Slip behavior of saw-cut shale fractures under fluid injection and periodical confining pressure

Abstract

Fluid-injection induced seismicity during geothermal energy and shale gas extraction has been proved, yet the mechanisms involved are still unclear. To explore such mechanisms, we conducted a series of triaxial shear-flow experiments with periodical confining pressure and different injection pressure on saw-cut shale fractures. Experimental results showed that both variable confining pressure and different injection pressure had significant influence on the slip behavior. The apparent friction coefficient was periodically changed with the variation of confining pressure. With high injection pressure, the apparent friction coefficient decreased compared with the case without fluid-injection, while the apparent friction coefficient increased with low injection pressure. The events of fracture stick-slip appeared more frequently and the corresponding stress drop was lower, which means lower seismic energy release, with the increase of fluid injection pressure and the periodical confining pressure. The results shed some lights on the theory of fluid-injection induced seismicity, and inspire people to develop the new injection approach to reduce the maximum seismic event.