Evolution of fluid transmissivity and strength recovery of shear fractures under hydrothermal conditions

Abstract

Geothermal systems rely on the presence of long-lived and high-volume, permeable fracture systems. The creation, reactivation, and sustainability of these systems depend on complex coupling among thermal, hydraulic, mechanical, and chemical (THMC) processes occurring in geothermal reservoirs. In part due to a paucity of experimental data, the evolution of fractures at geothermal conditions in response to THMC processes is poorly understood, particularly during the process of shear. We present preliminary results of triaxial slide-hold-slide experiments, with hold periods ranging in duration from 103 s to 106 s, to constrain rates and mechanisms of healing and sealing. Experiments were conducted on simulated fault gouge composed of Westerly granite and on bare surfaces of Westerly granite. The tests were run at temperatures of 22˚ and 200˚C with confining and average pore pressures of 30 MPa and 10 MPa, respectively. We used an axial displacement rate of 0.1 μm/s during sliding periods. Deionized water was continuously flowed along the simulated fracture so we could determine in-plane transmissivity during the tests. In gouge and bare surface experiments conducted at 200˚C, we observe significant decreases in transmissivity over the course of the experiments. For the hydrothermal gouge experiment we measured an order of magnitude net reduction in transmissivity from 1.73x10-18 to 0.17x10-18 m3, over the course of 220 hours while in the room temperature gouge experiment transmissivity only decreased by 0.35x10-18 m3 over the same amount of time. In the experiments, we observe an up to 16% recovery in transmissivity during sliding periods. At room temperature the friction data showed limited fault re-strengthening with time; healing rates are on the order of 0.1 MPa/decade. A similar healing rate was observed at 200˚C in the gouge but we observe an increase in the healing rate, to 0.75 MPa/decade, for a bare surface experiment at 200˚C. The differences in the healing rate of the gouge and bare surface experiments suggest that the generation of fine particles by grinding down of asperities on the bare surface promote quartz dissolution and reprecipitation at elevated temperatures. Further work is needed to test this possibility and provide better constraints on factors influencing the evolution of fluid transport properties and strength of shear fractures at geothermal conditions.