Coupled Hydromechanical Modeling of CO2 Injection Induced Seismicity at the Illinois Basin Decatur Project

Injection of CO2 into deep sedimentary formations causes pressure increase that may potentially lead to reactivation of pre-existing critically stressed faults and trigger earthquakes. At the Illinois Basin Decatur Project (IBDP), 1 million metric tons of CO2 were injected during 3 years into a high permeability Mount Simon Sandstone interval at a daily rate of about 1,000 metric tons. Nearly 20,000 induced microseismic events were detected, and approximately 5,000 were locatable, mostly occurring below the reservoir at the top of the underlying crystalline basement. Identified clusters form semilinear features oriented within 30 degrees of the direction of the maximum horizontal stress which suggests that the microseismic events are occurring along pre-existing basement discontinuities. In this work, we used TOUGH-FLAC to model spatiotemporal fluid pressure and stress changes to study the poroelastic effect of CO2 injection on modeled faults in crystalline basement rock below the injection zone. The model is coupled to a statistical rate-and-state seismicity framework to simulate seismicity rate occurring along basement faults. Our model is able to qualitatively reproduce the main features of the microseismic activity observed at the IBDP site.