  Understanding the controlling mechanisms underlying injection-induced
seismicity is important for optimizing reservoir productivity and addressing
seismicity-related concerns related to hydraulic stimulation in Enhanced
Geothermal Systems. Hydraulic stimulation enhances permeability through
elevated pressures, which cause normal deformations, and the shear slip of
pre-existing fractures. Previous experiments indicate that fracture deformation
in the normal direction reverses as the pressure decreases, e.g., at the end of
stimulation. We hypothesize that this normal closure of fractures enhances
pressure propagation away from the injection region and significantly increases
the potential for post-injection seismicity. To test this hypothesis, hydraulic
stimulation is modeled by numerically coupling fracture deformation, pressure
diffusion and stress alterations for a synthetic geothermal reservoir in which
the flow and mechanics are strongly affected by a complex three-dimensional
fracture network. The role of the normal closure of fractures is verified by
comparing simulations conducted with and without the normal closure effect.
