Opening Mode Stimulation or Shear Stimulation?

Abstract

Classically, hydraulic fracturing occurs through the propagation of new, opening mode fractures through intact rock. However, in many geothermal projects stimulation occurs primarily through induced slip of preexisting fractures and/or induced opening of preexisting fractures. Understanding, diagnosing, and predicting the mechanism of permeability enhancement is critically important because it affects every aspect of stimulation modeling and design. In this paper, we focus on induced slip of preexisting fractures. Theoretically, we discuss how, under certain conditions, induced slip could occur at a lower fluid pressure than needed to open preexisting fractures or propagate new tensile fractures. If induced slip causes a sufficient increase in permeability, the injection pressure could be prevented from increasing enough to cause opening of new or preexisting fractures. By postulating that the fluid pressure remains roughly constant at the edge of stimulated region, we calculated a threshold condition for the permeability-thickness that must be reached in order to prevent injection pressure from triggering opening mode deformation. We verified the threshold condition using a numerical model. We also discuss how the mechanism of stimulation propagation could have an important effect on the overall hydraulic behavior of the system.