Physical modeling of CO2-bearing rocks for reliable monitoring of geological carbon storage

Monitoring, verification and accounting (MVA) are key to ensuring safe and long-term geological carbon storage. Seismic monitoring is a major MVA technique that uses seismic data to obtain the elastic properties of CO2-saturated rocks. Reliable accounting of CO2 in subsurface reservoirs and potential escape zones requires an accurate model of rock physical properties. However, the commonly used model, based on the conventional Biot-Gassmann equation, may substantially underestimate the effect of CO2 saturation on seismic waves, leading to inaccurate accounting. We are developing an accurate model of the physical properties of rocks with CO2 that takes into account the effects of the stress dependence of seismic velocities in porous rocks and the weakening of CO2 on the rock framework. We validate our model using Kimberlina-1.2 (a previously proposed geological carbon repository in California) and build time-lapse models of elastic properties using this new method. We compare the results with the results obtained using the conventional Biot-Gassmann equation. Our innovative approach shows larger changes in elastic properties than the Biot-Gassmann results. Using our model, the reduction in shear wave velocity observed in the laboratory can be replicated. This new model increases the accuracy of time-lapse modeling of elastic waves and enables reliable accounting of CO2 using seismic monitoring. (More on nature.com)