A STRESS-DEPENDENT ROCK PHYSICS MODEL FOR TIME-LAPSE SEISMIC MONITORING OF GEOLOGIC CARBON STORAGE

Accurate rock physics modeling is crucial for time-lapse seismic monitoring of CO₂ storage reservoirs. Generating accurate seismic modeling data requires realistic rock physics theory to build elastic models of the subsurface. The Biot-Gassmann equation has been the standard approach in generating these models. Previous experiments illustrated that rock units filled with free-phase CO₂ demonstrate large changes in seismic velocities (~15% decrease for P-wave velocity [V_p] and ~16% decrease for S-wave velocity [V_s]). Estimating effects of CO₂ saturation with the Biot-Gassmann equation cannot replicate these large changes in seismic velocities because this approach only considers the impact of supercritical CO2 on the fluid properties and does not consider the justifiably proven evidence that rocks under these conditions are stress dependent. In this work, we expand the Gassmann equation with more accurate rock physics by including the stress dependence of seismic velocities because of compliant porosity, Krief's relations on fluid-saturated rocks, and CO₂ saturation effects on the rock framework. We create time-lapse elastic velocity models with our new method using flow simulations of injected CO₂ migration in the Kimberlina 2 model for the previously proposed Kimberlina CO₂ storage site in California. We compare results of CO₂-induced changes of elastic parameters obtained using the Biot-Gassmann equation and our new model leveraging stress-dependent rock physics. We demonstrate that our innovative approach produces larger changes in V_p, V_s, and density than those obtained using the Biot-Gassmann equation. Furthermore, these larger changes are more consistent with laboratory and field observations. Our more accurate rock physics model improves accuracy of time-lapse elastic-wave modeling for seismic monitoring of geologic carbon storage sites. This approach will potentially improve the quantification of a CO₂ plume within a storage reservoir and secondary plumes in overlying permeable formations via seismic imaging, inversion, machine learning, etc., since simpler theories (e.g., Gassmann equation) significantly underestimate the influence of CO₂ saturation on seismic waves.