Confinement pressure effect and influence mechanism of water injection-induced slip of shale fracture

Abstract

Water injection-induced fault slip is a prevalent phenomenon in shale gas extraction activities. To investigate the effects of confinement pressure on the slip behavior and its underlying mechanism, this study conducted water injection slip tests on shale samples with prefabricated fractures under varying confining pressures. The test results demonstrated significant confinement pressure effects on the slip characteristics of shale fractures. As confining pressure increased, the fracture openness decreased, and the initial slip water pressure rose, resulting in increased accumulated energy and the occurrence of significant “stick-slip” phenomena, which generated active acoustic emission (AE) signals. Additionally, an increase in confining pressure was accompanied by an elevation in the overpressure ratio, indicating a reduction in fracture permeability and an enhancement in fluid non-homogeneity. Furthermore, as confining pressure rose, the micro-projections interlocking the fracture surfaces underwent continuous breakage during slip, generating abundant rock debris. This debris accumulation subsequently caused a decrease in both the fractal dimension and roughness of the fracture surface. The research findings provide valuable insights for predicting and controlling fault slips and potential seismic activities induced by water injection during shale gas extraction.