The rise in induced seismicity resulting from the injection of produced water into aquifers poses operational challenges and raises public concern. Understanding the stick/slip motion of faults may offer strategies for mitigating this issue. The aim of this study is to investigate the effects of various lithologies and changes in effective stress on the magnitude of stick/slip events and the slow creep of rock. Knowledge of rock lithology and effective pressure management could be critical in mitigating induced seismic events.
Experiments were conducted on rock samples with different lithologies using a modified triaxial double-direct shear apparatus to quantify the frictional sliding, creep, and fracture permeability under controlled stress and pore-pressure conditions. A predefined normal load and pore pressure were applied and stabilized. After stabilization, the axial load or shear stress was increased monotonically to measure the magnitude of the induced slip along the fault and changes in fracture permeability. Samples with varying silica, calcite, and clay content were tested under different loading conditions to examine how the magnitude of stick/slip events, rock creep, and fracture permeability change.
The results clearly demonstrate that the magnitude of stick/slip events and slow creep of the fault depend on both lithology and loading conditions. The magnitude and frequency of stick/slip events were higher in rocks containing minerals with greater Mohs hardness, such as quartz. In contrast, rocks with high calcite content exhibited significantly smaller and less frequent stick/slip events, with smooth sliding and creep being predominant. The stick/slip motion of the rock increased with the hardness of the fault, which raised the static friction coefficient. The fracture permeability was also related to the hardness of the rocks studied. The permeability reduction due to shear slip of fractures in softer rocks was found to be greater than in hard rocks. The effective normal stress acting on the fault also played a crucial role, with larger magnitude events recorded at higher effective stress levels.
The ratio of shear stress to normal stress, plastic creep of the rock, and the permeability change of the fracture were all measured simultaneously and correlated with the rock mineralogy and stress conditions. Since stick/slip frictional sliding is the primary mechanism driving induced seismicity, and these parameters directly govern the propensity for unstable frictional ruptures, this characterization provides a potential method to determine where induced seismicity events are more likely to occur.