Experimental study on changes of pore structure and mechanical properties of sandstone after high-temperature treatment using nuclear magnetic resonance

Variations of the mechanical properties and the pore structures of rocks after high-temperature treatment play a significant role in ensuring the operation of underground coal gasification and prevent the pollution of groundwater. In this study, to quantitatively analyse the pore structure and mechanical properties of sandstone after treatment at different temperatures (25 degrees C, 100 degrees C, 200 degrees C, 300 degrees C, 400 degrees C, 500 degrees C, 600 degrees C, 700 degrees C, 800 degrees C, and 900 degrees C), an experiment was conducted based on nuclear magnetic resonance. The variation laws of the size and number of pores in sandstone after the high-temperature treatment were determined to investigate the development of cracks and the changes in the mechanical properties of the rock. At low temperatures, micropores account for over 90%, and the connectivity between pores and cracks is poor. In the temperature range of 200-500 degrees C, macropores and microcracks were found to increase the most, followed by mesopores, while micropores exhibited the smallest increase. Above 600 degrees C, the porosity and permeability of the sandstone improved significantly. Furthermore, the porosity and permeability increased sharply by 1.62 and 1.27 times, respectively, at 500-600 degrees C compared with those at room temperature. After the high-temperature treatment, the stress-strain curves of sandstone gradually became smoother, the post-peak residual stress intensified, and the brittleness weakened. The inflection point of sandstone strength occurs at 800 degrees C, which lags behind the porosity, permeability, and peak strain, because the trans-granular cracks penetrate the whole granule and connect with the surrounding fracture network.