Fatigue Behavior of Granite Subjected to Cyclic Hydraulic Fracturing and Observations on Pressure for Fracture Growth

This study aims to assess the fatigue behavior of granite subjected to cyclic hydraulic fracturing using cylindrical granite samples. Two sets of continuous and cyclic injection were carried out and monitored with acoustic emission (AE) sensors. In these experiments, the granite fatigue life or number of cycles to failure increased exponentially with decreasing maximum pressure during cyclic injection. Moreover, although cyclic injection induced more AEs, they were of lower energy compared to those under continuous injection. Similarly, the proportion of small-to-large-amplitude AEs, measured through the Gutenberg–Richter b value, was higher in cyclic injection cases compared to continuous injection cases. This implies a lower probability of observing large-amplitude AEs for cyclic injection at lower pressures. The damage process during cyclic injection was quantified with the cumulative absolute AE energy and depicted a three-stage process: an initial increase due to sample saturation, a steady linear increase before failure, and a nonlinear rapid increase leading to failure. Moreover, the slope of the cumulative energy during the second stage showed a relation with the pre-selected maximum pressure, increasing with increasing maximum pressure, and it was associated with the rate of stable enlargement of pre-existing microcracks that consequently induced fractures in the third stage. Finally, a decreasing trend of the pressure required for fracture initiation was considered to be analogous of field reopening pressures.