The classical concept of hydraulic fracturing is that a single, planar, opening mode fracture propagates through the formation. In recent years, there has been a growing consensus that natural fractures play an important role during stimulation in many settings. There is not universal agreement on the mechanisms by which natural fractures affect stimulation, and these mechanisms may vary depending on formation properties. One potentially important mechanism is shear stimulation, where increased fluid pressure induces slip and permeability enhancement on preexisting fractures. We propose a tendency for shear stimulation (TSS) test as a direct, relatively unambiguous method for determining the degree to which shear stimulation contributes to stimulation in a formation. In a TSS test, fluid is injected at a bottomhole pressure that is maintained below the minimum principal stress. Under these conditions, shear stimulation is the only possible mechanism for permeability enhancement (except perhaps thermally induced tensile fracturing). Standard pressure transient tests could be performed before and after the TSS test to estimate formation permeability. The flow rate rate transient during injection may also be interpreted to identify shear stimulation. Numerical simulations of shear stimulation were performed with a discrete fracture network (DFN) simulator that couples fluid flow with the stresses induced by fracture deformation. These simulations were used to qualitatively investigate how shear stimulation and fracture connectivity affect the results of a TSS test. Two specific field projects are discussed as examples of a TSS test, the Enhanced Geothermal Systems (EGS) projects at Desert Peak, Utah and Soultz-sous-Forêts, France.
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