Investigation on Injection-Induced Reactivation by Hydraulic Fracturing for Gypsum-Filled Flaw

We explore the reactivation of a laboratory fault approached by a fluid driven hydraulic fracture as representative of the stimulation of fractured reservoirs. In particular we seek to establish a correlation between the underlying injection-induced reactivation and the fracture pattern produced in hydraulic fracturing. A blind borehole is hydraulically fractured adjacent to a gypsum-filled fault at variable confining pressures to examine controls on fracture propagation in the proximity of a fault and the resulting style of fault reactivation. Experiments are run at constant axial stress (CAS) and constant axial displacement (CAD) modes to represent end members in tectonic loading. Modes of failure are deduced from focal mechanisms and reactivation displacement histories. Experimental results reveal that gypsum-filled flaws can arrest the propagating hydraulic fracturing, with fault states evolving from the a inactivated state at 2 MPa to a partially activated state observed at 10–20 MPa and a fully activated state at 30 MPa as increasing confining pressure. Moreover, the reactivation behavior of a partially activated flaw was particularly pronounced in CAS mode than in CAD mode. The non-uniform distribution of fluid pressure in a partially activated flaw leads to a difference in effective stress along the filled flaw, contributing to a driving force to fault reactivation. The deformation behavior of a fully activated flaw was significant, but the experimental system automatically compensates for stress reduction caused by fluid injection in CAD mode, leading to a less magnitude reactivation behavior. These findings provide insight into fracture–fault interactions and may improve strategies for safe hydraulic stimulation in geothermal and unconventional reservoirs.