ABSTRACT:

The majority of notable induced earthquakes happen on faults within the basement overlain by sedimentary layers. Pore pressure elevation and poroelastic effects have been considered as the driving mechanisms. In this article, a 3D numerical simulation is utilized to investigate the poroelastic response of faults extending up from the basement through overlaying sedimentary layer to fluid injection into an overlying reservoir. We study the direct role of pore pressure diffusion and the indirect poroelastic stress transfer effect on the failure of a favorably oriented strike-slip fault. The strike-slip stress regime is a common geological setting in many earthquakes associated with wastewater injection. We study the effect of mechanical properties of the reservoir and the fault zone on the response of the fault to the injection. We find that the fault failure tendency is higher in the shallow reservoir and basement in the case that the reservoir has a lower poroelastic stress coefficient (η = 0.25). Injection in the reservoir with a higher poroelastic stress coefficient (η = 0.39) results in a more fault reactivation tendency in the shallow basement. When α = 1, i.e., incompressible solid constituents, the CFS has a higher raise in the shallow reservoir than the basement. When considering a fault with an impermeable core but with a high-permeable damage zone, the injection fluid penetrates the deep basement and causes pore pressure elevation. Although this pore pressure elevation increases the total stress, the CFS increases in response to a decrease in Terzaghi effective stress.

Fluid injection into subsurface has become a common practice for many energy-related industrial projects such as disposal of coproduced oil/gas wastewater, reinjection of "flow-back" water from hydraulic fracturing operations, enhanced oil recovery (EOR) operations, disposal of nuclear waste, etc. In the 2010s, the central and eastern United States experienced raised seismic activities most of which occurred close to active industrial areas. Injection into the subsurface is a common industrial operation in these areas and strengthened the belief that these seismicity activities are attributed to the fluid injection. One might expect that these events occur in the target layer, however, most of these earthquakes occurred in the basement at depths of up to 11 km below the injection target layer (Horton, 2012; Kim, 2013; Keranen, 2013; Yeck et al., 2016; Yeck et al, 2017). Pore pressure and poroelastic stresses are two driving mechanisms that have been called upon to explain this phenomenon.

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