Temporal Variation in CT Values of Pore Water in Granite Fracture Subjected to Long-Term Confining Pressure and Temperature

Abstract

ABSTRACT: The groundwater flow in rock fractures and the chemical dissolution of rock minerals, which are greatly affected by the confining pressure and temperature, are important issues in the geological disposal of radioactive waste and geothermal studies. In order to investigate the long-term deformation and permeability of rock fractures, the authors conducted long-term permeability tests using a granite specimen with a single split fracture, with parameters of confining pressure and temperature. The saturated specimen was subjected to confining pressure of 3 MPa at 20°C for more than six months, and then the temperature was raised to 60°C. Microfocus X-ray computed tomography (CT) images were taken at predetermined times. The results showed that the CT values inside the fracture rapidly increased with time for 60 days at 20°C and 3 MPa, and then became stable. However, after the temperature was raised to 60°C, the CT values started to increase again. This evolution in the CT values implies pressure solution at the fracture interface due to the long-term confining pressure and temperature. 1. INTRODUCTION The groundwater flow in rock fractures and the chemical dissolution of rock minerals, which are greatly affected by the confining pressure and temperature, are important issues in studies on the geological disposal of radioactive waste (Tsang et al. 2012; Watson et al. 2018), enhanced geothermal systems (EGS) (Pruess 2006; Gerber and Marechal 2012), and petroleum extraction (Chen and Spiro 1994; Gan et al., 2009). In order to investigate the long-term deformation and permeability of rock fractures, the authors conducted long-term permeability tests using a granite specimen with a single split fracture, with parameters of confining pressure and temperature (Song et al. 2021). The saturated specimen was subjected to confining pressure of 3 MPa at 20°C for more than six months, and then the temperature was raised to 60°C. The results revealed that the permeability measured in the short-term tests showed reversible behavior and a dependence on the applied confining pressure and temperature. In contrast, the permeability measured in the long-term tests displayed irreversible behavior and an irregular change under constant confining pressure. In the experiment, microfocus X-ray computed tomography (CT) images were taken at predetermined times to observe the changes in the internal fracture structure.