Impact of injection rate on smooth and rough fracture activation in granite: Laboratory-scale acoustic emission analysis

Abstract

This study aims to understand the effect of injection rate on injection-induced fracture activation in granite. We performed water injection-induced slip tests on samples containing either a smooth or a rough fracture at four different injection rates under undrained conditions and monitored the acoustic emission (AE) signals during the tests. Experimental results reveal that the critical activation fluid pressure is related to the injection rate, pressure diffusion rate, stress state, and fracture roughness. For the smooth fracture, as the injection rate increases, the critical activation fluid pressure increases significantly, while the injection rate has little effect on the critical activation fluid pressure of the rough fracture. The quasi-static slip distance of fractures decreases as the injection rate increases, with rough fractures exhibiting a greater overall slip distance compared to smooth fractures. The number of AE events per unit sliding distance increases with the injection rate, while the global b value decreases. These results indicate that higher injection rates produce more large-magnitude AE events and more severe slip instability and asperity damage. We established a linkage between fluid injection volume, injection rate, and AE events using the seismogenic index (Σ). The smooth fracture exhibits a steadily increasing Σ with the elapse of injection time, and the rate of increase is higher at higher injection rates; while the rough fracture is featured by a fluctuating Σ, signifying the intermittent occurrence of large-magnitude AE events associated with the damage of larger fracture asperities. Our results highlight the importance of fracture surface heterogeneity on injection-induced fracture activation and slip.