Seismicity on Basement Faults Induced by Simultaneous Fluid Injection–Extraction

Large-scale carbon dioxide (CO 2 ) injection into geological formations increases pore pressure, potentially inducing seismicity on critically stressed faults by reducing the effective normal stress. In addition, poroelastic expansion of the reservoir alters stresses, both within and around the formation, which may trigger earthquakes without direct pore-pressure diffusion. One possible solution to mitigate injection-induced earthquakes is to simultaneously extract pre-existing pore fluids from the target reservoir. To examine the feasibility of the injection–extraction strategy, we compute the spatiotemporal change in Coulomb stress on basement normal faults, including: (1) the change in poroelastic stresses Δ τ s + f Δ σ n , where Δ τ s and Δ σ n are changes in shear and normal stress. respectively, and (2) the change in pore-pressure f Δ p . Using the model of (J. Geophys. Res. Solid Earth 99 (B2):2601–2618, 1994 ), we estimate the seismicity rate on basement fault zones. Fluid extraction reduces direct pore-pressure diffusion into conductive faults, generally reducing the risk of induced seismicity. Limited diffusion into/from sealing faults results in negligible pore pressure changes within them. However, fluid extraction can cause enhanced seismicity rates on deep normal faults near the injector as well as shallow normal faults near the producer by poroelastic stressing. Changes in seismicity rate driven by poroelastic response to fluid injection–extraction depends on fault geometry, well operations, and the background stressing rate.