Exploration of microstructure characteristics and mechanical behaviors of thermal-damaged argillaceous sandstone via LF-NMR and µ-CT technologies

Abstract

The effect of high temperature on the microstructure and mechanical behavior of rocks is a fundamental issue relevant to the exploitation of underground oil and gas resources, underground disposal of highly radioactive nuclear waste, and the development of geothermal energy. In this study, samples of argillaceous sandstone are subjected to high-temperature treatments (i.e., 25 ºC, 300 ºC, 600 ºC, 900 ºC, 1000 ºC, and 1200 ºC) followed by a series of uniaxial compression tests. Qualitative and quantitative analyses of the microscopic structures of the samples are carried out using scanning electron microscopy (SEM), low-field nuclear magnetic resonance (LF-NMR) and micron-scale computed tomography (µ-CT). Thermal damage defined by porosity is established and used to describe the evolution of rock damage over the temperature increase. After the high-temperature treatments, LF-NMR results indicate that small and medium pores consistently dominate the structure, with pore volumes of up to 98.8% to 100%. µ-CT test results show that the distribution frequency of medium pores is 16.5–28.3% and that of large pores is 71.7–83.5%. The µ-CT porosity ranges from 11.1 to 15.2% and the LF-NMR porosity ranges from 18.9 to 26.3%. When subjected to the same temperature, the µ-CT porosity is generally smaller than the LF-NMR porosity. When the temperature is increased, the peak stress and elastic modulus increases first and then decreases, while the peak strain decreases first and then increases. Test results of three mechanical parameters all indicate that 300 ºC is probably the threshold temperature of the test rock type, and the SEM, LF-NMR and µ-CT test results confirm this temperature. The relationship between thermal damage and temperature suggests that the argillaceous sandstone evolves through strengthening, damage derivation, and damage development as applied temperature increases.