Fluid‐Induced Fracturing of Initially Damaged Granite Triggered by Pore Pressure Buildup

Fluid flow-induced seismic activity has been observed in various environments; however, the relations among fluid migration, pore pressure buildup, and fracturing remain poorly understood. In the present study, we conducted fluid-induced fracture experiments on initially damaged granite. Acoustic events and macroscopic failure of the samples were triggered exclusively by a buildup of pore pressure, and fracturing occurred when the average effective stress reached a critical stress state (following the Mohr-Coulomb criterion). The spatiotemporal distribution of acoustic events is mainly controlled by the pore pressure diffusion front through the sample. Fluid migration velocity was dependent primarily on the initial stress state and permeability. Although the propagation of acoustic events was slower in our laboratory experiments than in natural seismic swarms, a similar parabolic relationship in space and time suggests that these microearthquakes are likely triggered by the passage of a critical pore fluid pressure through the damaged zones. Plain Language Summary Seismic activity induced by fluid injection has been observed in various environments; however, how fluid-induced rupture relate to the fluid migration and pore fluid pressure is not fully understood yet. In the present study, we conducted fluid-induced fracture experiments on initially damaged granite. Acoustic events and macroscopic failure were triggered exclusively by a buildup of pore pressure in the samples, and fracture follows the Mohr-Coulomb criterion when the average effective stress in the sample is equal to a critical stress state. Acoustic events were observed to migrate from the bottom to the top of the sample, reflecting the passage of a critical pore fluid pressure increase through the specimen, where fluid migration velocity are primarily dependent on the initial stress state of rocks. These laboratory data agree with behaviors of induced microseismicity in natural systems, suggesting that ruptures are mainly triggered by the fluid migration and buildup of pore pressure in the fracture zones.