Effect of Fault Architecture and Permeability Evolution on Response to Fluid Injection

Abstract

Injection-induced seismicity is thought to be due primarily to increase in fluid pore pressure, which reduces the shear strength of a nearby fault. We address the modeling and prediction of the hydromechanical response due to fluid injection, mainly as wastewater disposal. We consider the full poroelastic effects, including the changes in porosity and permeability of the medium due to changes in local volumetric strains. Our results consider effects of the fault architecture (low-permeability fault core and anisotropic high-permeability damage zones) on the pressure diffusion and the fault poroelastic response. We show that the high-permeable damage zone, the poroelastic response, and the permeability evolution can accelerate the pore pressure diffusion process during and after wastewater injection. By studying a geologically based model of the Guy-Greenbrier fault and of the earthquake sequence induced along it in Arkansas, United States, from October 2010 to July 2011, we show that the existence of highly permeable damage zones facilitates the pressure diffusion and results in a sharp increase in pore pressure at levels much deeper than the injection wells, while the anisotropic permeability in the damage zone can act as a barrier to cross-fault fluid flow. Furthermore, by computing the change ΔCFS of Coulomb failure stress, our simulations show that ΔCFS increases starting from the top of the Guy-Greenbrier fault and then propagates toward greater depth and toward the southwest direction, which is consistent with the seismicity migration.