ABSTRACT
Understanding fracture slip triggering during fluid injection is critical to optimize the strategies of hydraulic fracturing in geothermal energy extraction. We report a suite of triaxial shear-flow experiments on a sawcut fracture, a gouge-filled fracture, and a natural fracture under different combinations of effective normal stress and fluid injection rate. Our results show that the pore pressure measured at fracture slip exceeds that predicted by the Mohr-Coulomb failure criterion in the three fractures. The inapplicability of the failure criterion is due to the nonuniform distribution of pore pressure over the fracture planes. The results also show that the injection overpressure ratio increases with higher effective normal stress and greater fluid injection rate. Low permeable fracture and high injection rate can facilitate fluid overpressure build-up in rock fractures and increase the risk of fracture instability.
