Laboratory Fracture Slip and Seismicity Subjected to Fluid Injection-Related Stress and Pressure Paths

Causal mechanisms for fluid injection-induced seismicity are characterised by various stress paths to drive slippage of underlying fractures. Investigation into the role of stress paths on the fracture slip behaviour and seismic response is crucial for understanding causal mechanisms for induced seismicity. In this work, novel experimental stress and pressure paths were proposed to induce fracture slip in a fashion that decouples fracture normal and shear stresses, whilst maintaining the same rate to approach slippage (the same Coulomb stress change rate). Laboratory experiments were carried out on a shale sample to simulate slippage along a pre-existing fracture under three fluid injection-related stress and pressure paths, i.e. pore pressure elevation, fracture normal stress relaxation, and fracture shear stress increase. Seismic stable fracture slip characterised by frictional strengthening occurred in these experiments, and the time-varying friction of the fracture was reasonably described by the rate-and-state friction law. The pore pressure elevation and fracture normal stress relaxation paths at the same stressing rate caused similar fracture slip and seismicity behaviour, and can be considered to be equivalent. Results have shown a transition from quasi-static to dynamic fracture slip under both fracture effective normal stress relaxation paths, but not under the fracture shear stress increase path. This indicates that the fracture effective normal stress relaxation tends to cause accelerating slip, whilst the fracture shear stress increase initiates prolonged gradual slip. Accumulative seismic moment scales linearly with fracture slip duration, which implies that fracture shear stress increase causes larger seismic moment than fracture effective normal stress relaxation paths.