The Effect of Pressurization Rate and Pattern on Injection-Induced Seismicity in Highly Permeable Sandstone: An Experimental Study

Abstract

Effectively mitigating induced seismicity in subsurface engineering operations within highly permeable, porous geo-energy reservoirs requires a clear understanding of how fluid injection parameters influence the seismic response. In this study, we performed injection-driven fault reactivation experiments on highly permeable saw-cut Red Felser sandstone to provide new insight into the effect of injection pattern and rate on fault slip behavior and seismicity evolution. Three different pressurization rates were applied: high, medium, and low rates of 2, 1, and 0.2 MPa/min, respectively. Three injection patterns were also used: cyclic recursive, monotonic, and stepwise injections. Our results reveal that a high pressurization rate leads to increased slip velocity, more microseismic events, higher total acoustic emission (AE) energy, and a lower b-value compared to tests with low pressurization rates. We postulate that a high pressurization rate enhances the likelihood of a sudden reduction in effective normal stress, leading to fault opening and the disruption of asperity contacts. Furthermore, results from samples subjected to various injection patterns demonstrate that the cyclic recursive pattern exhibits a higher maximum slip velocity, more episodes of slow slip, and greater radiated AE energy than a monotonic pattern. In the case of the cyclic recursive pattern, increasing the number of cycles increases shear stress drop, shear slip, and maximum slip velocity. Our findings suggest that using a monotonic injection pattern and low pressurization rate may mitigate seismicity on pre-existing faults in a highly permeable, porous reservoir.