Bleed-off control on post-injection seismicity in enhanced geothermal systems

Deep geothermal reservoirs could provide widespread access to clean and renewable energy around the world. However, hydraulic stimulation of these reservoirs to create sufficient injectivity and heat extraction has frequently induced earthquakes during and, in particular, after reservoir stimulation, which raises public concerns. This study aims to provide a possible explanation for post-injection seismicity and understand how it responds to well bleed-off as a common industrial practice to control such seismic activity. To this end, we perform coupled hydromechanical simulations of reservoir stimulation in a conceptual model comprising a deep granitic reservoir intersected by a network of long fractures and a nearby, critically-stressed fault. We find a combination of mechanisms triggering post-injection seismicity with time delays of several months after stopping injection: (1) poroelastic stressing that transmits normal and shear stress and causes undrained pressure buildup on the fault, (2) fracture-dominated pore pressure migration toward the fault, and (3) long-lasting along-the-fault pressure diffusion toward pre-stressed fault patches, promoted by dilation-induced fault permeability changes. In this setting, bleed-off causes rapid pressure decline in the near-wellbore region but marginal pressure changes farther away. The resulting attenuations of pore pressure and shear stress on the fault plane may not be enough to prevent fault reactivation. Bleed-off may counterintuitively accelerate fault slip by rapid relaxation of normal stress on the fault, which not only brings the stress state closer to failure conditions, but also accelerates pore pressure diffusion along the fault by slightly increasing its permeability. We show that bleed-off can effectively control post-injection seismicity only if rupture initiates from a structure in close proximity and with sufficient hydraulic connection to the wellbore. Future research should be directed toward the optimization of stimulation and post-stimulation design in light of the involved triggering mechanisms and through effective combination with subsurface characterization to control post-injection seismicity.