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.
No abstract
To design well paths that minimize risk and avoid drilling hazards such as blowouts and stuck pipe, drilling engineers would like to have a quantitative understanding of the overpressure zones in the subsurface. Currently, pre-drill prediction of pore pressure is done using kinematically determined seismic velocity, which has a low resolving power in identifying various subsurface formations. In the rare examples where high-resolution velocity is used, the primary seismic input is inverted acoustic impedance. The acoustic impedance is converted into high-frequency velocity and density for effective stress and overburden stress computations. Both require transformation schemes, potentially causing additional uncertainty in pore-pressure prediction. In this paper, we present a method based directly on acoustic impedance. We thus avoid the additional, potentially error-prone step of converting impedance to velocity and density. We modify the methodology described in Rasolofosaon and Tonellot (2011). We call this the RT method in this paper. Using well log data, we first demonstrate that the RT method provides practically the same results as those using velocity and density data at the well location, and does it more efficiently. This leads us to suggest that the formation pore pressure itself can be written as a piece-wise continuous function of a single variable, acoustic impedance. This greatly simplifies the work steps in pore-pressure prediction methodology. This new method is then applied to well and seismic data in deepwater Gulf of Mexico (GoM) subsalt basins, predicting subsalt and salt-exit pore pressure. We compare the predicted results with measured pore-pressure data where available.
Objectives/Scope The deep water Bonga development is situated in block OML118 offshore Nigeria, . The Bonga Main Field was discovered in 1995 with first production in November 2005. The main reservoirs are channelized, unconsolidated, turbidite sandstones of Miocene age. While the field development has been successful, opportunities and challenges remain. Below the producing reservoir levels, there is potential for additional reservoirs - unlocking those deep hydrocarbons would require to drill beyond present well control. At the same time, drilling development wells cost effectively has remained challenging even for shallow intervals given subsurface heterogeneities, which often cause borehole stability issues. Methods, Procedures, Process This study introduces a novel workflow that allows the asset to leverage quantitative seismic interpretation, that is closely integrated with geomechanics modelling to address both the deep reservoir potential opportunity and the borehole stability related drilling cost challenge. Here we focus on the integration of the geomechanical and geophysical data and workflows rather than on the successful prediction of deep sand probabilities using seismic AvO inversion and Bayesian facies classification. As part of the seismic inversion, 3D dynamic Young's Modulus and Poisson's Ratio volumes were derived. In parallel, a finite-element mesh for geomechanical modelling was created from the structural interpretation and then populated with the seismic derived rock properties. The resulting field scale 3D geomechanics model helps to address production-related challenges such as top seal integrity, fault reactivation, compaction, subsidence, injection, depletion, borehole stability, and sand control. For this study, seismic data needed to be inverted over an interval from near seabed to deep targets below well penetration - some 3 seconds TWT or 10,000ft, a much larger window than normal for single reservoir-focused studies. Seismic AvO inversion was run using overlapping, time windows from shallow to deep, to account for wavelet transmission effects. The resulting inversion outputs, acoustic and shear impedance, were used to derive shale and sand probability volumes. Well based analysis was used to determine the best relationship between acoustic and shear impedance and Young's Modulus for both sand and shale facies. Using the facies probability volumes from seismic inversion, 3D dynamic Young's Modulus and Possion's Ratio volumes were calculated from the acoustic and shear impedance volumes. Results, Observations, Conclusions A 1D geomechanics model, calibrated against drilling experience, was used to convert from dynamic to static Young's Modulus. Finite-element geomechanical modelling was used to produce the 3D stress model combining pore pressure, structural information, seismic-based static rock properties, and far-field horizontal stresses. The final stage of stress analysis involved calculating stresses that honor local field measurements and incorporate regional trends. Novel/Additive Information Utilizing 3D finite element models constrained by seismic yielded a high resolution predictive model that will significantly improve wellbore stability predictions along the paths of future development wells. The business impact for the Asset is reduced development well costs by having a more predictable geomechanics model, fully constrained by lateral variations from 3D seismic data, and greatly reduced cycle times for borehole stability predictions for future wells.
Interpretation shares commonalities with GEOPHYSICS and the AAPG Bulletin in that it is a peer-reviewed journal. Unlike GEOPHYSICS and the AAPG Bulletin, Interpretation is built around special sections headed by a team of special-section editors who are either experts or particularly interested in the focused area. In addition to constructing a Call for Papers announcing their special section, the special-section editors also will solicit papers from colleagues, competitors, technology suppliers, and others that they believe may have contributions of interest to the Interpretation readership community. Submitted papers then are assigned by the special editors to three or more reviewers, many of whom are contributors to (and hence expert in) the same special-section topic. By design, the special section-structure of Interpretation reaches authors, editors, and reviewers who previously may not have been involved in the peer-review process. Recognizing this fact, in this article the standing editorial board attempts to summarize some of the more important qualities of what we find to be a good reviewer. Expectations of reviewersReviewing a paper takes time. However, the time invested helps not only the authors of the paper but also our profession, our professional societies, and the scientific community at large. The flowchart in Figure 1 summarizes the review process. The expectation is that reviewers will provide a timely, professional review. Reviewers are expected to critique but not correct poor English grammar. Simply circling a sentence or paragraph and writing "rephrase," "unclear," "unsubstantiated," or "garbled" is sufficient. A weak point of many papers, common to new authors but also to those who have not written a peer-reviewed paper in many years, is the linkage to other work. Here, the reviewers should ask the authors to add additional specific references that put their paper in context, or to reference papers that propose an alternative workflow or interpretation. A good reviewer may suggest some related and necessary references beyond that of their work group.By construction, many, if not most papers submitted to Interpretation are either multidisciplinary or they integrate diverse types of data. As a reviewer, you are not expected to be a content expert for every paper. If a paper has detailed algorithmic derivations in an appendix and you are uncomfortable with such arithmetic, simply tell the Associate Editor that you "leave the review of the equations to those who are more proficient in mathematics" and review the remainder of the paper. Alternatively, if you are comfortable with the mathematics or a detailed seismic processing workflow, but not with the geologic application, tell the editor that you have "reviewed the algorithms and workflows in detail, but leave the interpretation to those more familiar with this type of geologic problem." It is then up to the Associate Editor to choose a cross section of reviewers able to critique the paper in its entirety.There are multiple formats in const...
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