Impact of Fluid Type (CO2 vs Water) on Fault Rupture Induced by Underground Fluid Injection

Fluid injection can induce fault reactivation during subsurface engineering activities such as CO2 sequestration and wastewater disposal. This study examines how fluid type - CO2 versus water - affects induced rupture and seismicity in faults with curved geometries. Using the coupled multiphase fluid flow and geomechanical simulator TOUGH-FLAC, we analyze the temporal and spatial evolution of rupture in complex faults under different injection scenarios. In the simulations, the fault zone is modeled as volume elements in TOUGH and as interface grid points in FLAC3D. We compare two cases, injecting supercritical CO2 and water at the same mass rate. The injection well is located in a deep reservoir, 450 meters from a naturally curved fault dipping at 60 degrees. The results show that fault rupture occurs in both cases. However, within the first five years, rupture nucleation is delayed for water injection compared to CO2 injection due to a more rapid and significant pressure increase in the CO2 case. After five years, fault rupture is larger and lasts longer with water injection, driven by greater pressure changes resulting from water's higher viscosity compared to CO2.