Characterizing Pressure Transients in Shear-Stimulated Fracture Networks

Abstract

The classical theory of hydraulic fracturing is that injection creates a single, planar fracture that propagates through the formation. However, a variety of observations indicate that injection can create a network of fractures, especially in Enhanced Geothermal Systems (EGS) and in shale. Since 1980s, it has been recognized that an important mechanism of stimulation in EGS is shear stimulation, the stimulation of natural fractures from induced slip on preexisting fractures. The tendency for shear stimulation (TSS) test has been proposed as a way of measuring the ability of a formation to experience shear stimulation. In a TSS test, fluid is injected into a well while maintaining the bottomhole fluid pressure modestly lower than the minimum principal stress. Under these conditions, injection pressure is high enough to cause shear stimulation but low enough to avoid propagating hydraulic fractures through the formation. This test isolates the effect of shear stimulation because it should be the only possible mechanism for increasing permeability (unless thermal fracturing occurs). In this study, a discrete fracture network simulator, CFRAC, is used to study pressure transients in shear-stimulated fracture networks. CFRAC takes into account both fluid flow and the stresses induced by fracture deformation. We investigate pressure transient techniques specifically designed to interrogate properties of shear-stimulated fracture networks in unconventional resources. Analytical techniques developed to characterize pressure transients and properties of shear-stimulated fracture networks are compared against the synthetic data sets generated by CFRAC.