The shear behavior of sandstone joints under different fluid and temperature conditions

Understanding the shear behavior of a sandstone joint under the influence of different fluids and temperature conditions is essential for evaluating the risks of wastewater injection and CO2 geological storage. To reveal the shear behavior, a series of direct shear tests were conducted on the sandstone joints with a similar joint roughness coefficient at temperatures range from 30 to 120 degrees C and under dry, H2O-injected, or CO2-injected conditions. Experimental results indicated that the temperature range from 30 to 120 degrees C does not significantly affect the shear strength and the friction coefficient of the sandstone joint, but it markedly decreases the shear stiffness and increases the peak shear displacement. With the increase in temperature, the shear strength is reduced by a maximum of 7.96% under dry conditions (at 90 degrees C), 3.17% under H2O-injected conditions (at 90 degrees C), and 2.41% under CO2-injected conditions (at 90 degrees C); the shear stiffness is reduced by a maximum of 65.8% under dry conditions (at 120 degrees C), 56.22% under CO2-injected conditions (at 120 degrees C), and 23.07% under H2O-injected conditions (at 120 degrees C). The mean friction coefficients under dry, H2O-injected, and CO2-injected conditions are 0.729, 0.696, and 0.714, respectively. The pore pressure could reduce the shear strength, and this effect follows Terzaghi's effective stress law for a sandstone joint. In addition, water and CO2 have a weakening effect on the friction coefficient. In the short term, water and CO2 lead to a reduction in the friction coefficient of approximately 4.53% and 2.06%, respectively. These results provide a basic insight into projects regarding the injection of wastewater and CO2 geological storage. However, the long-term effects of fluids on shear behavior need to be investigated in the future.