Microphysical Modelling of Frictional Slip in Hydrothermal Conditions
Authors: Mei Cheng , Rudnicki John W. 
Published: September 2023

ABSTRACT
Injection of fluids for geothermal stimulation has often caused seismic events that have raised concern and resulted in discontinuing the operations. Consequently, a safe and effective geothermal operation requires an assessment of the potential for induced seismicity. Rate and state friction (RSF) is well-established as a robust description of rock friction. It is, however, strictly empirical and extension to hydrothermal conditions relevant to geothermal stimulation is problematic. We extend a microphysical model to hydrothermal conditions by incorporating thermally activated processes in fault gouges. We use a simple spring-slider model to simulate steady-state friction at a typical range of temperatures and load-point velocities. Numerical simulations show that, consistent with experimental observations, the simulated steady-state friction of granite gouges in wet conditions rises slightly with temperature, and the friction rate parameter (a − b) transitions from velocity-strengthening at low temperatures to velocity-weakening behaviors at high temperatures of about 300°C. This transition indicates the possibility of change from stable to unstable (seismic) slip near this temperature. These results also suggest that the dominant deformation mechanisms may evolve from frictional granular flow with intergranular pressure solution at low temperatures to grain boundary sliding (GBS) with solid-state diffusion creep at high temperatures.

INTRODUCTION
Induced seismicity in enhanced geothermal systems is generally detected in a depth range from a few hundreds of meters to several kilometers below the Earth surface where temperature and pore fluid pressure are elevated (Tomac and Sauter, 2018; Johnson et al., 2016; Atkinson, 2015; Eberhart-Phillips and Oppenheimer, 1984). The correlation between induced seismicity and geothermal production has been proposed based on many field observations, such as the Utah FORGE geothermal system (Moore et al., 2019), the Reykjanes and Svartsengi geothermal areas (Keiding et al., 2010), the Geysers geothermal field (Eberhart-Phillips and Oppenheimer, 1984), and the Geoven deep geothermal site (Schmittbuhl et al., 2021). The injection of cold fluids and extraction of hot fluids from the subsurface may cause the pressure imbalance and trigger the seismic slip in the seismogenic zone (Xing et al., 2020). Extensive studies suggest that the aseismic-seismic transition in the seismogenic zone is thermally controlled (Currie et al., 2002; Tichelaar and Ruff, 1993; Blanpied et al., 1995; Niemeijer et al., 2016; Den Hartog and Spiers, 2013; Toy et al., 2010). In addition, the seismic slip behaviors are probably linked with temperature dependence of effective frictional properties, rock composition, porosity, and plasticity in fault gouges (Tullis and Yund, 1980; Boettcher et al., 2007; Giger et al., 2007; Barbot, 2022; Scholz, 2019). The complex frictional behaviors are often captured by a standard RSF that uses a state variable to represent the structural evolution within the fault zone (Dieterich, 1979; Ruina, 1983).