Pressure rate effect on the fluid injection-induced earthquake magnitude and frequency

Fluid injection-induced earthquakes present a significant challenge for geo-energy applications, such as geothermal systems and CO2 storage. Understanding the earthquake magnitude and frequency in response to fluid injection is of vital importance. Both the pressure and pressure rate are considered dominant parameters for the occurrence of induced earthquakes. Theoretical analyses of a spring-block model [1] have demonstrated that the reservoir response depends on the nondimensional pressure rate, defined as the ratio of the characteristic time of frictional slip to that of pressurisation. The pressure rate effect is most pronounced when this ratio falls within a narrow range of 10-4 to 10-3. These results have contributed to the interpreting laboratory experimental observations, however, the correlation between the injection rate and induced earthquakes at the field scale remains poorly understood.
This work develops a coupled hydro-mechanical model to simulate constant-rate fluid injection into a reservoir adjacent to a sealing, steeply-dipping, rate-and-state frictional fault, aiming to evaluate fault activation behaviour and the associated induced earthquake magnitude and frequency. The fault is modelled as a rate-and-state frictional contact with Mohr-Coulomb fault strength, and deemed to be reactivated when the shear traction on the fault exceeds the fault strength (frictional resistance), which depends on the fault sliding velocity. Rate-and-state fault slip dynamics are resolved using frictional contact modelling through a fully implicit, monolithically coupled finite element formulation. The fault is characterised by velocity-weakening frictional properties, allowing it to slip multiple times during fluid injection. A seismicity rate model is used to simulate the induced seismicity rate along the fault during fluid injection. Given that the nondimensional pressure rate depends on both the critical slip distance (related to the characteristic time of frictional slip) and the injection rate (related to the characteristic time of pressurisation), different combinations of the two parameters within the typical range of field values are examined to investigate the pressure rate effect on induced earthquake magnitude and frequency and seismicity rate.
Results have shown that the critical slip distance affects both the magnitude and frequency of induced earthquakes, whilst the injection rate primarily controls the frequency of induced earthquakes in typical field conditions. Notably, the induced earthquake frequency, along with the induced seismicity rate, shows a positive correlation with the pressure rate. However, the maximum induced earthquake magnitude does not appear to be significantly affected by the pressure rate within the typical range of field conditions. Based on the model results, the range of nondimensional pressure rate, within which the pressure rate effect is significant in field reservoir conditions, is identified. Outcomes of this work may provide valuable guidance for regulating injection rates to mitigate fluid injection-induced earthquakes in geo-energy applications.

Reference
[1] Rudnicki, J.W. and Zhan, Y., 2020. Effect of pressure rate on rate and state frictional slip. Geophysical Research Letters, 47(21), p.e2020GL089426.