Improving the Monitoring, Verification, and Accounting of CO₂ Sequestered in Geologic Systems with Multicomponent Seismic Technology and Rock Physics Modeling

Abstract

Research done in this study showed that P-SV seismic data provide better spatial resolution of geologic targets at our Appalachian Basin study area than do P-P data. This finding is important because the latter data (P-P) are the principal seismic data used to evaluate rock systems considered for CO 2 sequestration. The increase in P-SV 1 resolution over P-P resolution was particularly significant, with P-SV 1 wavelengths being approximately 40-percent shorter than P-P wavelengths. CO 2 sequestration projects across the Appalachian Basin should take advantage of the increased resolution provided by converted-shear seismic modes relative to P-wave seismic data.In addition to S-wave data providing better resolution of geologic targets, we found S-wave images described reservoir heterogeneities that P-P data could not see. Specifically, a channel-like anomaly was imaged in a key porous sandstone interval by P-SV 1 data, and no indication of the feature existed in P-P data. If any stratigraphic unit is considered for CO 2 storage purposes, it is important to know all heterogeneities internal to the unit to understand reservoir compartmentalization. We conclude it is essential that multicomponent seismic data be used to evaluate all potential reservoir targets whenever a CO 2 storage effort is considered, particularly when sequestration efforts are initiated in the Appalachian Basin.Significant differences were observed between P-wave sequences and Swave sequences in data windows corresponding to the Oriskany Sandstone, a popular unit considered for CO 2 sequestration. This example demonstrates that S-wave sequences and facies often differ from P-wave sequences and facies and is a principle we have observed in every multicomponent seismic interpretation our research laboratory has done. As a result, we now emphasis elastic wavefield seismic stratigraphy in our reservoir characterization studies, which is a science based on the concept that the same weight must be given to S-wave sequences and facies as is given to P-wave sequences and facies. This philosophy differs from the standard practice of depending on only conventional P-wave seismic stratigraphy to characterize reservoir units.The fundamental physics of elastic wavefield seismic stratigraphy is that Swave modes sense different sequences and facies across some intervals than does a P-wave mode because S-wave displacement vectors are orthogonal to Pwave displacement vectors and thus react to a different rock fabric than do P waves. Although P and S images are different, both images can still be correct in terms of the rock fabric information they reveal.ii