Abstract
The flow behavior of rock fractures under varying temperatures is one of the critical issues in deep geo-energy projects such as radioactive waste repositories and enhanced geothermal systems. We performed permeability experiments on four cylindrical fractured Beishan granite samples with a diameter of 50 mm and a height of 100 mm to investigate the effect of temperature on fracture permeability. Argon was used to test the fracture permeability to avoid the probable water-rock interaction on fracture surfaces. Three of the samples were tested under a confining pressure of 5 MPa and temperatures of 22, 90 and 150 °C, respectively, measuring aperture change caused by creep. The fracture aperture under constant temperatures decreases with time, and the descending rate also decreases with time until a steady phase at about 10 days. The eventual reduction magnitudes of fracture aperture under the temperature of 90 °C and 150 °C are larger than that under 22 °C. The rest sample was tested under three temperatures cycles between 30 and 150 °C. Temperature has a more significant influence on fracture aperture compared with the effect of creep. Fracture aperture decreases gradually as the temperature increases from 30 °C to 150 °C, and then the fracture aperture remains almost unchanged as the temperature decreases from 150 °C to 30 °C. The fracture aperture cannot recover to the initial value after a thermal loading cycle due to the plastic deformation of asperities. The similar tendencies also appear in the following two temperature cycles, while the changes in fracture aperture decreases with increasing temperature cycles. Asperity degradation was observed with the analysis of three-dimensional surface topography of the fracture samples.
It is well known that fractures in rock masses often provide major conduits through which groundwater can flow, and fracture aperture can be altered by mechanical loading induced by excavation, thermal expansion and tectonic movements or chemical water-rock interactions. The effect of cyclic heating and cooling on the permeability of granite fractures likely to exist in the deep underground engineering, such as underground nuclear waste repositories, CO2 sequestration, and enhanced geothermal systems is the focus of this paper.
