Injection induced seismicity on basement faults including poroelastic stressing

Abstract:

Large magnitude induced earthquakes associated with fluid injection occur on basement faults. The faults communicate both hydraulically and mechanically with the target reservoir. In this two-dimensional plane strain numerical study, we examine the full poroelastic response of basement faults to fluid injection, considering two factors: (1) permeability of the fault zone, and (2) hydraulic connectivity of the faults to the target formation. Given the formation properties, we obtain the spatiotemporal distribution of the Coulomb stress change Δτ(x,t). We separate Δτ into two parts: (1) the poroelastic stress change Δτ_s+fΔσ_n, where Δτ_s and Δσ_n are changes in shear and normal stress (+Δτ_s favors slip in the direction of the fault and +Δσ_n weakens the fault by extension), and (2) the pore pressure change fΔp. Our result shows that direct diffusion of pore pressure into high permeability faults plays the dominant role in determining their mechanical stability. However, for hydraulically isolated faults, poroelastic stresses are transferred to deeper basement rocks and can trigger earthquakes, even without direct impact of elevated pore pressure. We predict the seismicity rate on basement faults using the seismcity rate model of Dieterich [1994]. High seismicity rate is observed in the hydraulically connected faults due to direct pore-pressure diffusion. The seismicity rate is lower in the isolated faults, caused by poroelastic stressing on the deeper fault zone.