Temperature-Dependence of Frictional Properties for Hydrothermally-Altered Granitic Rocks: Implications for Geothermal Applications

Granitic rocks in geothermal reservoirs often contain clay minerals such as kaolinite and illite as the products of hydrothermal alteration. The clay minerals affect permeability as well as the frictional properties of the rocks, thereby significantly influencing the performance of geothermal reservoirs and the potential for induced seismicity during their operation. When investigating rock properties important for geothermal applications, it is critical to consider their temperature-dependence. Here we show results from friction experiments on kaolinite-rich rock powders, a material found in the hydrothermally-altered granites of the geothermal province in Cornwall, UK. The experiments were performed at temperatures up to 180 C to establish the temperature-dependence of friction and frictional-stability parameters. Steady-state frictional strength of vacuum-dried kaolinite-rich powders sheared at a normal stress of 60 MPa decreases with increasing temperature (from ss~0.4 at T70 C to ss~0.3 at T120 C), while the contrary is observed for fluid-saturated kaolinite-rich powders (where ss~0.23 at T70 C and ss~0.25 at T120 C), although some slip-strengthening and -weakening at 120 C and 180 C, respectively, hinders the exact determination of the frictional strength at these higher temperatures. Therefore, the potential for hydro-shearing of favourably-oriented, fluid-saturated fractures in the hydrothermally altered rock mass might decrease with increasing temperature as the frictional strength of kaolinite-rich material lining the fractures increases. The frictional-stability behaviour of our experimental powders changes from velocity-strengthening, i.e., stable sliding, at T70 C to velocity-weakening, i.e., potentially unstable sliding, at T~100 C. In fluid-saturated powders, velocity-strengthening behaviour is recovered at T=180 C. Thus, the potential for induced seismicity is likely highest at a critical temperature around ~100 C. Our results are crucial as input parameters for models of the geothermal-system response to high-pressure fluid injection into the reservoir and thus for the design of pumping protocols.