Permeability and porosity in damaged salt interlayers under coupled THMC conditions

Abstract

Salt rock is a potential medium for the underground storage of hydrocarbons and nuclear waste. In China, salt deposits are typically composed of layered rocks, usually sandwiched by insoluble interlayers. These salt rock–gypsum interlayers significantly affect the integrity of bedded salt caverns. Therefore, it is important to assess how the permeability and porosity of salt interlayers in an excavation damage zone (EDZ) evolve at high temperatures and triaxial stress/seepage pressures. To address this, in this study, we have developed a triaxial testing system that uses thermo-hydro-mechanical-chemical (THMC) coupling. Then, the permeability and porosity of salt rock–gypsum interlayers with 0.6–0.9 degrees of damage (i.e.after being subjected to a triaxial stress of 60–90% of the triaxial compressive strength; the confining pressure is 25 MPa and experimental temperature is 20 °C) under a coupled THMC environment were measured. The results show that the permeability and porosity of gypsum with the four different damage degrees do not increase monotonically with increasing temperature under the coupled THMC condition. Especially for the 0.6 and 0.8 damaged gypsum specimens, the permeability and porosity tend to decrease in the temperature range of 300–500 °C. For the 0.8 and 0.9 damaged gypsum specimens, a vertical and catastrophic increase in permeability and porosity is observed at 550 °C and 300 °C, respectively. The variation trends of the permeability and porosity of the damaged specimens are closely related to the combined effects of the triaxial stress, seepage pressure, thermal stress, and degree of damage.