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78th EAGE Conference and Exhibition 2016
- Conference date: May 30, 2016 - June 2, 2016
- Location: Online
- Published: 30 May 2016
201 - 250 of 1034 results
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Prestack Seismic Data Inversion for Shale Gas Reservoir Characterization in China
Authors G. Yu, Y.S. Zhang, X.M. Wang, X. Liang, U. Strecker and M. SmithSummaryAn integrated study of well Zhao-104 and surrounding wide-azimuth 3D seismic data volume within the shale gas reservoir in South China has been conducted with the objective of generating shale formation properties related to fracture orientation and intensity in the area and deriving such reservoir rock properties. Well data, structural seismic information and prestack inversion products were combined in an integrated interpretation. Seismic gather conditioning improved seismic data quality prior to prestack inversion by improving signal/noise ratio, removing NMO stretch and aligning reflection events. Fracture strike and P wave anisotropy were calculated using the RMO updated sector velocity fields in elliptical velocity inversion, while inversion for P and S impedance and derivative attributes produced volumes that relate to rock properties such as brittleness and rigidity that are likely to impact fracturing. The inversion for P and S impedance and derivative attributes produced volumes that relate to rock properties such as brittleness and rigidity that are likely to impact fracturing. Seismic attribute analysis of anisotropy from elliptical velocity inversion indicates that anisotropy varies horizontally and vertically, and that it is dominantly controlled by stress azimuth, which conforms to the current day stress field as independently determined from borehole break-outs.
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Application of Iterative Inversion for Organic Rich Shale Prediction - A Case Study in Xiuwu Basin
More LessSummaryOrganic rich shale and non-organic rich shale are usually developed together in Xiuwu Basin, and well logging data reveals that they can be differentiated by integrated elastic attributes. However, previous explorations show that the pre-stack method is not satisfying due to the poor quality of seismic data in this area. Furthermore, it is always a difficult task to conduct an AV O inversion precisely due to two factors: severe influence of initial Vp/Vs model for linear AV O inversion and intensive tendency of easily falling into local optimization for non-linear AVO inversion. For a better organic rich shale prediction, an integrated pre-stack method including advanced data conditioning and iterative inversion is elaborately designed and successfully applied. Therefore, the proposed approach and its corresponding application are of significant importance to current knowledge for organic rich shale prediction.
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Application of Seismic Liquid Identification Method in Shale Gas Sweet Spots Prediction in Baojing Area of South China
Authors Y.D. Yang, H.D. Huang, Y.N. Luo, Y.X. Miao, J.L. Zhang, S. Zhang and D. YangSummaryThe shale gas resource is abundant in China, but the exploration level is relatively low. Because of the shortage of drilling and well logging data, the research on high precision seismic fluid identification method is of crucial importance. Conventional post-stack seismic inversion method can identify the shale formations, but cannot distinguish the gas-bearing shale and gas-free shale. However, the liquid mobility factor can effectively characterize the permeability of reservoir. In this paper, we propose a practical method for the prediction of shale gas sweet spots, which combines liquid mobility factor with high precision time-frequency analysis method. Firstly, we analyse the time-frequency characteristics of fluid based on the matching pursuit method, then calculate the liquid mobility factor and quantify its range in shale gas sweet spots based on logging data. Eventually, the distribution of shale gas sweet spots can be predicted. Application of this method in Baojing shale area of Hunan province in South China shows that the prediction results are consistent with the drilling, gas testing and well logging data, which achieves a good exploration effect and may play a significant guiding role in the shale gas exploration.
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Geomechanical Investigations for a North American Shale Gas Play
Authors N. Hummel, M. Parotidis, J. Wheeler and J. GrahamSummaryFor a multi-well multi-stage hydraulic fracturing campaign of a North American shale gas play we investigate geomechanical effects on drawdown management to improve our understanding of declining production rates. We calibrate a geomechanical model by determining and constraining rock properties, pore pressures and stresses along the well trajectory as well as demonstrate its role in estimating the rock strength (UCS) for significant underbalanced drilling conditions. In this way, we find the area under investigation to be quite heterogeneous, as data from offset wells show different UCS magnitudes. To better understand the mechanism of production decline rates, we analyse the productivity of corresponding wells in line with their choke size and drawdown control. This identifies a positive production-related choke and drawdown management effect for a distinct and simultaneous decrease in both choke size and drawdown pressures. Finally, we utilize the geomechanical model to investigate injection and depletion effects on critically stressed fractures. Our results suggest that with depletion the number of critically stressed fractures significantly decreases. Hence, a successful proppant settlement is critical for minimising loss of production.
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Organic Geochemistry and Pore System Characterization of Shales from Middle Permian Barren Measures Formation, India
More LessSummaryShale gas characteristics of core shale samples from Middle Permian Barren Measures Formation have been assessed for this study. The samples were collected from Raniganj, West Bokaro and South Karanpura sub-basins within Damodar Valley Basin, eastern India. Organic geochemical characteristics such as total organic carbon content (TOC), type of organic matter and thermal maturity have been investigated using Rock-Eval pyrolysis. Biomarker analysis, using gas chromatography – mass spectrometry (GC-MS), has been performed in order to understand the source and depositional environment of organic matter. Pore space distribution and pore morphology are inferred from the images from field emission scanning electron microscopy (FE-SEM) and mineral identification as well as semiquantitative mineralogy of the same has been analysed using X-ray diffraction analyses. The results show that the shale samples contain a mixture of Type-II and Type-III organic matter with oil to wet gas thermal maturity. The TOC values range from 2.66–9.38%. The biomarker data reveal that the organic matter is dominated by terrestrial higher plant input, deposited under sub-oxic environment. The FE-SEM images suggest intergranular mineral matrix pores to be the dominant pore type. The inorganic part to the shale is composed of quartz, micas, kaolinite, illite, feldspars and siderite.
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Analysis of the Heterogeneity of the Polish Shale Gas Formations by Factor Analysis on the Basis of Well Logs
Authors K. Wawrzyniak-Guz, J.A. Jarzyna, M. Zych, M. Bała, P.I. Krakowska and E. PuskarczykSummaryThe paper presents result of Factor Analysis applied to Upper Ordovician and Lower Silurian shale gas sediments from three wells located in Baltic Basin, Poland. Jantar Member and Sasino Formation, considered as sweet spots, were especially investigated with regard to their heterogeneity. The study confirms huge diversity of these formations – they differ between each other, between adjacent fine clastic deposits and they exhibit changeability from one well to another. Applied Factor Analysis also showed which petrophysical parameters play important role in characterization and description of complicated nature of these formations.
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Elemental and Isotopic Chemostratigraphy of the Vaca Muerta Formation, Neuquén Basin, Argentina
Authors E. Hernandez Bilbao and J.F. SargSummaryThe Vaca Muerta Formation is the main source rock in the Neuquén basin and is now being targeted as an unconventional reservoir. Chemostratigraphic signatures of five facies from the lower Vaca Muerta are used to calibrate elemental and isotopic chemostratigraphic signatures identified for the rest of the Formation. High frequency stratigraphic sequences identified in core are related to lower frequency sequences on a broader correlation based on log signature combined with elemental and isotopic data. Well correlation is based on a sequence stratigraphic interpretation where HST’s are characterized by large Ca positive excursions and TST’s contain high Mo and Ni peaks, especially towards the bottom of the interval. Element-TOC correlations show Ni and Cu as best proxies for TOC. In addition, correlations of TOC with Si, Ni, Mo and Ca depict elemental cutoff values for low (<2 wt.%) moderate (2–5 wt.%) and high (>5 wt.%) TOC values.
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Integration of Static and Dynamic Data in Flow Simulation of Carbonate Reservoirs
Authors M.G. Correia, C. Maschio and D.J. SchiozerSummaryLinking geostatistical modeling of multiscale carbonate heterogeneities with representative simulation flow models is challenging as conventional upscaling procedures often disregard the complex dynamic behavior in reservoir simulation. This work presents a methodology to build robust flow simulation models considering: (1) multiscale geological scenarios based on carbonate reservoirs; (2) accurate representation of static and dynamic data in reservoir simulation; (3) an optimized production prediction based on enhanced well completion. The development of this work follows three main steps: (1) hierarchical upscaling procedure by flow units, (2) integration of flow units into a reservoir-scale simulation model, (3) validation of simulation model. The methodology we present here is useful for multidisciplinary areas of expertise as it integrates geostatistical modeling of carbonate reservoir heterogeneities with construction of robust simulation models considering the development of complex carbonate reservoirs.
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Dispersivity Estimation in Real Pore Scale Samples
Authors F. Collin-Bastiani, R. Guibert, P. Horgue, M. Deckers, T. Clemens and G. DebenestSummaryThe aim of this work is the evaluation of numerical methods to estimate longitudinal effective dispersion coefficients on pore-space images using a Lagrangian Particle Tracking method. The pore-space imaging is done using a X-ray Microscope Computed Tomograph. Equations are then directly solved in the mesh respecting the binary image using the open-source platform OpenFOAM.
After a short reminder of the LPT method and a description of the samples used, the LPT algorithm is validated in idealized media. The use of simple 2D and 3D geometries allow the comparison to theoretical laws. But in order to ensure the validity of our method, we also cross compare our results with the Volume Averaging Method. The impact of the mesh quality on statistical moments will be also presented showing some effects on the first moment.
The main part of the presented study focus on the comparison of three different estimators of dispersion coefficients which treat boundary conditions differently. This allows to determine dispersivity in non periodic samples.
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Efficient Calculation of Flux Conservative Streamline Trajectories on Complex and Unstructured Grids
More LessSummaryWe have generalized the streamline trajectory calculations and a novel global isoparametric coordinate system from hexahedron cells to other unstructured cells (prism, PEBI and tetrahedron). We have also developed a novel and flexible flux conservative double boundary layer construction which has sufficient degrees of freedom for flux consistency with the adjacent cells, even for complex faulted or embedded local grids. The solution has been implemented for both corner point and unstructured grids. In all cases the lowest order velocity model provides analytic solutions for trajectories and transit times. The construction at faults is completely determined by the flux across the non-neighbor connections, and supports both physical NNC’s, with a finite cell overlap area, and simulator NNC’s where no such area need exist. The results are demonstrated on simple type models, in near-well models, and in full field applications.
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Impact of In Situ Reactivity on Scale Management in Sandstone and Carbonate Reservoirs
More LessSummaryReactive transport simulators are used to investigate the impact of geochemical reactions on in situ brine compositions, and to predict the behaviour of produced brines. These calculations are then used to inform scale management decision making, and to calculate risk of damage and cost of treatment. The authors describe the various types of reservoir interactions that occur during reservoir displacement processes, and then use examples of reactive flow simulations to illustrate the impact of these reservoir processes on scale management decision making and cost, using examples with field data from sandstone and carbonate reservoirs. Barium stripping in sandstone reservoirs undergoing seawater flooding is shown to reduce scaling tendency at the producers, while sulphate stripping due to anhydrite precipitation in a chalk field has an even greater impact, delaying the onset of scaling and reducing chemical treatment costs. The field examples are preceded by a discussion of the velocity at which various saturation, component and thermal fronts propagate, and how mixing regimes are established.
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Implementation of a Vertex Centered Method inside an Industrial Reservoir Simulator - Fault Modeling Aspects
By P. SamierSummaryCorner point grids and pillar based unstructured grids do not provide an effective workflow for translating earth models into simulation models. It requires grids that allow an accurate representation of the near well flow and preserving geological accuracy and offering flexible resolution control. Hence a 3D unstructured approach is required.
Significant work has been done for generating unstructured grids. Coupling with geomechanics and hydraulic fracture flow for gas shale simulation have given a new impulse for unstructured gridding. Recent methods such as VAG or more mature ones such as MPFA provide a numerical scheme based on multipoint stencil more physical than two point flux methods.
Paper presents the application of three multi-point stencil methods (namely VAG, MPFA-O, MPFA-L) on a faulted full field model. Main issues addressed are the fault discretization using unstructured grid and the benefits compared to the usual structured grid and fault multipliers approach.
Results, accuracy and performance of multipoint scheme methods on unstructured grids are compared with TPFA methods on structured CPG grids and PEBI method on unstructured grids.
Two simulation examples are presented and compare results, accuracy and performance. The first test case is a Y fault model and the second one is a full field faulted model.
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Accelerating Large-scale Reservoir Simulations Using Supercomputers
More LessSummaryNowadays, simulations of advanced recovery processes applied by the petroleum industry have been becoming more and more complicated. In the meantime, finer geological models for these processes are being utilized. Their numerical simulations can take days or even longer to complete one run using regular workstations. Fast computational methods and computer techniques should be investigated.
We are dedicated to developing new reservoir models and fast reservoir simulation techniques, including new discretization methods, efficient nonlinear methods, linear solver and peconditioner methods, and parallel computing techniques.
Numerical results show our reservoir simulations are accelerated thousands of times faster by using a supercomputer and parallel computing is a powerful tool. Supercomputers also have huge memory and extremely large reservoir models can be computed.
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Improving Model History Match Using a Novel Streamline-based Approach - A Field Study from Saudi Arabia
Authors A.A. Al-Najem, A.A. Al-Turki, R. Batycky and M. ThieleSummaryThe work presents the application of a novel streamline-based history matching workflow called EVOLVE™ to improve on the history match quality of a highly complex carbonate reservoir in Saudi Arabia with hundreds of wells and many years of historical field data. EVOLVE™ is a linear step-wise approach that carries ensemble of models from geology to forecasting. In this study, an ensemble of geological models were generated by varying key geo-parameters. Clustering on “distance matrix” of breakthrough profiles helped identify a subset of geo-models that retains models diversity which were then carried forward for sensitivity on key flow parameters. Only simulation models that bracketed field production history while retaining parameters diversity were kept. However, to improve on well-level matches, historical time range was divided into equal time intervals to guide permeability update in between wells and minimize total oil rate mismatch for all wells. After history matching each model, there were minimal changes to field-level match, but with 30% improvements in well-level matches.
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Challenges of a Complex Mature Oil Reservoir Simulation
Authors E.M. Bisso Bi Mba, A. Yadav, A. El-Hawari and E. OmaraSummaryThe building of dynamic simulation models to analyse and predict fluid behaviour in mature fields has become an important process for supporting operational and investment decisions. In order to validate the simulation model for prediction, it is necessary to match historical field production and pressure profiles. In this study, the field under consideration is an offshore mature oilfield with 32 years of production history. A robust simulation model with fair history match is required to identify by-passed oil and poorly swept areas, delineate infill well positions and optimize actual water injection.
The challenges encountered during the dynamic simulation process can be grouped into two categories: complexities of static and dynamic data. Significant issues such as K-index error and floating cells above the reservoir were faced due to the use of stair step grid geometry for the 3D geological model. Due to limited downhole measurements during commingled production, there are high uncertainties in rate allocation and only limited single pressure points are available. This represents an important constraint when trying to history match the field. Therefore it is recommended to simplify static and dynamic model as much as possible to achieve the study objectives within a reasonable timeframe.
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A New Method for Calculating Permeability Using NMR T2 Distribution
More LessSummaryA theoretical equation of calculating permeability through NMR transverse relaxation time (T2) distribution is derived according to Poiseuille’s equation, Darcy’s law, Archie’s equation and the relationship between NMR T2 distribution and pore throat radii, on condition that the porous media are disconnected tortuous capillary tubes with different radii. The average value of effective T2 which controlled the permeability was calculated to obtain permeability using the new equation derived in this paper. Results from NMR experimental data and NMR logging data in tight sandstone reservoirs confirmed that this method has a good performance in permeability estimation.
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Permeability Estimations Based on Internal Surface and NMR T2 for Chesapeake Bay Impact Structure (Eyreville Cores)
Authors S.I. Mayr and H. WilhelmSummaryHydraulic permeability is not only necessary for hydraulic purposes, but also for heat flow determinations. Often no measurements of hydraulic permeability are available, therefore it is necessary to go back on estimations. The choice of the estimation method depends on the available measured physical properties, a comparison of more than one estimation is advisable. Estimations by means of internal surface and porosity (PaRiS-model) as well as NMR-relaxation time and porosity are frequently used petrophysical models.
We compare estimations of permeability by means of these models for the post impact (clay) and Exmore section (diamicton, clay and sandstone) from the corehole Eyreville (Impact structure Chesapeake, USA), and compare them with some literature data. We indicate that both methods don’t account for closed porosity and anisotropy. For the rocks of the Exmore section the PaRiS-model leads to considerable overestimation for the sandstones and underestimation for the clay samples. Besides these limitations both methods give results which lie in the same order of magnitude. We estimate mean bulk permeability values on sample scale between 0.1 mD and 1000 mD. On field scale the permeability will be higher. Therefore, advective mass transport and a convective contribution to heat transport is to be expected.
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Clay Mineral Effect in Sandstone Reservoir toward Usage of Fluid Drilling Type. Study Case - Lisa Field, Tarakan Basin
Authors F.A. Abdulah, M.S.A.A. Akbarsyah and Y.Y. YuniardiSummaryPorosity and permeability analysis of the rock is an important factor in assessing the quality of a reservoir, which is generally controlled by the type of rock that covers the composition and texture of the rock. However, the primary porosity and permeability will change with diagenesis process which works on the reservoir rock. One basin in Indonesia that have good prospects is Tarakan Basin, East Kalimantan. There are 14 oil and gas fields in Tarakan Basin. An integrated core study of reservoir has been done for optimizing exploration and production
In exploration and production, injection fluid in well is considered because it affects mechanisms that occur in the reservoir. An example is in Sangatta field, after the use of Oil Base Mud began to be used, the oil production in Sanggata exceeds 3000 BOPD, Compared to when using a water base mud. One of the main reason the using of oil-based mud is clay sensitivity problem at the reservoir in Sangatta. Beside, clay minerals are usually assumed to be detrimental to sandstone reservoir quality because they can plug pore throats as they locate on grain surface in the form of films, plates and bridge and some clay minerals promote chemical compaction
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Two-step Upscaling Method Applied to Non-Darcy Flow
Authors R. Guibert, P. Horgue, J.F. Thovert and G. DebenestSummaryIn this study, a two-step upscaling method, already validated for absolute permeability determination, is extended to non-Darcy flows.
Inertial regime is generally observed near wellbores.
The upscaling process consists in evaluating the Forchheimer factor and induces several simulations in each spatial directions.
The method is applied on two different porous media (sphere packing and rock sample). These configurations are used to quantify the accuracy and demonstrate the convergence of the method.
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Numerical-simulation-based Determination of Relative Permeability in Laminated Rocks
Authors M. Sedaghat, S.K. Matthai and S. AzizmohammadiSummaryReservoir simulation using the extended Darcy’s law requires relative permeability curves derived either via analytic saturation functions (Corey models etc.) or from special core analysis (SCAL). Since such experimental exploration of the space of influential parameters (pore geometry and wettability) is costly and time consuming, establishing ways to extract ensemble relative permeability from numerical simulation, kri, over the entire range of water saturation is highly desirable.
Our work focuses on finding accurate ways to compute ensemble kri(sw) for layered rocks when both capillary and viscous forces are strong. Two methods are proposed: an unsteady state saturation variation (USSV) method and a steady state saturation variation (SSSV) technique. To evaluate these approaches, SCAL data was extracted numerically from a real mm-scale layered sample. Results obtained with a Finite Element-Centered Finite Volume (FECFM) simulator, suggest that either of the approaches work significantly better than conventional unsteady state and JBN (Johnson-Bossler-Naumann) methods. Also, investigating saturation and velocity profiles within the sample indicates that bed-by-bed variations in wettability influence the flow pattern along/across interfaces making equipermeable layers behave like zones with different flow velocity. This dramatically challenges conventional relative permeability models and is addressed in terms of a new variable called relative permeability index.
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A Visual Framework for Reservoir Connectivity Analysis
Authors R. Cabral Ramos Mota, H. Hamdi, M. Costa Sousa, E. Sharlin and Z.X. ChenSummaryStatic connectivity measures have been proposed for quick evaluation of reservoir performance to provide a potentially important link between the reservoir characterization and the simulation studies. These measures are easy in concept and inexpensive in execution, and create an important, intermediate level in the assessment of reservoir productivity. This paper proposes a framework for static connectivity analysis in reservoirs that use water flooding technique and pressure propagation fronts as it used in well testing. The software uses a fast marching method and a shortest path algorithm that both are sensitive to geological heterogeneities which can give some insights into finding the connective geobodies. An illustrative example is shown to describe the software interface and to present a simple but systematic connectivity analysis scenario. The distinct tasks contained in a typical reservoir development workflow may be benefited from the addition of connectivity analysis, such as the assessment of optimum well placements for injection-production wells and the evaluation of features of stratigraphic architectures that affect the recovery. The proposed tool is towards providing a geoengineering approach to use the geological knowledge for proposing better production scenarios.
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An Investigation on Simulation Capabilities of Regular Based Pore Networks
Authors S. Aghabozorgi Nafchi and B. RostamiSummaryThe main constrain of pore network modelling simulations is to develop a network structure which is close enough to the real structure of porous media. Although recent approaches can capture the real structure of medium, but performing sensitivity analysis is cumbersome due to the complexity of developed networks. For many years, the results of using regular based networks has not been promising but one reason for their inefficiency might be the simplicity of equations and ignorance of wetting layers. In this study, it is tried to understand can these networks be a good representation of porous media if advanced equations are used in the calculations? To achieve this aim, a simple regular cubic network is distorted by removing some of throats randomly. The describing parameters of network are tuned to match macroscopic properties of real porous media. The developed network is validated using the experimental data available for Bentheimer sandstone. This study shows that such a simple network can be adequately representative of a real rock structure if advanced equations and concepts (eg. Shape factor and wetting layers) are used in calculations. This simple network can be used for sensitivity analysis of pore-scale mechanism with acceptable results.
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A New Model For Calculate Capillary Pressure And Relative Permeability In Reservoir Rocks Based On Pore Network Modeling
Authors J. Deylami, S.H. Mousavi and M.C. PoppelreiterSummaryTo gain insight in relationships among data like capillary pressure and relative permeability in reservoir rock, we have developed a model for petroleum reservoir rocks.
In petroleum engineering we are limited in capillary pressure and relative permeability data. We need capillary pressure and relative permeability data in multiphase reservoirs to have better recognition about phase behavior in reservoir rock and that’s very important in reservoirs simulation. We have shown that our model produce distinctly different curves for capillary pressure and relative permeability.
We have investigated some basic issues such as effect of network size, network dimension, and different trapping assumptions in the two networks. We have generated pore network of reservoir according cubic pore network model. After that we have studied fluid behavior in our network and also we have applied transport phenomena equations in fluids that are present in our network as a reservoir rock
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Predicting the Geological Structure Ahead of a Tunnel Using Full Waveform Inversion - A Blind Test
Authors K. Musayev, A. Lamert, K. Hackl, W. Friederich and M. BaitschSummaryLooking ahead of a tunnel is an important issue in order to have a smooth and safe construction process. Seismic methods are used to locate the geological anomalies ahead of a tunnel; the waves reflected from the geological changes carry information about the location and mechanical properties of the objects. Different migration techniques and first arrivals of the reflected waves are commonly used to achieve the goal. Full waveforms carry more information about the geological structure of a domain than the first traveltimes of the reflected waves. This work emerges from the curiosity to see whether full waveforms can successfully image the velocity field in tunnel applications. There are two parties in this study. The first party chooses a synthetic model and provides the second party with the precisely calculated waveforms in the time domain. The second party has no prior information about the synthetic model except for the waveforms and the inversion is carried out in the frequency domain as a blind test. The aim is to see whether the velocity field can be successfully imaged with the full waveform inversion.
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Research on the Applicability of MASW to Detect a Near Vertical Fault in a Near Surface Synthetic Model of the Otway Pro
By B.N. SeiveSummaryIn this research, we tested the applicability of the MOPA and MOAA methods to detect a near vertical fault location in the specific case of the Otway geosequestration geology in Victoria, Australia. A near vertical fault was added on a geological model of the first 500m of the Otway site and different case scenarios were studied by finite difference modelling. The project tested the influence of fault dip angles, fault offsets and choice of sources (passive or active) on the results of the MOPA and MOAA methods. The results show that the MOAA method, although very simple in nature, is effectively detecting the fault location. However, the results of the MOPA method were more mitigated with an effect markedly hidden behind a much stronger phenomena believed to be reflections on the fault plan
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About Data-driven Integration of Ill-posed Geophysical Tomography and Geotechnical Logging Data
By H. PaascheSummaryWe employ a recently developed data-driven approach to exemplary infer a probabilistic 2D sleeve friction model constrained by ill-posed geophysical tomographic imaging and laterally sparse cone penetration logging data. The integration and inference approach is based on fuzzy concepts and can fully cope with unknown and even non-unique inter-relations between geotechnical parameters, such as sleeve friction, and multiple physical properties imaged by fully non-linear geophysical tomography, e.g. ensembles of equivalent seismic or radar velocity models. Such data-driven integration and inference approaches can be applied to complex databases and do not require the a priori selection of tomographic data sets believed to be particularly closely linked to the target parameter, e.g., sleeve friction and seismic shear wave velocity tomograms. However, in the sense of error propagation incorporation of all available tomographic data sets may inflate the range of the final probabilistic prediction, which is not desirable. In turn, discarding data sets not expected to be physically linked to the target parameter may hamper predictions and potentially result in overseeing weak and yet unrecognized, but eventually existing, physical links, which could have improved the inference of probabilistic geotechnical target parameter models.
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Most Frequent Value Based Factor Analysis of Engineering Geophysical Sounding Logs
Authors N.P. Szabo and G.P. BaloghSummaryA multivariate statistical approach is presented to estimate water saturation in shallow heterogeneous formations. An improved factor analysis algorithm is developed to process engineering geophysical sounding data in a more reliable way. Resistivity and nuclear data acquired by cone penetration tools equipped with geophysical sensors are processed simultaneously to give an estimate to factor logs. The new factor analysis procedure is based on the iterative reweighting of data prediction errors using the highly robust most frequent value method, which improves the accuracy of factor scores in case of non-Gaussian data sets. A strong exponential relationship is detected between water saturation and the first factor log. Tests made on penetration logs measured from a Hungarian well demonstrate the feasibility of the most frequent value based factor analysis approach, which is verified by the results of local inverse modeling.
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3D Seismics and Isotopic Analysis Provides Constraints on the Origin of Methane in the Earths Deep Subsurface.
Authors M. Manzi, B. Sherwood Lollar, T. Onstott and E. van HeerdenSummaryUnderstanding and quantifying the sources of methane (CH4) in sedimentary environments is critical for studies of climate change, biogeochemical cycling, energy exploration, and for the mitigation of risks posed by methane explosions in deep mining environments. It was first suggested that the gases in deep gold mines of the Witwatersrand Basin (South Africa) were biogenic, that is, they originated from shallow coal deposits in the basin and were transported as dissolved phases in groundwater into the underlying strata via cross-cutting faults and igneous dykes. However, the integration of 3D seismic data with underground mapping data, gas compositions, hydrogen and carbon isotope signatures provide the new evidence for the abiogenic origin of the methane gas. This holistic approach has further demonstrated that the faults, dykes and fracture systems, as well as compositional variation, hydrogen and carbon isotopic analysis play an important role in unravelling the origin of methane gas in Earth’s deep subsurface, as well as in understanding the methane-water-transportation mechanism.
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Exploration of Deep Carbonate Aquifers by Magnetotellurics
Authors F. Sumanovac, J. Oreskovic, S. Kolar and N. BalaskoSummaryElectrical tomography is often considered as the main geophysical method in groundwater investigations, especially in complex geological models, but in case of deep exploration it cannot achieve deeper aquifers and the electrical sounding cannot ensure complete and dense space coverage. In such a case the use of the magnetotelluric method can ensure determination of deep targets, as well as a good space sampling. The capabilities of the MT method are shown in a common hydrogeological model in Croatia which can be found in both regions, in the Pannonian basin and Dinarides. The carbonate aquifer is underlying at depths greater than 100 m below impermeable package of clastic deposits. Two filed examples of groundwater investigations are presented, Gotalovec (north-western Croatia) and Baška (karstic Adriatic island Krk) areas. A main conclusion can be drawn that the MT method provides the most complete data on lithological and structural relationships in the considered model, but it is very sensitive to surface and urban noises. The results of the other geophysical methods can improve the resolution (seismic reflection) and reduce the interpretation ambiguity (electrical tomography).
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ERT and IPT Surveys to Check the Integrity of the Geomembrane in the Landfill of Bellolampo (Palermo, Italy)
Authors R. Martorana, P. Capizzi, A. D’Alessandro and D. LuzioSummaryIn landfills, changes in resistivity and chargeability can be related to the characteristics of the waste and they can be abrupt and considerable within short distances. These physical properties are function of generation, mobility and degree of saturation of the leachate, gas generation, compaction density and variability. These relationships mean that it can be possible to get an overall image of the quantity and characteristics of the waste from surface electrical measurements over the landfill. In this paper, we present and discuss the results of three electrical tomographies carried out in the landfill site of Bellolampo (Palermo, Italy). The main aim of these surveys was to check the integrity of the geomembrane. The application of the geoelectrical methods have allowed to obtain useful information to check the integrity of the geomembrane at the base of the new landfill built in the waste site of Bellolampo, unfortunately on fractured limestones and at high risk of pollution. The comparison between ERT and IPT, performed upstream, above and downstream of the landfill, allowed to identify the electrical properties of the rock, waste and leachate, and detect the possible presence of plumes of pollutant nearby and below the landfill.
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Small Scale Conductivity Estimation Using DC Resitivity and Well Logs Data - Lalor Case Study
Authors A.B. Bouchedda, S.T. Tirdad, E.G. Gloaguen and B.G. GirouxSummaryThis paper proposes to estimate a small scale conductivity model by inverting both DC resistivity and well logs data. We have considered an objective function that contains DC resistivity data fitting term and respects the semi-variogram of resistivity well logs data. To solve this problem, we used derivative-free optimization SGSD algorithm. The estimated model shows the same spatial variability than the one of the Iron measure along the boreholes.
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Dispersion and Attenuation for the Drained/ Undrained Transition - Modelling the Experiment
Authors L. Pimienta, J. Borgomano, J. Fortin and Y. GuéguenSummaryThe frequency effects in rocks have been theoretically shown to originate from fluid flow at different scales. In order to check experimental results recently reported, a 1D poroelastic model has been developed. Four properties may be predicted using the model: the rock elastic response, in terms of bulk modulus and attenuation, and the hydraulic response, in terms of fluid over-pressure and phase shift. This model aims at better understanding and comparing the experimental measurements. Two aspects are addressed by the model’s predictions: the comparison between local and global measurements, and the role of the boundary conditions on the measurement. It is shown that, for a given rock submitted to the same experimental conditions, global and local measurements will lead to different results. Moreover, the dead volume at both sample’s ends is shown to strongly affect the results obtained under fluid saturation at lowest frequencies.
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Multiscale Measurement for Wave Dispersion in Consolidated Sandstones
More LessSummaryA multi-band measurement techniques system has been designed and built at CNPC rock physics lab. From measured Young’s modulus and Poisson ratio, the velocities in rock samples can be obtained. In order to verify our system, we use two tests (Aluminum and Lucite) to demonstrate the performance of the multi-band measurement techniques system. Tests with two consolidated sandstone samples, we measured and compared the properties under different confining pressures and partially water-saturated conditions, to investigate the influence of pressure and water-saturation on the multi-band dispersion.
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Reciprocity and Microinhomogeneity in Poroelasticity
Authors T.M. Mueller and P.N. SahaySummaryIn Biot’s theory of poroelasticity the potential energy is a function of two kinematic variables which does not include the porosity.
Under this assumption the number of independent poroelastic parameters can be reduced by one. This is sometimes referred to as reciprocity property.
A thermodynamic consideration shows that the porosity is a kinematic variable in poroelasticity. Since the potential energy function has to be constructed from all kinematic variables, this implies for the specific case of a poroelastic medium that the porosity must be included in the deformational potential energy expression.
It also implies that the parameter reduction assumed by Biot does not hold and that one more poroelastic parameter is required to fully describe the volumetric deformation of porous rocks.
In the framework of the generalized poroelasticity framework this additional parameter appears in the form of the microinhomogeneity parameter.
It can be expressed in terms of measurable poroelastic parameters.
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Velocity-saturation relation in partially saturated rocks: Modelling the effect of injection rate changes
Authors J.W. Liu, T.M. Muller, Q.M. Qi, M. Lebedev and W.T. SunSummaryThe relationship between P-wave velocity and fluid saturation in saturated porous media is of importance for reservoir rock characterization. Forced imbibition experiments in the laboratory reveal rather complicated velocity-saturation relations including rollover-like patterns induced by injection rate changes. Poroelasticity-theory based patchy saturation models with a constant fluid patch size are not enough to describe these velocity-saturation relations. Therefore, we incorporate a saturation-dependent patch size function into two models for patchy saturation. With this, we can model the observed velocitysaturation relations obtained at different injection rates well. The results indicate that there can exist a distinct relation between patch size and injection rate. Moreover, we assess the relative importance of capillarity on velocity-saturation relations and find that capillarity stiffening impairs the impact of patch size changes. Capillarity stiffening appears to be a plausible explanation when a decrease of the injection rate is expected to boost the importance of capillarity.
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A Set-up for Dispersions and Attenuations Measurements in Fluid-saturated Rocks
Authors L. Pimienta, J. Fortin and Y. GuéguenSummaryInvestigating experimentally the frequency dependence of the elastic properties of fluid-saturated rocks remains challenging. A new set-up using the stress-strain method has been tested and applied to measure elastic properties of fluid saturated rocks. Calibrations have shown that measurements of Young’s modulus and Poisson’s ratio may be obtained with an accuracy of about 1%. Dissipation has been measured down to values of 0.01. The frequency range is [10–3;102] Hz.
The method is applied to the study of Fontainebleau sandstone samples saturated by different fluids. Over the range of frequency, two dispersion/dissipation phenomena are measured. Using the existing theories, those two transitions are shown to be the drained/undrained and the undrained/unrelaxed transitions.
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A Dual Porosity Solid Substitution Recipe for Heavy Oil Rocks
Authors S. Glubokovskikh, B. Gurevich and N. SaxenaSummaryHeavy oils production involves heating of the reservoir, which results in significant reduction of the pore fill viscosity. Seismic monitoring of these subsurface operations requires a model relating elastic properties of the heavy oil rocks at different temperatures. To this end, we developed a simple solid substitution scheme. Key feature of the model is division of porosity into stiff matrix pores and compliant cracks-like pores, which is important because presence of a solid material in compliant pores or cracks stiffens the rock to much greater extent than its presence in stiff pores. We approximate a typical compliant pore as a circular disc confined between rigid plates and surrounded by empty pores. The stiff pores are then embedded using generalized Gassmann’s equations. When the pore fill is solid, the predictions of the scheme are close to the predictions of the solid-squirt model recently proposed by Saxena and Mavko. However, the present scheme also gives a continuous transition to the classic Gassmann’s equations for a liquid pore fill at low frequencies and squirt theory at the high frequency limit.
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Clay Distribution Effects on the Joint Elastic-electrical Properties of Shaly Sandstones
Authors N. Aladwani, A.I. Best, L.J. North and T.A. MinshullSummaryA clay distribution classification scheme was applied to a joint ultrasonic and resistivity datasets for shaly reservoir sandstones, allowing us to identify pore-filling and load-bearing shale and their effects on P- and S-wave velocity, attenuation and electrical resistivity at 60 MPa effective pressure. Reservoir sandstones with load-bearing shale form a distinct trend to clean and pore-filling shale sandstones in resistivity-velocity cross plots, and show decreasing attenuation with increasing resistivity. The results give useful information for improved hydrocarbon reservoir characterisation from joint property inversions.
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An Investigation into the Non-Newtonian Behavior of Polymer Flow in Porous Medium
Authors S. Aghabozorgi Nafchi and B. RostamiSummaryPolymer injection has been of great interest in petroleum engineering as an enhance oil recovery (EOR) method. In order to have a reliable injection plan, an accurate estimation of polymer behaviour is required to control the fluid flow in porous media. However it is difficult to predict the flow properties of polymer solutions - known as non-Newtonian fluids – due to variable viscosity at different shear rates. In the present work, pore network modelling has been used to investigate the behaviour of polymer solutions in porous media. A pore network is constructed which adequately represents the real structure of porous medium. The network parameters are tuned to match the experimental properties of reservoir rock samples reported in the experiments. To calculate the non-Newtonian apparent viscosity, an iteration method has been applied since the viscosity is a function of shear rate while the viscosity should be known to determine the shear rate. The estimated non-Newtonian viscosities are in good agreement with the experimental data. Finally, the effects of polymer adsorption on variable viscosity of polymer solutions has been investigated. The results reveals that the apparent viscosity highly depends on the pore radius which can be reduced because of the polymer adsorption.
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Resonant Seismic Wave Interaction with Acoustic Cavities
Authors F.M. Schneider, S. Esterhazy, I. Perugia and G. BokelmannSummary“Resonance seismometry” will be a feature for the verification of the future Comprehensive Nuclear Test Ban Treaty (CTBT) during On-Site Inspections (OSI). To better understand the scientific underpinning of that field, we apply an analytical solution for plane elastic wave interaction with a spherical acoustic inclusion derived by Korneev (1996) using the model dimensions of an cavity created by an underground nuclear test. For a fluid-filled cavity small changes of the frequency can cause strong variations of the scattered field, and clear resonances potentially occur. For the OSI-application, the strongest peaks appear in an observable frequency range at around 15 Hz and 30 Hz. This would apply for the case that the cavity is filled by ground water after the explosion. We also investigate the case of a gas-filled acoustic cavity, and show that no resonances are observed. The spectral characteristics of the cavity scattering can be described by dominant S-wave scattering in a broad frequency range (0–95 Hz) and a monotonous increase of total scattered energy with frequency.
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Stiffness Reduction Method for Finite-element Scheme Elastic Wave Modelling in Heterogeneous Media - An Alternative to PML
More LessSummaryAn effective absorbing boundary condition is imperative to rule out spurious reflections at the artificial boundaries. Among all the absorbing boundary techniques, PML is one of the most developed and popular. However, PML is complicated and computational expensive for elastic wave modelling based on the finite-element scheme, e.g., finite element method (FEM) and spectral element method (SEM). Hence, we introduce a stiffness reduction method (SRM) in elastic wave simulation and extend this method to fit heterogenous media. Thus, we build a rugged elastic heterogenous medium and test three possible scenarios to implement SRM by using FEM. Finally, we compare these three solutions with the one obtained by using PML. Numerical results suggest that SRM can yield comparable performance to that of PML, yet put up an easier implementation.
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Optimized Staggered-grid FD Method for Elastic Wave Modeling Based on Elastic Plane Wave Solution
Authors P. Yong, J.P. Huang, Z.C. Li, L.P. Qu and Q.Y. LiSummaryNumerous optimized finite-difference (FD) schemes have been presented for scalar wave modeling by minimizing time-space domain dispersion relation errors.While optimization FD schemes for elastic wave numerical simulation are relatively less. In this paper, we develop an optimized FD schemes with equivalent staggered grid for elastic modeling based on the elastic plane wave, which is derived by eigenvalue decomposition method in wavenumber domain. Specifically, we obtain a new time-space dispersion relationship with analytical plane wave solution. The L2-norm of the relative dispersion errors is minimized by the Gauss-Newton algorithm. Dispersion analysis and numerical results have demonstrated that the presented method can achieve high accuracy in time and space domain.
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A Strategy for Elastic Wave Simulation Based on Pseudo-analytical Operator Differencing
More LessSummaryThe approach implemented to wave propagation is one of the key factors that will not only determine the accuracy but also influence the computational efficiency. In this paper, by introducing coupled velocity, predictor-corrector strategy for elastic wave simulation is proposed to extend the k-space method from acoustic wave equation to elastic wave equation. Considering the massive computational cost while solving k-space method in spectral domain directly, we adopt optimal finite-difference coefficients based on k-space method to improve the efficiency. Numerical example demonstrates that the proposed strategy can suppress numerical dispersion both in space and time effectively. Furthermore, this method allows us to use large sampling step both in space and time, thus reducing the amount of calculation.
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Lowrank Finite-difference Method for Elastic Wave Propagation
More LessSummaryWe propose a new lowrank finite-difference method for elastic wave propagation in time domain. Firstly, based on elastic wavefield vector decomposition, we derive recursive integral time extrapolation operators for P-and S-wave, respectively. And we employ lowrank decomposition method to approximate these extrapolation operators. Then, we design the corresponding finite-difference scheme. Compared with conventional finite-difference method, the lowrank finite-difference method has higher accuracy and allows us to use large time-step in elastic wave simulation. Numerical experiments confirm that the proposed method can be an effective tool to simulate elastic waves.
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Efficient Quasi-P Wavefield Extrapolation Using an Isotropic Lowrank Approximation
Authors Z. Zhang and T. AlkhalifahSummaryUsually the computational cost of the quasi-P simulation depends on the complexity of the medium, and specifically the anisotropy. The effective-model method splits the anisotropic dispersion relation to an isotropic background and a correction factor that depends on the gradient of the wavefields. As a result, the computational cost is independent of the nature of anisotropy, which makes the extrapolation efficient. A dynamic implementation of this approach decomposes the original pseudo-differential operator into a Laplacian, handled using the low-rank approximation of the spectral operator, and an angular dependent correction factor applied in the space domain to correct for anisotropy. We analyze the role played by the correction factor and propose a new spherical decomposition. The proposed method provides accurate wavefields in phase and a more balanced amplitude. Also, it is free of SV-wave artifacts. Applications to a simple homogeneous VTI model and the revised Hess VTI model demonstrate the effectiveness of the approach.
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Application of Perturbation Theory for P-wave Eikonal Equation in Orthorhombic Media
Authors A. Stovas, N. Masmoudi and T. AlkhalifahSummaryThe P-wave eikonal equation for orthorhombic media is the sixth-order PDE with complex and time-consuming numerical solution. We propose to approximate the solution of this equation by applying multi-parametric perturbation method. We also investigate the sensitivity of traveltime surface in orthorhombic media with respect to three anelliptic parameters. A simple moveout equation valid for orthorhombic media is derived.
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Twin Wavefront Construction Approach to Simulation of Shear Waves in Transversely Isotropic Layered Media
Authors E. Iversen and T. KaschwichSummaryWe present a novel approach to simulation of shear (S) waves in transversely isotropic layered media, based on the wavefront construction method. Each ray code specifying S-wave simulation will give rise to two runs of the wavefront construction process, one run for each elementary (twin) S wave. For one S-wave twin the polarization, in local anisotropy coordinates, is assumed mainly of type SV; for the other twin the polarization is mainly SH. The method allow for models that consists of heterogeneous elastic layers separated by generally shaped interfaces. Inside the layers the type of elasticity is either isotropic or transversely isotropic. The symmetry axis of transverse isotropy may have arbitrary orientation and is allowed to change direction smoothly inside the layers. We use standard ray theory for transversely isotropic media, not an average type of ray computation. This ensures accurate traveltimes in situations of strong anisotropy. The introduction of a ‘fat singularity’ guarantees continuous polarization directions in a region around an S-wave singularity. One may optionally apply coupling ray theory for continuation of amplitude along the rays.
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Valhall Case Study - Value of Seismic Technology for Reducing Risks in a Reactive Overburden
Authors N. Haller, R. Flateboe, C. Twallin, V. Dahl-Eriksen, P. Heavey, E. Kjos, R. Milne and W.E.A. RietveldSummaryIn 2015, the Valhall field benefited from a rejuvenated multi-component seismic processing sequence from a large OBC survey (permanent LoFS array supplemented with extra-seabed cables) covering the full field. These new high-resolution compressional and converted reflectivity pictures were decisive in understanding the structural framework, the distribution of sands units and the presence of gas in the overburden. Combining the updated overburden description with an extensive effort of reviewing the wells history will reduce risks for future wells and P&A, while demonstrating the value of long-term seismic strategy and integrated multi-disciplinary approach.
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Interpretation-guided Image Enhancement Using RTM Vector Image Partitions
Authors R.G. Gu, O.Z. Zdraveva, M.H. Hegazy and S.B. BuzzellSummaryA subsalt seismic image is challenged by complex salt bodies and poor illumination, in spite of great technology advances achieved in recent years. We developed an automated method referred to as Interpretation-Guided Image Enhancement (IGIE), to optimize and enhance the subsalt image by using Reverse Time Migration (RTM) Vector Image Partitions (VIPs). Starting from a few interpretation horizons, IGIE boosts signals from a specific shooting direction that illuminates the target events and suppresses noise from other directions. To demonstrate the effectiveness of our method, we present a case study from the Gulf of Mexico. The results show that IGIE successfully reveals steeply dipping subsalt events that are hidden under noise
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A Quantitative Approach to Reconcile Subsalt Images from Overlapping Surveys with Different Geometries
Authors L.P. Letki and M. VieSummaryWith exploration focusing on more complex targets, there is a high demand for high-quality interpretable seismic images in challenging geological settings such as subsalt areas, combined with an ever increasing interest in multipurpose seismic volumes, adequate for both qualitative and quantitative interpretation. Recent advances in subsalt imaging have focused on qualitative methods to address the main challenge of inadequate subsurface illumination, caused by the complex overburden and seismic acquisition geometry. This paper discusses how point spread functions and depth domain inversion can open the door for new quantitative approaches.
The depth domain inversion workflow is applied to two surveys with different acquisition geometries covering the same area. The survey-specific space-, depth- and dip-dependent illumination effects are captured by the respective grids of point spread functions. The outputs are reflectivity volumes corrected for illumination effects. In areas illuminated by both surveys, the strong similarity between the results obtained independently from each survey proves that this approach removes the survey-specific seismic experiment imprint to unravel the common subsurface response. This opens the door for new quantitative workflows to merge different surveys or partial images and improve both the structural and quantitative interpretability of subsalt images.
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