Converted seismic waves (specifically, downgoing P‐waves that convert on reflection to upcoming S‐waves are increasingly being used to explore for subsurface targets. Rapid advancements in both land and marine multicomponent acquisition and processing techniques have led to numerous applications for P‐S surveys. Uses that have arisen include structural imaging (e.g., “seeing” through gas‐bearing sediments, improved fault definition, enhanced near‐surface resolution), lithologic estimation (e.g., sand versus shale content, porosity), anisotropy analysis (e.g., fracture density and orientation), subsurface fluid description, and reservoir monitoring. Further applications of P‐S data and analysis of other more complicated converted modes are developing.

/pdf/converted-wave-seismic-exploration-applications-3a2klst9ax.pdf

Converted‐wave seismic exploration: Applications

SUMMARY Time-lapse seismology is important for monitoring subsurface pressure changes and fluid movements in producing hydrocarbon reservoirs. We analyse two 4-D, 3C onshore surveys from Vacuum Field, New Mexico, USA, where the reservoir of interest is a fractured dolomite. In Phase VI, a time-lapse survey was acquired before and after a pilot tertiary-recovery programme of overpressured CO2 injection, which altered the fluid composition and the pore-fluid pressure. Phase VII was a similar time-lapse survey in the same location but with a different lowerpressure injection regime. Applying a processing sequence to the Phase VI data preserving normal-incidence shearwave anisotropy (time-delays and polarization) and maximizing repeatability, interval-time analysis of the reservoir interval shows a significant 10 per cent change in shear-wave velocity anisotropy and 3 per cent decrease in the P-wave interval velocities. A 1-D model incorporating both saturation and pressure changes is matched to the data. The saturation changes have little effect on the seismic velocities. There are two main causes of the time-lapse changes. Any change in pore-fluid pressures modifies crack aspect ratios. Additionally, when there are overpressures, as there are in Phase VI, there is a 90 ◦ change in maximum impedance directions, and the leading faster split shear wave, instead of being parallel to the crack face as it is for low pore-fluid pressures, becomes orthogonal to the crack face. The anisotropic poro-elasticity (APE) model of the evolution of microcracked rock, calculates the evolution of cracked rock to changing conditions. APE modelling shows that at high overburden pressures only nearly vertical cracks, to which normal incidence P waves are less sensitive than S waves, remain open as the pore-fluid pressure increases. APE modelling matches the observed time-lapse effects almost exactly demonstrating that shear-wave anisotropy is a highly sensitive diagnostic of pore-fluid pressure changes in fractured reservoirs. In this comparatively limited analysis, APE modelling of fluid-injection at known pressure correctly predicted the changes in seismic response, particularly the shear-wave splitting, induced by the high-pressure CO2 injection. In the Phase VII survey, APE modelling also successfully predicted the response to the lower-pressure injection using the same Phase VI model of the cracked reservoir. The underlying reason for this remarkable predictability of fluid-saturated reservoir rocks is the critical nature and high crack density of the fluid-saturated cracks and microcracks in the reservoir rock, which makes cracked reservoirs critical systems.

/pdf/processing-modelling-and-predicting-time-lapse-effects-of-1218k5f96z.pdf

Processing, modelling and predicting time-lapse effects of overpressured fluid-injection in a fractured reservoir

Differences in the frequency response of horizontal and vertical detectors (vector infidelity) in ocean bottom cable (OBC) surveys can cause problems for multicomponent processing, such as S-wave birefringence and amplitude variation with azimuth (AVA) analyses, and combining vertical and hydrophone data for water-born multiple suppression. One source of this problem is poor detector coupling with the seabed that produces resonances and phase distortions. Coupling and data quality are generally excellent for the inline component. However, the crossline component often exhibits low-frequency resonance compared to the inline. Also, OBCs are susceptible to rotational modes about the cable axis that produce spurious S-waves on the vertical component. I derive a method for correcting the crossline and vertical components based on a model of OBC detector coupling, and design vector operators in the frequency domain from shots over many offsets and azimuths. The crossline data are corrected,relative to the inlin...

Detector coupling corrections for vector infidelity of multicomponent OBC data

Vinton salt dome is located in Southwestern Louisiana, in Calcasieu Parish. Tectonically, the piercement dome is within the salt dome minibasin province. The field has been in production since 1901, with most of the production coming from Miocene and Oligocene sands. The goal of our project was to develop and calibrate new processing and interpretation technology to fully exploit the information available from a simultaneous 3-D surface seismic survey and 3-C, 3-D vertical seismic profile (VSP) survey over the dome. More specifically the goal was to better image salt dome flanks and small, reservoir-compartmentalizing faults. This new technology has application to mature salt-related fields across the Gulf Coast. The primary focus of our effort was to develop, apply, and assess the limitations of new 3-C, 3-D wavefield separation and imaging technology that could be used to image aliased, limited-aperture, vector VSP data. Through 2-D and 3-D full elastic modeling, we verified that salt flank reflections exist in the horizontally-traveling portion of the wavefield rather than up- and down-going portions of the wavefield, thereby explaining why many commercial VSP processing flow failed. Since the P-wave reflections from the salt flank are measured primarily on the horizontal components while P-wave reflections frommore » deeper sedimentary horizons are measured primarily on the vertical component, a true vector VSP analysis was needed. We developed an antialiased discrete Radon transform filter to accurately model P- and S-wave data components measured by the vector VSP. On-the-fly polarization filtering embedded in our Kirchhoff imaging algorithm was effective in separating PP from PS wave images. By the novel application of semblance-weighted filters, we were able to suppress many of the migration artifacts associated with low fold, sparse VSP acquisition geometries. To provide a better velocity/depth model, we applied 3-D prestack depth migration to the surface data. The reflector dip calculated from these images were used to further constrain the depth images from the less well sampled VSP data. In spite of the above technical success, we were less than pleased with our final VSP images. Since no extra sources are used, simultaneous recording of the surface and VSP data were believed to provide a cost-effective means to acquire 3-D VSP data. However, the subsurface sampling associated with the VSP is quite different from that associated with the surface seismic data. After our analysis, we find that considerable shot infill would result in a better, unaliased subsurface image. We were able to ascertain that the subsurface illuminated by the VSP was extremely small, with the PS image being even smaller than the PP image. One-way wave equation extrapolators do not work well for the VSP geometry, where we wish to extrapolate energy sideways (from the VSP well towards and away from the salt dome) as well as vertically (away from the shots on the earth surface). Merging separately-generated images proved to be both cumbersome and error-prone. Alternative, advanced multiarrival traveltime calculations that we obtained from research colleagues at other institutions could not be easily modified to image rays that had an upgoing component. In the end, we used a simpler first-arrival Eikonal-based traveltime algorithm with its well-known limitations. While the surface acquisition using radial receiver lines and concentric shot lines provided good illumination of the salt dome, this unconventional geometry proved to be particularly difficult to process using commercial software, where the lack of ''shot lines'' and ''receiver lines'' necessary for dip filtering, residual statics, and residual velocity analysis proved to be nearly intractable. We also learned that while commercial software available at UH works well for a bootstrapped velocity model computed from the seismic data alone, it was severely limited in its ability to include the dense well control available at Vinton Dome. To more accurately estimate velocities, we developed a 3-D turning wave tomography algorithm adapted to the VSP geometry employed at Vinton Dome. We were able to determine that there is about 10% anisotropy at Vinton Dome, with the axis of transverse isotropy perpendicular to the geologic formations deformed by the diapirism. At the time of this final report, we have not yet integrated traveltimes from the surface data into the tomographic inversion to better constrain the velocity model, nor developed an anisotropic migration algorithm to image the 3-D 3-C VSP (objectives well beyond the original scope of the project). As a secondary objective, we developed a suite of new 3-D volumetric attribute algorithms and image enhancement algorithms. We estimate volumetric dip and azimuth using a multiwindow Kuwahara approach that avoids smoothing amplitude and dip across faults.« less

Development and Calibration of New 3-D Vector VSP Imaging Technology: Vinton Salt Dome, LA

Anisotropy and fracture characterization in individual layers is realized through iterative layer stripping corrections of four, converted-wave (PS-wave) synthetic reflection seismic data sets, generated from azimuthally anisotropic (HTI and TTI) models, and a four component (4-C) data set from the Teal South, Gulf of Mexico. The corrections were applied on a layer-by-layer basis to evaluate the efficacy of constant polarization rotation and time-shift operators. Equivalent isotropic models were compared to anisotropic models after layer-stripping corrections using rms amplitude and shear-wave-splitting time-difference maps to quantify and identify inherent errors in estimating seismic polarization parameters. For HTI media radial and transverse components of PS data that have had layer-stripping corrections applied, exhibit incorrect symmetry and orientations. This may adversely affect inversion and/or amplitude-variation with angle offset (AVO) and amplitude versus azimuth (AVA)analysis. Layer-stripping...

J.E. Gaiser

Papers

Applications for Vector Coordinate Systems of Converted Waves Obtained by Multicomponent 3-D Data