The Effect of Pressurization Rate and Pattern on Injection‐ Induced Seismicity in Highly Permeable Sandstone

Abstract

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.

Plain Language Summary
Human activities involving subsurface fluid injection projects, such as geothermal energy recovery and/or gas storage (CO2, H2 or methane), are widely acknowledged to cause earthquakes occasionally. This is a cause for public concern. Although several studies demonstrate that injection patterns and rates can play an essential role, the underlying physical mechanisms responsible for induced earthquakes still need to be better understood. Therefore, we performed laboratory tests on highly permeable Red Felser sandstone containing a simulated geological fault. We pumped water from the bottom of the sample using different pressurization rates and patterns while monitoring the effects on fault movement behavior. Our results showed that faster fluid injections tend to cause more rapid fault slips and generate more laboratory micro-earthquakes compared to slow injections. Among the injection patterns, the cyclic injection pattern resulted in the highest slip velocity and higher earthquake activity, indicating that the pattern of injection can impact fault movement. Our results can help improve the design of fluid injection projects to minimize the risk of inducing small earthquakes, especially in areas with pre-existing geological faults.