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81st EAGE Conference and Exhibition 2019
- Conference date: June 3-6, 2019
- Location: London, UK
- Published: 03 June 2019
61 - 80 of 1010 results
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Seismic Interferometry Using Walkaway DAS VSP Data: CO2CRC Otway Project Feasibility Study
Authors E. Sidenko, R. Pevzner, A. Bona and K. TertyshnikovSummaryComing Stage 3 of the CO2CRC Otway project will focus on well-based CO2 monitoring techniques. Although time-lapse active source seismic is probably the most reliable geophysical method for monitoring of the injected CO2 it has some potential disadvantages: cost and invasiveness. Use of Distributed Acoustic Sensing (DAS) for permanent monitoring and utilization of the non-primary wavefield, such as free-surface multiples, can help to alleviate these issues. Permanently installed boreholes DAS receiver arrays can provide a better optimisation of active source effort, meanwhile, interferometric imaging of VSP multiples increases the illumination of the subsurface. We performed a feasibility test of interferometric imaging on the Otway synthetic walkaway VSP dataset and this test demonstrated that the proposed approach is viable for use on DAS data. Subsequently, we performed a test on the field walkaway VSP-DAS data. Both tests have shown that utilization of non-primary wavefield can significantly increase the efficiency of walkaway VSP providing more detailed subsurface image. Experiment on the field data has proved that DAS can be sensitive enough to deal with the secondary wavefield. Finally, the combination of interferometric and standard imaging for walkaway VSP could provide a subsurface image comparable to surface seismic.
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Virtual Shear-Wave Source Delivers a Reliable S-Wave Velocity Model for VSP Imaging
Authors A. Aldawood, I. Silvestrov and A. BakulinSummaryLand walkaway VSP data acquired with vibroseis data contains abundant amounts of converted waves. Their imaging requires an accurate shear-wave velocity model that is often not available. Here, we apply seismic interferometry to create a virtual shear-wave checkshot with first arrivals representing the direct downgoing shear wave along the borehole. Testing on a field data example demonstrates the ability of the redatuming approach to reconstruct accurate virtual gathers and deliver reliable shear-wave velocity from first-break time picks. Using the retrieved shear-wave velocity profile, the mode-converted VSP field data were migrated to obtain a reliable subsurface image consistent with the image obtained from PP data.
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Multiscale Constrained Full Waveform Inversion for Borehole Multicomponent Seismic Data
Authors M. Charara and C. BarnesSummaryFull-waveform inversion for borehole seismic data is an ill-posed problem and constraining the problem is crucial. Constraints can be imposed though covariance matrices. Usually, they are set to a diagonal matrix. For the model space, horizontal and vertical spatial correlations using a Laplace distribution can be used to fill the model space covariance matrix. This approach reduces the degree of freedom of the inverse problem. Strong horizontal spatial correlation distances favor a tabular geological model whenever it does not contradict the data. The relaxation of the spatial correlation distances from large to small during the iterative inversion process allows the recovery of geological objects of the same size, which regularizes the inverse problem. Synthetic constrained and unconstrained inversions for 2D-2C crosswell data show the clear improvement of the inversion results when constraints are used.
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Joint Least-Squares RTM of Surface Seismic and Seismic-While-Drilling Datasets
Authors N. Kazemi, D. Trad, K. Innanen and R. ShorSummaryLeast-squares migration can, in theory, reduce the acquisition footprint and improve the illumination of the subsurface structures. However, in complex subsurface structures, rays or the wave energy will penetrate poorly in some regions, e.g., subsalt region, and that region will be a shadow zone to a typical surface seismic acquisition. The shadow zone is in the null space of the migration operator and the subsurface information in that region will not be recovered even by posing imaging as an inverse problem. To rectify this, we use another set of data, along with surface seismic dataset, whose ray paths are different from the surface seismic. Seismic-while-drilling (SWD) dataset are complementary to surface data, and it brings an opportunity to address seismic illumination issue by adding new measurements into the imaging problem. Accordingly, in this research, we formulate the joint least-squares reverse time migration of surface seismic and SWD datasets and explore its potential in imaging the parts of the model that is in the shadow zone of the surface seismic acquisition. Presented results on the BP-Model94 show that the joint least-squares migration outperforms the single surface and single SWD least-squares migrations in improving the illumination of subsurface structures.
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Enhancing VSP Imaging Quality by an Efficient Dip Filter Consistent with Velocity Model
Authors M. Lou and H. SimpsonSummaryVSP data usually provides a higher resolution seismic image than surface seismic data, but its imaging result often suffers severe migration artifacts due to the fact that there are fewer, and vertically-fixed, borehole receivers and therefore low and uneven CDP folds in VSP survey. A common method to suppress VSP migration artifacts is to reduce migration aperture to a small angle. However, a small aperture angle can fail to image significantly dipping structures. Here, we develop an efficient dip filter consistent with the local velocity model in VSP migration, which preserves true dip structures and in the meantime suppresses migration artifacts. The dip filter consists of two major parts: (1) we first efficiently calculate the wave propagation angles from both source and receiver positions to each image point based on a travel time table computed by an Eikonal solver in conventional Kirchhoff migration, and (2) we then accordingly perform the dip filtering for each image point based on the possible dip angles defined by the bisects of the source and receiver propagation angles to the image point. Both synthetic and field VSP data demonstrate that the dip filter consistent with the local velocity model effectively enhances VSP imaging quality.
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Bayesian Full-Waveform Tube-Wave Inversion for Effective Hydraulic Fracture Aperture and Mechanical Fracture Compliance
Authors J. Hunziker, A. Greenwood, S. Minato, N. Barbosa, E. Caspari and K. HolligerSummaryThe hydraulic characterization of fractures is crucial for geothermal energy production, hydrocarbon exploration, CO2-sequestration, and nuclear waste disposal. We propose to estimate the effective hydraulic aperture and the mechanical compliance of isolated fractures intersecting a borehole through a Bayesian inversion of full-waveform tube-wave data recorded in a vertical seismic profiling setting. Following the successful application of the proposed inversion procedure to synthetic data, we show in this study initial results of an application to real data.
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Magnetotelluric Forward Modelling in General Anisotropic Media Using Open-Source Package Esys-Escript
More LessSummaryAnisotropy in the electrical conductivity of the subsurface has been recognized as one of the major difficulties in interpreting Magnetotelluric (MT) data; as it can lead to inaccurate inversion results under simple isotropic assumption. A robust modelling code producing accurate simulation results for anisotropic cases is much needed for the MT community, in order to extract more information from MT data in the inversion process. We present open-source Python package esys-Escript. It is designed to solve mathematical modelling problems using the finite element method (FEM). Its key idea is to formulate the problem in terms of general partial differential equations (PDEs) with proper boundary conditions. Its object-oriented programming style creates a flexible and easy-to-use interface for users to tackle their tasks. esys-Escript has been used to simulate 2D isotopic MT responses with a good agreement with published analytical solutions. Extending from this work, we present an implementation of the MT forward model for anisotropic media using Escript. In order to validate our code, we show anisotropic MT modelling results with the comparison of analytical solutions under an axially anisotropic setting. Then we add a general anisotropic case displaying Escript MT modelling results, validating against published anisotropic MT modelling code.
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GPU-Accelerated Transient Electro-Magnetic Modelling
Authors Y. Liu, M. Hagdorn and A. ZiolkowskiSummaryWe present a GPU-accelerated transient electromagnetic modelling method implemented in CUDA C where the numerical efficiency can be improved by a factor of over 100 compared with the serial C code. The method uses the Fourier pseudo-spectral method to solve the spatial derivatives, and uses the rapid expansion method (REM) to solve the temporal evolution of the field by a summation of Chebyshev polynomials. The results are free of numerical dispersion and accurate to the Nyquist in both space and time. We show that if the model consists only of 1-D or 2-D structures, the computational effort to solve a 3-D field can be reduced by up to two orders of magnitude without loss of accuracy. We demonstrate the accuracy of the code by a 2.5-D modelling example in a VTI anisotropic half space. The significant improvement in numerical efficiency generalizes the use of REM in large-scale, time-domain EM modelling or inversion problems.
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Curvelet-Based 2-D Inversion for MT Data
More LessSummaryIn this paper, we propose a new method for two-dimensional (2-D) magnetotelluric (MT) inversion based on the curvelet transform. Unlike the conventional inversion methods that apply constraints on the model in the space-domain, the method presented in this paper is based on the sparse constraint by the curvelet transform, and we directly invert the curvelet coefficients instead of the model conductivities in the space-domain. The curvelet transform is a multiscale sparse scheme that transforms the model parameters into the curvelet coefficients at multiple scales. The basis function of the curvelet transform is the “wedge base” that satisfies the anisotropic scale relationship (width∝length2) and has the characteristic of arbitrary directivity. Thus, it has the capability to “optimally” represent the edge of the target objects. To achieve a sparse constraint, we use L1-norm of the curvelet coefficients for the inversion. This can help extract the features of target objects more sparsely and get high-resolution inversion results. We compare the results of our curvelet-based inversion with those based on the traditional L1-norm and L2-norm inversions. The experiments with theoretical data demonstrate that the sparse constraint inversion based on the curvelet transform can better reveal the boundaries of target objects.
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3D inversion of Large Fixed-Loop Transient Electromagnetic Data with Topography
More LessSummaryIn this paper, we propose a new method for the inversion of large fixed-loop transient electromagnetic (TEM) data. The unstructured tetrahedral grids with flexibility to delineate the rugged topography are adopted to discretize the inverse model. The transmitting source is divided into short electric dipoles to simulate its practical shape, size and attitude. In the numerical simulation, we adopt the edge-based finite-element method (FEM) to discretize the governing equation in space, and the unconditionally stable second-order of Backward Euler (BE) scheme to discretize the time. The final forward modelling equation is solved for all time channels iteratively from an initial field. Since the full-channel forward modelling matrix is asymmetric, we use a backward recursive method to solve the adjoint forward problem to compute the Jacobian matrix. Considering the limited power of personal computational devices, we apply the limited-memory BFGS optimization method to accomplish our three-dimensional (3D) inversions. The inversion algorithm is tested firstly on a synthetic example. The inversion results show that the proposed inversion scheme can well invert TEM data with sharp topography. We further test our inversion code on a field survey data and obtained results that agree well with the geology and other geophysical information.
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Calculation of Optimal Noise Levels for the Detection of Conductive Lenses in Permafrost with Radar Scans
Authors K. Van den Doel and G. StoveSummaryWe present results from numerical simulations of reflection scans with a low frequency pulsed radar system through permafrost host rock with embedded target layers of highly conductive sulfides. The goal of the simulations is to determine the signal to noise ratio needed to detect targets at various depths. Sulfide layers are of interest for mining as they often contain minerals. These layers are typically mined at depths up to a kilometer or more, and the question arises if they can be detected from the surface with a remote sensing method. At these depths seismic methods are not feasible because the targets do not have a strong density contrast, and therefore generate no appreciable reflections. However, as permafrost is highly resistive, the host rock is almost transparent to electromagnetic waves, suggesting a survey with a pulsed radar system. Feature detection down to several kilometers below the surface has been achieved with low frequency pulsed radar surveys in resistive environments such as Martian rock, ice, and permafrost. The simulations are based on the emission and detection properties of the Adrok radar system in a permafrost environment in which we place a conductive reflector (representing the mineral target) at various depths.
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Delineation of Buried Paleochannel Using EM Induction in Eastern Banda Aceh, Indonesia
More LessSummaryThe delta of the Aceh River dominates the coastal landscape around Banda Aceh. The areas were constructed by paleochannel activities in the past. We propose to learn the past history of these formation, so that we might provide a better understanding of paleotsunami impact around coastal area of Banda Aceh. As a preliminary study, we have conducted electromagnetic induction method measurement in the suspected of paleochannel area in Rumpet village, eastern of Banda Aceh. The EM induction measures an apparent electrical conductivity at the surface, which represents a weighted average of the electrical conductivity distribution over a certain depth range. The measurements were performed along a profile with 250 meters length and 10 meter spacing between stations crossing the suspected buried paleochannel. The apparent conductivity data collected along the profile clearly reflect geometry of the expected paleochannel in the area. Compared to electrical resistivity methods, the electrical conductivity data of the EM induction methods reveal almost the same trend as showed in the electrical resistivity model. Therefore we propose that the EM induction method is potentially used for paleochannel investigation, especially in preliminary study since the EM induction method can be operated fast and costly effective in large areas surveys.
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A New Petrophysical Modeling Workflow for Fractured Granite Basement Reservoir in Cuu Long Basin, Offshore Vietnam
Authors H. Vo Thanh, Y. Sugai, R. Nguele and K. SasakiSummaryGranite fractured basement reservoir contributes higher 40% of the world's oil and gas reserves. However, geological modeling of fractured reservoirs is complex and presents unique challenges in comparison with conventional reservoirs. It is extremely difficult to achieve the best results for a future development plan. This research presented the new workflow to enhance the accuracy of porosity and permeability models for a fractured reservoir in offshore Vietnam by using Artificial Neural Network (ANN) and co-kriging method. ANN was employed to solve problems that conventional modeling has not been successful. The seismic attributes selection was used for initial ANN generation. Then, the prediction property model was established through ANN training process. Well log data was used for correlation to cross-validation the predictive models. Next, the co-kriging algorithm was created the porosity and permeability models. Also, the Drill Stem Test (DST) data was used for history matching models to confirm the Co-kriging approach. The history matching was iterated until the geological model achieved the best matching with DST data. The history match shown the excellent fitting between simulation model and measurement data. Overall, we conclude that ANN and co-kriging are useful method for developing reliable workflow in fracture basement reservoir.
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Integrating Structural Restorations into Basin and Petroleum System Modeling: A Study from the Gulf of Mexico
Authors W. AlKawai, K. Mesiling and T. MukerjiSummaryThis study documents the importance of incorporating sequential structural restorations when constructing basin and petroleum system models across structures characterized by significant lateral stratigraphic translation. The results of the study show that constructing a basin model using only vertical backstripping across a structure, which laterally displaces stratigraphic layers produces a markedly different layer paleogeometry compared to a model incorporating sequential kinematic structural restoration. The paleogeometry obtained through pure vertical backstripping does not accurately represent the evolution of the basin because it does not show progressive lateral slip and rotation of bedding during structural evolution. Differences between these two methods of constructing paleogeometry translate into differences in simulated pressure and thermal history. The differences in thermal history can be significant when studying the maturation of a source horizon and the corresponding evolution of the petroleum system. The results emphasize the need to incorporate sequential structural restorations into basin and petroleum system modeling to properly simulate pressure and thermal history in the vicinity of structures associated with significant lateral displacement of stratigraphic layers.
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Geostatistical Electrofacies Calculation in Non-Stationary Cases
Authors J. Chautru, T. Romary, H. Binet and M. BourgesSummaryA common issue in geological modeling is to determine facies along each well. It is easy in cored wells, but difficult in uncored wells where only electric logs are available. The issue is usually solved by defining “electrofacies,” calculated from logs and calibrated with core data to ensure their geological consistency.
The most common clustering approaches generally consider all the log measurements as equivalent, and rarely care about the spatial correlation between the data points. However, accounting for the spatial relationship between samples in clustering will improve the realism and the geological consistency of the electrofacies.
This paper presents a case study made with a clustering algorithm which accounts for the spatial relationship between samples, allowing a balance of the influence of the different sources of information: log responses, lithological description and sample locations.
Working with a search neighbourhood to define the pairs of samples to be used for the hierarchical clustering, combined with the influence of coordinates, allows one to manage a large range of non-stationary effects. By comparing the statistics calculated on the classification results to the sedimentological model or to Vertical Proportion Curves computed from cored wells, optimal input parameters for the clustering algorithm can be determined.
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Principles for a Hierarchical Earth Model Representation Aiming Towards Fit-For-Purpose Grid Resolution
Authors E. Suter, T. Kårstad, A. Escalona, H.A. Friis and E.H. VefringSummaryPoor control with grid resolution has been identified as one of the major limitations within current earth modelling technologies. A main reason is the inflexibility of the structural model that the grid is adapted to. Aiming for fit-for-purpose optimization of earth model grid resolution, we summarize the principles of a hierarchical, nested representation of geological interfaces and volumetric regions in correspondence with geological principles. When generating an earth model grid, each region can be either discretized or further subdivided using the geological interfaces in its interior in a recursive manner. Control with the resolution of the geological structure thus enables better control with the resolution of the grid.
The method targets improved local control with the trade-off between model resolution and time spent for computation in grid-based modelling and simulation exercises. This is achieved by allowing a higher model resolution where it matters most to the application at hand while gradually ignoring details elsewhere. Also uncertainties in the structural geometries and topology are managed. Applications of particular interest include workflows where computation time is a major limiting factor, such as geosteering and interactive geological interpretation. However, a successful development could potentially benefit any grid-based subsurface workflow.
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Seamlessly Linking Static and Dynamic Models Through a Single High-Fidelity Grid System
Authors J. Riou, A. Ganzo, D. Gilmour and B. Arief WibowoSummaryOne major challenge with the current subsurface modeling workflows is the difficulty to transfer the high complexity of static geological models to simulation. This paper describes an improved gridding technology that overcomes the shortcomings of existing corner-point stair-step and pillar-based grids in capturing the complex geological features of hydrocarbon reservoirs. It also highlights the seamless use of this new gridding technology in flow simulation.
After reviewing the different types of grids commonly used in geological modeling, the benefits of the new approach will be detailed, while highlighting its compatibility with commercial flow simulators.
Common modeling practices that use corner-point stair-step and pillar-based grids fail to preserve the geometry of most geological objects. The new gridding algorithm presented here addresses three major shortcomings of the current approaches by providing an efficient way to:
- Accurately represent any type of geological structures in a 3D grid.
- Capture complex sedimentological properties across geological structures.
- Connect the grid to commercial flow simulators to preserves any type of structure and sedimentology in dynamic simulations.
This high-accuracy gridding system is fully simulator agnostic, giving simulation engineers a unique opportunity to run models without the need for any explicit grid system conversion.
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Uncertainty Handling of Reservoir Facies Modeling Using Stochastic Inversion
Authors V. Rotar, N. Cardozo, S. Manral and L. SchulteSummaryStochastic inversion delivers multiple alternative impedance cubes of higher resolution compared to deterministic inversion while honoring the seismic data. Consequently, it offers an alternative approach to capture the uncertainty in the property distribution because it allows addressing the impact of the limitation in the seismic resolution on the modelled reservoir property. This study compares the uncertainty estimation based on stochastic inversion with the Gauss simulation technique applied to facies modelling. Its final goal is to understand and quantify the benefit of using stochastic inversion in facies modeling and uncertainty estimation.
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Integrating Seismic Inversion in Static Uncertainties
Authors F. Piriac, P. Biver and S. HalfaouiSummaryResources estimation has always been the main challenge for the oil and gas industry. The uncertainties accounting for this estimation are split into Structural, Static and Dynamic uncertainties and each component can have a major impact on the final resources estimation. This paper is dealing with the static uncertainties component coming from seismic inversion and proposes a workflow to derive stochastic multi-realizations from deterministic inversion results through geostatistical simulations. In order to validate the approach, the described methodology is first compared to a stochastic seismic inversion of elastic properties performed on a real data set (turbiditic deep offshore, Australia). Then a generalization of the workflow is proposed for litho-seismic attribute in order to constrain the geomodel in-filling. The obtained results show that deterministic inversion results could be integrated in a global uncertainty workflow and thus contribute to the range of the stock-tank original oil in place volume (STOOIP).
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New Aspects of Rock Typing
Authors R. Moghadasi and M. MotieSummaryIt has been tried to analyse different rock typing methods for their performance and accuracy. To do this, a set of capillary pressure, porosity, and permeability data were collected. It was revealed that he FZI* method is capable of identifying different rock types based on heir dynamic/flow properties. However, its accuracy is affected by wettability effects/differences.
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