Strength recovery and sealing under hydrothermal conditions

While there is significant evidence for healing in natural faults, geothermal reservoirs, and lab experiments, the thermal, hydraulic, mechanical, and chemical interactions that influence healing are poorly understood. We present preliminary results of triaxial slide-hold-slide experiments to constrain rates and mechanisms of healing. Experiments were conducted on gouge composed of Westerly granite and on bare surfaces of Westerly granite and Eureka quartzite. Tests were run at 22, 100, and 200˚C. In some experiments, we also determined the in-plane fluid transmissivity. In bare surface experiments we observe that restrengthening depends on both time and temperature. At 200˚C the simulated fractures restrengthen at a rate of ∆µ/∆log(t_hold) = 0.009/decade while at 22˚C the healing rate is ~ 0.002/decade. In the gouge experiments restrengthening appears to be independent of temperature. This may be related to the heterogenous mineral composition and thickness of the gouge layer which could allow shearing to be accommodated in unhealed zones. In the experiments, an overall reduction in fluid transmissivity is observed but sliding periods are often associated with increases in the fluid transmissivity. The transmissivity reduction tends to be greater at 200˚C relative to room temperature. Our preliminary results suggest that multiple healing mechanisms are operating under hydrothermal conditions.