Post-stack Inversion Research Articles

Delineation of gas-charged geobodies at Solar gas discovery offshore West Nile Delta, Egypt

Gas accumulation has been proved at the Solar Channel offshore West Nile Delta, Egypt. It is produced as an important exploration target. It was important to carry out a detailed study in searching for gas play within the Solar Channel. The spectral-decomposition technique is applied to the imaging and mapping of bed thickness, geologic discontinuities, and channel delineation at Solar discovery. From the study, two distinctively different channels (the gas-bearing “Red Channel” and water-bearing “Yellow Channel”) are delineated in the area, proved by the drilled well. Rock-physics analysis (P-impedance versus S-impedance crossplot) of well-log data revealed that it could separate gas-bearing sands from water-bearing sands and shale. Poststack inversion of seismic data has been carried out to invert for P-impedance volume. Therefore, a lithofacies cube has been generated after applying P-impedance cutoffs to discriminate among gas-bearing sand, water-bearing sand, and shale. In the end, a gas-charged geobody is extracted at the Solar Channel.

Application of waveform tomography at the Campos Basin field

Campos Basin field has been continuously character¬ised at the Reservoir Characterization Project (RCP) in Colorado School of Mines. Past research includes poststack and prestack joint inversions of PP and PS data which increased the reservoir resolution and could predict a porosity map. To further improve characterisation of the Campos Basin field, waveform tomography (WT), or full waveform inversion (FWI), is performed for a 2D line from the 2010 ocean bottom cable (OBC) data under a 2D acoustic isotropic medium assumption. The goal is to bring high resolution and accuracy to the P wave veloc¬ity model for better-quality reservoir imaging. In order to achieve the best results the application of WT to the 2D dataset required defining the suitable parameters to these data, where the main options in the inversion are the type of objective function, the time domain damping, and the fre¬quency discretisation. Waveform inversion has improved the final velocity model, as verified by migrated images showing more continuous and focused horizons at the reservoir depth. The improved seismic image and velocity model are possible inputs, respectively, to a new geological interpretation and to acoustic/elastic attributes inversion. Waveform tomography (WT), or Full Waveform Inversion (FWI), has been the subject of studies and conceptual devel-opment over the past 30 years (Virieux and Operto, 2009). Furthermore, research groups and the oil industry around the world have shown the improvement that WT can bring to the velocity model, especially in terms of high resolution in comparison with conventional techniques, such as traveltime tomography (TT) or migration velocity analysis (MVA). These techniques can only build a smoothed version of the velocity model, containing the kinematics of the wavefield, even though they are able to generate a good quality seismic image. WT represents a more advanced approach that attempts to completely describe the complex interaction of the propagating waves and the earth, in which the phase of the whole waveform is used in the model reconstruction (Pratt, 2013).

Poststack, prestack, and joint inversion of P- and S-wave data for Morrow A sandstone characterization

Exploration for oil-bearing Morrow sandstones using conventional seismic data/methods has a startlingly low success rate of only 3%. The S-wave velocity contrast between the Morrow shale and A sandstone is strong compared with the P-wave velocity contrast, and, therefore, multicomponent seismic data could help to characterize these reservoirs. The SV and SH data used in this study are generated using S-wave data from horizontal source and horizontal receiver recording. Prestack P- and S-wave inversions, and joint P- and S-wave inversions, provide estimates of P- and S-wave impedances, and density for characterization of the Morrow A sandstone. Due to the weak P-wave amplitude-versus-angle response at the Morrow A sandstone top, the density and S-wave impedance estimated from joint P- and S-wave inversions were inferior to the prestack S-wave inversion. The inversion results were compared with the Morrow A sandstone thickness and density maps obtained from well logs to select the final impedance and density volume for interpretation. The P-wave impedance estimated from prestack P-wave data, as well as density and S-wave impedance estimated from prestack SV‐wave data were used to identify the distribution, thickness, quality, and porosity of the Morrow A sandstone. The stratal slicing method was used to get the P- and S-wave impedances and density maps. The S-wave impedance characterizes the Morrow A sandstone distribution better than the P-wave impedance throughout the study area. Density estimation from prestack inversion of SV data was able to distinguish between low- and high-quality reservoirs. The porosity volume was estimated from the density obtained from prestack SV-wave inversion. We found some possible well locations based on the interpretation.

Gas hydrate saturation from seismic data constrained by log data in the Krishna-Godavari Basin

A strong bottom simulating reflector (BSR) was observed along seismic line transect across site NGHP-01-05 having characteristic reverse polarity with respect to seafloor, cross-cutting of dipping strata, and high reflectivity below the BSR. The observed BSR depth along seismic profile varies from ~100 to ~125 mbsf. Gas hydrate was recovered during coring and drilling at this site during National Gas Hydrate Drilling Expedition-01 of India. Recovered pressure cores at discrete depths within gas hydrate stability zone from site NGHP-01-05 shows ~0.6–9.4 % pore volume of methane hydrate. Gas hydrate saturation was estimated along seismic profile constrained by observed velocity and density log to introduce model-based acoustic impedance inversion. It was observed that each stacked trace amplitude converted to acoustic log as a product of velocity and density. During the impedance inversion process inverted velocity log was observed at each and every trace act as an individual log. Gas hydrate saturation was estimated from inverted velocity coupled with effective medium rock physics modeling. The estimated gas hydrate saturation from the post-stack acoustic impedance inversion velocity along 2D seismic profile varies up to 15–20 % of the pore space. The gas hydrate saturation estimated from electrical resistivity log and core data shows maximum 20 % pore spaces saturated with gas hydrate from anisotropic modeling, which is comparable to gas hydrate saturation along seismic profile estimated from inverted velocity constrained by logs.

Quantitative interpretation of CO2 plume from Sleipner (North Sea), using post-stack inversion and rock physics modeling

Sleipner gas field in the North Sea is the world's first industrial scale CO2 injection project designed specifically to reduce the emission of greenhouse gas. Here CO2 separated from natural gas produced at Sleipner gas field is injected into the Utsira sand, which is a major saline aquifer in the North Sea basin. From time-lapse (4D) seismic data, CO2 plume is observed as a number of bright sub-horizontal reflections within the reservoir. Correlation of log data with the seismic data indicates that CO2 accumulates within a series of interbedded sandstones and mudstones beneath a thick cap rock of mudstone. Nine reflective horizons have been mapped within the reservoir on the six seismic surveys from 1999 to 2008. Comparison with the baseline seismic survey of 1994 (pre-injection) provides clear impression of the migration of CO2 plume. In this paper, we attempt to model CO2 distribution quantitatively within the reservoir by applying a pressure-dependent differential effective medium (PDEM) theory using 4D seismic data. Pre- and post-injection acoustic impedances are calculated by inverting post-stack seismic data of 1994 and 2001 using a model-based inversion technique. A 3D CO2 saturation volume is estimated using PDEM theory from inverted acoustic impedance of the year 2001 taking the reference of that from the results of pre-injection data of the year 1994. Since the gas distribution type is seldom known, we estimate the saturation distribution using both a homogeneous and a patchy distribution pattern in our rock physics model. We estimate saturation for homogeneous distribution of CO2 to be 0–20% and for CO2 as patches of gas as 0–80% of the total porosity within ∼200m thick reservoir unit. 5–7% uncertainty in the predicted CO2 saturation is estimated using a Monte-Carlo simulation technique. Our results indicate that a large amount of CO2 is accumulated as patches of gas within sand layers capped by mud layers, though some amount of gas may have mixed uniformly with water.

Application of elastic impedance: A case study from Mahanadi Basin, India

Over the last decade, seismic inversion has acquired the status of being a routine interpretation step during exploration as well as development of fields. For detailed reservoir characterization, prestack inversion is desirable; however, poststack inversion is a reasonably effective tool. The inversion starts with calibration of the well with seismic data. The synthetic seismogram is tied with stacked seismic data on the basic premise that the seismic data (full stack) mimics the zero-offset data and therefore should match with the zero-offset synthetic seismogram. Generally, a good match is obtained. However, during the course of many inversion exercises, it is observed that the synthetic-to-seismic match is not good in parts of the drilled interval. There could be many reasons for this partial mismatch. One of the important reasons is that the well data represent one-dimensional data, whereas the seismic data are stacked over an angle range. In other words, the well data are equivalent to zero-offset data, whereas seismic is not. The poor calibration at the well location leads to lack of confidence in inversion results. The possible solution, such as simultaneous or full-waveform inversion, involves a lot of computation time and hardware requirements. An alternative approach is introduced which uses elastic impedance instead of normal P-impedance for synthetic match and inversion of stacked data. The synthetic seismic generated from elastic impedance shows an improved match with seismic data. The approach is quick, reliable, easy, and not heavy on computational resources. The result is inverted volumes with better hydrocarbon delineation and characterization ability.

Incorporating 3C seismic data quantitatively for enhanced geologic detail in an oil sands reservoir

A workflow incorporating converted-wave (PS) data in an integrated quantitative interpretation (QI) was illustrated with a case study in the Canadian oil sands in which multiple data types were available including high-quality 3D multicomponent data, dipole sonic logs, and a multicomponent walk-away vertical seismic profile (VSP). In an area with unconventional rock property behavior and complex fluid distributions, the dipole sonic logs provided the data necessary for robust deterministic rock-physics templates. The VSP was essential in depth-registering the P-wave surface seismic with the PS-wave data and in determining the appropriate phase rotation of each data set. The 3D multicomponent seismic was used to derive a variety of separate and joint attributes incorporating amplitude variation with offset, prestack and poststack inversion and multiattribute processes. Finally, all elements of the workflow were combined in an interactive classification procedure for optimum representation of geology in the seismic volume. Results of the QI workflow with and without the PS data were compared with each other, and ultimately, to blind wells to assess the potential benefits of including PS data. The comparison showed that better prediction of fluid properties, without compromising P-wave data resolution, was possible when PS data were included.

Reservoir characterization of the Montney Shale – integrating seismic inversion with microseismic

Understanding the optimization of heterogeneity of unconventional shale reservoirs prior to hydraulic fracturing is important for optimizing hydrocarbon production and recovery. Early prediction of geomechanical heterogeneity impacts the efficiency of horizontal well placement and completion design. Heterogeneity within shale reservoirs is influenced by composition and textural variation of the rock, ie, the rock quality. Rock quality can be evaluated or predicted with seismic-derived rock properties. This study shows how seismic data are used to determine rock quality through a multi-attribute analysis of wells logs integrated with post-stack and pre-stack inversion to characterize the Montney Shale at Pouce Coupe, Alberta (Duenas, 2014). The heterogeneity analysis combined with microseismic data and production profiles of the two horizontal wells in the area shows that lithology has a major influence on the rock quality of the Montney interval. The combined interpretation of this work with an understanding of the natural fracture system and the stress state of the reservoir can provide a rock quality index (RQI). This RQI can aid in future exploration and operational development of the Montney and other shale reservoirs worldwide.

Multimodal reservoir porosity simulation: An application to a tight oil reservoir

Abstract At appraisal stage of a reservoir characterization, a key step is the inference of the reservoir static properties, such as porosity. In this study, we present a new nested workflow that optimally integrates 3D acoustic impedance and geophysical log data for the estimation of the spatial distribution of reservoir porosity, which is applied to a tight sandstone oil reservoir located in Quebec, Canada. In this workflow, 3D seismic is the main source of spatial information. First, the statistical petrophysical relationship between acoustic impedance and reservoir porosity is established using collocated geophysical log data. Second, a conventional least-squares post-stack inversion of the impedance is computed on the seismic grid. The fit between well log data and numerically computed traces was found to be inaccurate. This leads to the third step, involving a post-stack stochastic impedance inversion using the same seismic traces not only to improve well and trace fit but also to estimate the uncertainty on the inverted impedances. Finally, a Bayesian simulation algorithm adapted to the estimation of a multi-modal porosity distribution is used to simulate realizations of porosity over the entire seismic grid. Results show that the over-smoothing effect of least-squares inversion has a major impact on resource evaluation, especially by not reproducing the high-valued tail of the porosity distribution. The adapted Bayesian algorithm combined with stochastic impedance inversion thus allows a better reproduction of the porosity distribution and improves estimation of the geophysical and geological uncertainty.

Quantitative application of post-stack inversion of organic-rich shale based on rock physics linear relationship and 3D seismic data, Perth Basin, Western Australia

Unconventional resources such as shale gas have been an extremely important exploration and production target. To understand the seismic responses of the shale gas plays, the use of rock physical relationship is important, which is constrained with geology and formation-evaluation analysis. Since organic-rich shale seismic properties remains poorly understood, seismic inversion can be used to identify the organic-rich shale from barren shale. This approach helps identify and map spatial distributions and of the organic rich shales. This study shows the acoustic impedance (AI), which is the product of compressional velocity and density, decreases nonlinearly with increasing total organic carbon (TOC) content. TOC is obtained using Roc-Eval pyrolysis for more than 120 core shale samples for the Perth Basin. By converting the AI data to TOC precent on the seismic data, we therefore can map lateral distribution, thickness, and variation in TOC profile. This extended abstract presents a case study of the northern Perth Basin 3D seismic with application of different approaches of seismic inversion and multi-attribute analysis with the rock physical relationships.

Application of Post-Stack Seismic Inversion in Discription of the Reservoir Distribution

Seismic inversion includes different methods. Acoustic impedance inversion is an ordinary method to find reservoir in oil and gas exploration, but in some area the Acoustic impedance inversion is not effective on different geologic setting. In this paper we found the Gamma Ray is effective on distinguish the sand and shale bed by using P-wave and Gamma Ray crossplot analysis. The final Gamma Ray seismic inversion results show that Gamma Ray seismic inversion can improve the vertical resolution. A satisfactory inversion effect is obtained in HM area.

Improved unconventional reservoir characterization using multi-azimuth stratigraphic inversion, case study on the Fort Worth Basin

The development of the exploration and exploitation of unconventional hydrocarbons requires innovative skills to allow a better characterization of natural fracture networks and fracability of the rocks at depth. Within this context, we bring into focus a multi-azimuth inversion methodology to give clues about the sweet spots characterization through the Barnett Shales formation. To achieve our goal we used an isotropic model-based stratigraphic seismic inversion, considering a series of azimuthal sectors to build partial seismic stacks required as input of the workflow. The initial seismic survey was recorded over a surface of approximately 100 square miles to image the Barnett Shales formation, of about 150 ft thickness only and localized in a transition zone between the oil and gas window. A preliminary processing task was to define limits of azimuthal and incidence angle sectors to build several multi-azimuth post-stack and pre-stack seismic datasets. Two exploration wells with a set of log data are available in the interest area helping in the model a priori building and in the well-to-seismic calibration, both crucial steps to apply the inversion workflow. We first perform full post-stack and pre-stack usual inversion to help in finding sets of best inversion parameters, then we do the multi-azimuth post-stack and pre-stack inversion workflow using the same previous sets of parameters for each azimuthal sector. Consequently a series of acoustic impedance, P- and S-wave elastic impedance cubes are computed for each azimuthal sector. Another aspect of the study is the quantification of azimuthal variations using an ellipse fitting algorithm giving, for each bin of the interest volume, the ratio (between major and minor axes) and tilt (i.e. azimuth from the East) of the ellipse. Results showed up to 10% of anisotropy for impedances in the Barnett Shales. We go a step further computing geomechanical features like Poisson's ratio and brittleness coefficient within the Barnett Shales and we quantified their azimuthal variations. Some areas present lower Poisson's ratio and higher brittleness than other parts. It may suggest that the latter are sweet spots that could be more easily stimulated by induced fracturing.

Multiscale rock-physics templates for gas detection in carbonate reservoirs

The heterogeneous distribution of fluids in patchy-saturated rocks generates significant velocity dispersion and attenuation of seismic waves. The mesoscopic Biot–Rayleigh theory is used to investigate the relations between wave responses and reservoir fluids. Multiscale theoretical modeling of rock physics is performed for gas/water saturated carbonate reservoirs. Comparisons with laboratory measurements, log and seismic data validate the rock physics template. Using post-stack and pre-stack seismic inversion, direct estimates of rock porosity and gas saturation of reservoirs are obtained, which are in good agreement with oil production tests of the wells.

Delineating Hydrocarbon Bearing Zones Using Elastic Impedance Inversion: A Persian Gulf Example

Reservoir characterization plays an important role in different parts of an industrial project. The results from a reservoir characterization study give insight into rock and fluid properties which can optimize the choice of drilling locations and reduce risk and uncertainty. Delineating hydrocarbon bearing zones within a reservoir is the main objective of any seismic reservoir characterization study. In the current study, using limited well control and seismic data, an attempt was made to predict the productive zones of a reservoir using elastic impedance inversion. Elastic impedance logs at near and far angles of incidence have been crossplotted to find the desired productive parts of the formation. Two partial angle stack seismic data have been inverted using a model-based post-stack seismic inversion. The crossplot of the two inverted volumes is interpreted based on the results from the well location. Finally, the hydrocarbon bearing zones of the reservoir was delineated according to the seismic crossplot analysis.

A Two-Dimensional Post-Stack Seismic Inversion for Acoustic Impedance of Gas and Hydrate Bearing Deep-Water Sediments Within the Continental Slope of the Ulleung Basin, East Sea, Korea

A post-stack inversion of 2D seismic data was conducted to estimate the spatial distribution of acoustic impedance associated with gas and hydrates in the Ulleung Basin, East Sea, Korea constrained by logs from three boreholes drilled on its continental margin. A model-based inversion was applied to a Plio-Quaternary succession composed of alternations of unconsolidated mass-flow deposits/turbidites. A comparison of seismic reflections and synthetic data computed from impedance logs is shown for two zones. An upper (steep) slope zone contains a moderately continuous, possibly bottom-simulating reflector feature along the corresponding section. This feature may be associated with a lithology boundary near a drill site in addition to, or instead of, a stability boundary of gas hydrates (i.e., gas below and hydrates above). The lower (gentle) slope zone has locally cross-cutting reflection patterns that are more likely to be attributed to gas- and hydrate-related physical phenomena than to spatiotemporal changes in lithology. This seismic inversion is informative and useful, making a contribution to enhance the interpretability of the seismic profiles for a potential hydrate recovery.

Stochastic and deterministic seismic inversion methods for thin-bed resolution

Results from a typical post-stack or pre-stack seismic inversion are band-limited primarily due to missing low and high frequencies in the wavelet. Thin beds that are of primary interest in reservoir characterization are generally poorly resolved. Inversion methods are designed to incorporate a priori information in several different ways. In this paper, we examine and compare performances of two such methods: a stochastic method that draws a starting broad-band model based on some statistics, derived from well logs, and a deterministic method that incorporates constraints using a model norm, based on the basis pursuit decomposition. We apply a stochastic method employing very fast simulated annealing for optimization, using fractal-based starting models, and a basis pursuit inversion (deterministic method) to a single post-stack dataset to assess the performance of the two algorithms. We find that both methods are able to derive a broad-band acoustic impedance from the band-limited post-stack seismic data. The results show a similar structure and therefore result in possible increasing confidence in the interpretation. The essence of our study is that the calibration of results from two different methods can provide an interpreter with more information in interpreting broad-band inversion results hitherto not derived from standard deterministic algorithms.

Elastic impedance variation with angle inversion for elastic parameters

Elastic impedance (EI) and amplitude variation with offset or angle (AVO/AVA) inversion are two cardinal methods to estimate elastic parameters underground with reflection seismic data. Conventional EI inversion as a kind of pre-stack and post-stack joint inversion method has been widely applied in the industry because of its high efficiency and high stability of wavelet extraction; however, the robustness of extracting elastic parameters in conventional EI inversion is still controversial. The robustness of three-term AVO inversion has improved a lot; however, it is still challenging to extract reasonable space variant wavelets for each offset or incident angle. In this paper, a robust three-parameter estimation method, named elastic impedance variation with angle (EVA) inversion, is proposed in the Bayesian framework, which can estimate elastic parameters directly from EI. This method supposes that the parameters to be inverted are Cauchy distributed and it is implemented based on a normalized EI equation in a logarithmic domain which can reduce the nonlinearity of inversion. Application of a covariance matrix to decorrelate the parameters and constraint of well log curves introduced in an objective function enhances the robustness of EVA inversion. A model test shows that the proposed EVA inversion method enables one to estimate reasonable elastic parameters with extremely smooth initial models and moderate Gaussian noise. A real data example shows that the inverted P-wave velocity, S-wave velocity and density are identical to well log interpretation results, which shows the validity of the proposed method.

Incorporation of geology with rock physics enables subsalt poststack inversion: A case study in the Gulf of Mexico

One of the main goals of seismic inversion is to obtain high-resolution relative and absolute impedance for reservoir properties prediction. We aim to study whether the results from seismic inversion of subsalt data are sufficiently robust for reliable reservoir characterization. Approximately [Formula: see text] of poststack, wide-azimuth, anisotropic (vertical transverse isotropic) wave-equation migration seismic data from 50 Outer Continental Shelf blocks in the Green Canyon area of the Gulf of Mexico were inverted in this study. A total of four subsalt wells and four subsalt seismic interpreted horizons were used in the inversion process, and one of the wells was used for a blind test. Our poststack inversion method used an iterative discrete spike inversion method, based on the combination of space-adaptive wavelet processing to invert for relative acoustic impedance. Next, the dips were estimated from seismic data and converted to a horizon-like layer sequence field that was used as one of the inputs into the low-frequency model. The background model was generated by incorporating the well velocities, seismic velocity, seismic interpreted horizons, and the previously derived layer sequence field in the low-frequency model. Then, the relative acoustic impedance volume was scaled by adding the low-frequency model to match the calculated acoustic impedance logs from the wells for absolute acoustic impedance. Finally, the geological information and rock physics data were incorporated into the reservoir properties assessment for sand/shale prediction in two main target reservoirs in the Miocene and Wilcox formations. Overall, the poststack inversion results and the sand/shale prediction showed good ties at the well locations. This was clearly demonstrated in the blind test well. Hence, incorporating rock physics and geology enables poststack inversion in subsalt areas.

Ocean Temperature and Salinity Distributions Inverted from Combined Reflection Seismic and XBT Data

AbstractSeismic oceanography has been widely used to detect many kinds of oceanographic features as a new oceanographic prospecting method. Inversion for temperature and salinity distributions from seismic data is a very important research field in seismic oceanography. We analyzed low frequency seismic data from one multichannel seismic (MCS) line acquired in the scope of the European GO Project (line GOLR‐12) combined with the simultaneously acquired XBT (eXpendable BathyThermograph) and CTD (Conductivity‐Temperature‐Depth) data, which are located to the southwest of the Iberian Peninsula. We used a post‐stack constrained impedance inversion method to derive temperature and salinity distributions of sea water and the result demonstrated that this method can indeed provide reliable temperature and salinity distribution profiles every 6.25m along the seismic line, with the temperature resolutions of 0.16°C. This method applied to good quality MCS data can provide high lateral‐resolution temperature and salinity profiles for oceanographic research, using only a few oceanographic measurements to constrain the inversion procedure.

Post-stack stratigraphic inversion workflow applied to carbon dioxide storage: application to the saline aquifer of Sleipner field‡

In the Norwegian North Sea, the Sleipner field produces gas with a high CO2 content. For environmental reasons, since 1996, more than 11 Mt of this carbon dioxide (CO2) have been injected in the Utsira Sand saline aquifer located above the hydrocarbon reservoir. A series of seven 3D seismic surveys were recorded to monitor the CO2 plume evolution. With this case study, time-lapse seismics have been shown to be successful in mapping the spread of CO2 over the past decade and to ensure the integrity of the overburden. Stratigraphic inversion of seismic data is currently used in the petroleum industry for quantitative reservoir characterization and enhanced oil recovery. Now it may also be used to evaluate the expansion of a CO2 plume in an underground reservoir. The aim of this study is to estimate the P-wave impedances via a Bayesian model-based stratigraphic inversion. We have focused our study on the 1994 vintage before CO2 injection and the 2006 vintage carried out after a CO2 injection of 8.4 Mt. In spite of some difficulties due to the lack of time-lapse well log data on the interest area, the full application of our inversion workflow allowed us to obtain, for the first time to our knowledge, 3D impedance cubes including the Utsira Sand. These results can be used to better characterize the spreading of CO2 in a reservoir. With the post-stack inversion workflow applied to CO2 storage, we point out the importance of the a priori model and the issue to obtain coherent results between sequential inversions of different seismic vintages. The stacking velocity workflow that yields the migration model and the a priori model, specific to each vintage, can induce a slight inconsistency in the results.