Numerical investigation of the use of hydraulic stimulation to mitigate fault slip risk in deep mines

Abstract

Fault slip and associated seismicity present a significant risk to the safety of underground mines. This paper investigates whether hydraulic stimulation, defined as the injection of pressurised fluid into a rock mass in advance of mining, has the potential to mitigate such risks. Fluid injected in the proximity of a critically-stressed pre-existing fault plane decreases the effective normal stress acting on the fault, initiating slip. Based on this concept, hydraulic stimulation could be used to trigger the release of built-up strain energy in advance of mining. Numerical investigations are carried out in a two-dimensional distinct element code to model the effect of injection on the slip of a pre-existing fault located adjacent to a conceptualised open stope mining operation. Injection is accompanied by slip and a significant decrease in shear stress acting on the fault. When no injection is performed, excavation triggers a series of slip events in response to the changing abutment stresses, generating a maximum Mw 1.3 event. Conversely, when hydraulic injection is performed prior to the mine excavation sequencing, the total number of slip events is reduced, and a lower maximum magnitude of Mw 1.1 is observed. Longer injection durations are observed to trigger larger areas of slip and greater total shear displacements. However, a consequence of injection activities is that stress becomes concentrated outside of the fluid propagation front, potentially triggering seismicity in areas where it was not originally anticipated.