Mechanisms for microseismicity occurrence due to CO (sub 2) injection at Decatur, Illinois; a coupled multiphase flow and geomechanics perspective

A striking feature of the Decatur, Illinois, pilot CO (sub 2) injection site is the lack of temporal correlation between seismicity and injection rate. We investigate this issue by performing coupled flow and geomechanics modeling to assess the hydromechanical conditions that may have led to the occurrence of microseismicity during CO (sub 2) injection. We construct our model by developing a 3D structural model that includes the main stratigraphic intervals and a set of fault surfaces interpreted using surface seismic data and microseismicity locations. We calibrated our flow simulations using reservoir pressure measurements recorded during injection and we performed sensitivity analysis to investigate the impact of different hydraulic properties on the model's pressure response. Our results indicate that good agreement with measured pressures requires significant pressure diffusion into the basement section, as opposed to pressure compartmentalization in the injection interval. We constrained the range of initial stresses for the geomechanics simulations using field measurements of the minimum horizontal stress magnitude and orientation. We then quantified fault slip tendency prior to injection. Our results show that faults exhibiting intense microseismicity are critically stressed and favorably oriented to slip.Our coupled flow and geomechanics modeling results indicate that poroelastic stresses are small and, in the absence of pore pressure, tend to stabilize the basement faults. Our model also shows that pore pressure diffusion along basement faults is essential to explain fault destabilization in the distant areas where intense microseismicity is located. From a sensitivity analysis, we find that the severity of fault destabilization is very strongly dependent on the along-fault permeability and the degree of fault connectivity, a result that shows the importance of fault networks to transmit pore pressure to areas far away from where the injection takes place.