Investigating the Correlation Between Post-Injection Trailing Seismicity and Wellhead Pressure Decay in Enhanced Geothermal Systems

Injection of fluids during reservoir stimulation aims to enhance reservoir permeability but induces seismic activity that persists for several hours to several months after injection has ceased. Physical and hybrid models have been successfully applied to reproduce and forecast observed seismicity rates during and after injection. However, these models are typically site-dependent, raising the question of whether a general relationship between pressure and seismicity decay can be observed across different sites and operations.

In this study, we investigate the correlation between post-injection pore pressure decay and the decrease in seismicity rate using data from multiple EGS injection operations that share similar properties. First, the performance of several empirical statistical models is evaluated to describe the decrease in seismicity rate. Second, wellhead pressure decay is shown to be best described by a simple exponential model. Lastly, we introduce a time-to-fraction metric to compare the pressure and seismicity evolution after shut-in. We show that the times required to reach a given fraction of the initial rate for both pressure and seismicity are correlated, with pressure evolution being slower than seismicity rate evolution. No correlation is observed between seismicity decay and injection parameters such as injected volume, average injection pressure, or injection duration. These observations suggest that pore pressure has a limited influence on seismicity decay, which has strong implications for reservoir management.