The Illinois Basin – Decatur Project is important for its carbon capture and storage (CCS) effort in the Mount Simon Formation. Characterizing the rock mechanical properties and quantifying the fluid effect are important in monitoring CO2 migration and minimizing the risk of leakage. We are motivated to conduct a rock physics investigation using 16 rock samples from verification well #2, covering both the Eau Claire seal and the Mount Simon storage units. Given that seismic anisotropy is prevalent in many samples, we characterized the five pressure-dependent frame moduli and sample anisotropy using 1 MHz ultrasonic waves under a range of confining pressures (5 to approximately 70 MPa) under the assumption of a transversely isotropic medium. We found that seismic velocities increase and anisotropy decreases as the effective pressure increases. Hence, we expect that anisotropy would increase with increasing pore fluid pressure due to CO2 injection. The highest P- and S-wave anisotropy measured in the Eau Claire Formation at in-situ conditions is approximately 31% and approximately 29%, respectively. Among the anisotropy values measured in the Mount Simon sandstone samples at in-situ conditions, the highest P and S anisotropy values measured were approximately 8% and 6%, respectively. Using the anisotropic form of Gassmann's fluid substitution and our measured dry-frame moduli, we then predicted brine-saturated seismic wave velocities which matched well with the sonic logs of the nearby well CCS#1, demonstrating that future quantitative time-lapse monitoring of CO2 plumes is possible.

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