Shear slip on the natural fractures is proposed as a viable stimulation mechanism in unconventional geothermal and petroleum reservoirs. Fracture mechanics studies show that the propagation of mechanically closed fractures often involves both mode I and II propagation. Wing cracks are triggered by shear deformation at the tip of the natural fractures. These wing cracks tend to reorient and extend in the direction of maximum compressive stress. In addition to mode I propagation (i.e., wing cracks), natural fractures may also propagate in mode II in a plane approximately parallel to the pre-existing natural fracture. A displacement discontinuity method with Mohr-Coulomb elements is used in this paper to study the response of natural fractures to water injection. Modeling results indicate that the onset of fracture slip occurs when the initial shear stress exceeds the shear strength of the Mohr-Coulomb contact elements. The results show that injection into a single natural fracture may lead to the coalescence of multiple natural fracture which can be regarded as an important advantage of this stimulation technique. However, it was found that network connectivity and fracture coalescence is less likely achievable through simultaneous injection into multiple natural fractures mainly due to the compressive stress shadow in the vicinity of the neighboring fractures caused by the shear slip.
EGS design concept in a number of field projects (Soultz, Desert Peak, Newberry) relied on the conceptual model of permeability increase by slip on natural fractures due to water injection. The conceptual model envisions injection pressures below the minimum in-situ stress to cause slip on the critically-stressed fractures and or induce shear failure of the rock mass. However, Jung (2013) presented a review of the results and observations from a number of EGS experiments and suggested that the hitherto adherence to stimulation by shear or “hydro-shearing” is the main reason for the poor progress in the EGS success. Based on interesting interpretations of a number of phenomena, Jung argued that tensile fracturing and not shear slip or propagation is the main mechanism of stimulation, recommending a return to the conventional stimulation concept. In this paper, we review the concept of wing-crack propagation and show this mechanism is in fact, an integral part of the shear slip stimulation mechanism and that a shear propagation mode is also plausible and can contribute to permeability and MEQ. The process is similar to the shear failure in laboratory triaxial compression tests on rock whereby tensile and shear cracks coalesce to form a macroscopic shear crack or fault across the sample. And although individual tensile cracks do form in the process, the failure is referred to as shear failure.