Experimental investigation of fault reactivation mechanism and related seismic responses induced by hydraulic fracturing in mining areas

Fault reactivation in mining areas can cause significant hazards, including rock bursts and mining-induced seismic events. Water injection and fracturing shows a great potential for fault reactivation regulation and seismicity mitigation. However, the mechanical driving processes of fault reactivation induced by water injection and fracturing and the associated seismic response patterns remain unclear. An investigation is conducted into fault slip induced by hydraulic fracturing under true triaxial stress conditions. Two field scenarios are considered: The water injection borehole either connects or disconnects with the fault plane. Initially, two types of 45°-inclined fault specimens were prepared: connected and disconnected specimens, both measuring 150 mm × 150 mm × 150 mm. A self-developed true triaxial stress loading system was used to simulate critically stressed fault, with stress conditions of 10, 12, 20 MPa for the connected specimens, and 5, 6, 10 MPa for the disconnected ones. During the experimental process, parameters including fault slip displacement, water injection rate, water injection pressure, and acoustic emissions (AE) were continuously monitored and recorded. Additionally, the three-dimension CT scan was employed to observe internal cracks development for the post-fracturing disconnected specimens. By analyzing the spatiotemporal distribution of AE events, focal mechanisms, CT scan results, and mechanical processes, the pore pressure-dominated and poroelastic coupling-dominated mechanisms of fault reactivation/slip induced by hydraulic fracturing are confirmed, along with associated seismic response patterns. Results show that poroelastic coupling-dominated fault slips can be induced during the whole hydraulic fracturing process. In the initial stage, fractures initiates at the bottom of borehole due to stress concentration, which then propagate along with the initial fractures and develop in the direction perpendicular to the tangential stress, until intersect with fault plane and resultantly induce pore pressure-dominated fault slip. The main focal mechanism of fault slip is compressive-shear, whereas that of hydraulic fracturing damage is tensile-shear. Their corresponding AE events are both mainly distributed near the fault plane and the bottom of borehole. The fault friction coefficient promptly responds to changes in water injection rates, so as to maintain the stable fault slip. Such response shows an increase in the first and then decrease with the increase of water injection rates, which indicates a feasibility of regulating fault slip rates via injection rate control. Moreover, direct water injection into faults proves to be more effective for fault slip regulation in low-permeability formations, whereas hydraulic fracturing works better in high-permeability formations. These findings could provide important insights for seismic signal identification induced by hydraulic fracturing near faults and for induced seismicity regulation.