An experimental investigation into fluid injection-induced fault slip and fracturing under true triaxial stress

Abstract

This study investigates fluid injection-induced fault slip under true triaxial stress, focusing on cases where injection boreholes are either hydraulically connected or disconnected from the fault. Through stress and displacement monitoring, acoustic emission analysis, spectral and source mechanism inversion, and CT scanning, three stages of fluid injection-induced fault slip were identified: injection-induced disturbance, hydraulic fracturing, and pore pressure buildup within the fault zone. The results reveal the dynamic evolution of fracture initiation, pressure accumulation, and fracture propagation leading to fault slip, and show that the fault slip state significantly influences pressure buildup. The observations confirmed a poroelastic coupling mechanism: as pore pressure within the fault zone rises, slip velocity initially increases with the injection rate and subsequently decreases. Source mechanism analysis indicates that fault slip is dominated by compressive-shear motion, whereas hydraulic fracturing exhibits tensile-shear characteristics. Rock mechanical strength, permeability, and stress field evolution analyses showed that fault reactivation precedes hydraulic fracturing of surrounding rock in hard rocks, whereas hydraulic fracturing occurs before fault slip in soft rock. In addition, for low-permeability faults, direct fluid injection is more effective in controlling slip than that for high-permeability faults.