Effective Medium Modeling of CO2-Sequestered Carbonate Reservoir

An appropriate rock physics model to monitor seismic properties of carbonate reservoir sequestered with CO2 imposes a grand challenge as carbonate rocks have complex microstructure and chemically active fluid-rock system. Existing rock physics theories based on Gassmann's theory are not suitable for modeling a dynamic system (system in which rock matrix changes with time due to rock-fluid interaction). On the other side, self-consistent approximation (SCA), differential effective medium (DEM), etc. theories based on Eshelby's inclusion model do not incorporate pressure, which is very important for gas saturated reservoirs. The DEM theory modified for pressure (PDEM), which can explain rocks' heterogeneous microstructure and squirt flow, has been applied successfully to match the laboratory-measured (ultrasonic) elastic properties of CO2-rich water-saturated carbonate rocks. The PDEM theory correlates well with the laboratory experiment that shows reduction in P- and S-wave velocities of saturated rocks as a result of permeability and porosity enhancement due to dissolution of carbonates by acidic saturating fluid. This theory can be up-scaled from laboratory to field measurements demonstrating the predicted elastic properties in the seismic frequency range decrease significantly. The joint effect of free CO2 gas saturation and chemical dissolution on velocities of rock show that both attenuation and dispersion decrease from complete water to gas saturation. This theory can be applied to identify variations in elastic properties of CO2-saturated carbonate reservoir and monitor the movement of CO2 gas. These important factors guarantee that CO2 storage will not destroy subsurface geology and sequestration is safe for the environment.