance of several low power sources in conjunction with fibre optic cables as the receivers. We have tested the effectiveness of a small weight drop, a percussion rod and a parker using optical fibre cemented behind the casing as sensors. Results demonstrate that such inexpensive sources with minimal environmental impact are suitable for downhole seismic with DAS. This combination is highly applicable to the wide range of tasks in mining exploration, near-surface investigations and monitoring, cross-hole studies.
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