EXPERIMENTAL STUDY ON FRICTIONAL PROPERTIES OF FLUID-BEARING SANDSTONE FRACTURES AT DIFFERENT TEMPERATURES

Seismic activity caused by fluid injection in sandstone reservoirs has been associated with the frictional properties of embedded faults or fractures. In order to study the frictional characteristics of fluid-bearing sandstone fractures under different temperature conditions, velocity stepping tests were carried out at varying temperature and pressure conditions (a temperature range of 25 °C ~ 140 °C and an effective normal stress range of 4 ~ 12 MPa) on dry, water saturated and CO ₂injected sandstone fractures (obtained by saw cutting), respectively. The experimental results show that: (1) For dry sandstone fractures, increasing effective normal stress and increasing temperature can both increase the initial friction coefficient of fractures, while varying effective normal stress has no obvious effect on the frictional stability of fractures. An increase in temperature is found to enhance the frictional stability of fractures. (2) For sandstone fractures saturated by water, the initial friction coefficients of fractures also increase with the effective normal stresses, but they can be weakened by the rising temperatures, and increasing effective normal stress and temperature can both favor the frictional instability of fractures; (3) For the CO ₂injected sandstone fractures, the initial friction coefficients of fractures are affected by the change in effective normal stress and temperature, which is opposite to that of water-saturated sandstone fractures. The frictional stability of fractures is affected by the ambient temperature, seemingly independent of the effective normal stress. To sum up, these experimental results suggest that the frictional characteristics of sandstone fractures are jointly controlled by the effective normal stress, temperature and the injected fluid type. These experimental results may provide a better understanding of earthquakes induced by fluid injection.