Effects of temperature and CO2 on the frictional behavior of simulated anhydrite fault rock

Abstract

The frictional behavior of anhydrite‐bearing faults is of interest to a) the safety and effectiveness of CO2 storage in anhydrite‐capped reservoirs, b) seismicity induced by hydrocarbon production, and c) natural seismicity nucleated in evaporite formations. We performed direct shear experiments on simulated anhydrite fault gouges, at a range of temperatures (80‐150 °C) and sliding velocities (0.2‐10μms−1), under a fixed effective normal stress of 25 MPa. Four types of experiments were conducted: 1) dry experiments, 2) experiments pressurized with water, 3) dry experiments pressurized with CO2, and 4) wet experiments pressurized with CO2. Fluid pressures of 15 MPa were used when applied. Over the temperature range investigated water‐saturated samples were found to be up to 15% frictionally weaker than dry equivalents. Wet samples containing CO2 were also up to 15% weaker than CO2‐free equivalents. Dry sample strength without CO2 was independent of temperature, whereas wet samples without CO2 strengthened 10% from 80 to 150 °C. Samples containing CO2 weakened by 4% (dry) and 10% (wet) from 80 to 150 °C. Under the P‐T conditions investigated, only dry anhydrite fault gouge showed velocity‐weakening behavior above 120 °C, required for faults to potentially generate earthquakes. Assuming natural fault gouges are wet in‐situ, seismicity is unlikely to nucleate in anhydrite‐rich faults, though the presence of dolomite or reaction‐produced calcite may change seismic potential. CO2 penetration into wet anhydrite‐rich faults may trigger slip on critically stressed faults due to the observed short‐term CO2 weakening effects (excluding possible formation of secondary minerals), but is not expected to influence slip stability.