Acoustic signatures of fault reactivation and healing under hydrothermal conditions

Faults and fractures heal and seal over time, decreasing along-fault permeability, and increasing reactivation stress. This presents a dilemma in geothermal reservoirs as maintaining permeability is crucial for reservoir longevity, but the reactivation of faults to increase permeability can also cause hazardous seismicity. The healing rate of faults is temperature-dependent and shows significant differences under wet and dry conditions. We investigate the healing behaviour of a bare, water-saturated fault surface at temperatures up to 163 °C through slide-hold-slide experiments. The gneiss sample from the UtahFORGE geothermal demonstration project is continuously actively probed with P-waves and monitored for passive acoustic emissions radiating from the fault. Our data show that with increased temperatures, the fault surface friction decreases, healing rate increases, and the fault becomes more prone to unstable slip. The decrease in friction and increase in healing rate we measure are larger and occur at lower temperatures than previously demonstrated. P-wave amplitudes and P-wave velocities increase during healing, with amplitudes sensitive to temperature but velocities conversely insensitive. We attribute this to a sensitivity of the P-wave amplitude to changes in contact area with P-wave velocity correlating with mechanical compaction, off-fault microcracks, and the formation of wear products during sliding. The sample continues to creep throughout holds during our hotter experiments, but the creep motion does not erase continuous healing. Acoustic emissions spike upon slip reactivation, where higher event rates and higher slip velocities occur as healing progresses—after longer hold times and at higher temperatures. The amplitude of the P-wave, as well as the acoustic emission rate, show precursory signs of spontaneous reactivation and therefore might have potential in forewarning slip.