Laboratory investigations on activation characteristics of fracture induced by fluid injection and unloading of normal stress

The activation of fractures and faults associated with stress perturbations is frequently encountered in various engineering applications. Understanding the hydraulic response mechanisms governing fractures and faults activation is vital to mitigate the risk of seismicity induced by deep resource extraction. To characterize the fracture activation behavior and explore the hydraulic response mechanisms, laboratory experiments on fracture activation induced by fluid injection and unloading normal stress were conducted. The results demonstrate that fracture activation induced by fluid pressurization and unloading normal stress present quasi-dynamic sliding and dynamic slip, respectively. Due to the severe damage of fracture asperities under fluid injection, the 3D roughness index shows a decreasing trend with increase of normal stress, suggesting that the governing deformation mechanism of fracture activation exhibits a transition from dilation to slip. The fracture frictional instability characteristics were further discussed through fluid pressurization rate variations. Under the condition of unloading normal stress, the fracture activation is mainly controlled by the slip mechanism, and the magnitude of fracture deformation is larger than that of fluid injection. Compared with the heterogeneous stress perturbation caused by fluid injection, unloading normal stress triggers faster slip rates and is accompanied by greater energy release, indicating that the unloading-induced fracture activation is more efficient and also has a higher potential to induce seismic hazards for natural faults.