Influence of high-temperature thermal cycles on the pore structure of red sandstone

Analyzing variations in rock pore structure under high-temperature thermal cycles is of great significance for the development of deep resource. In this paper, red sandstone from Linyi, Shandong Province, was heated to 200 °C, 400 °C, 500 °C, and 600 °C, followed by five cold thermal cycles. The transverse relaxation times (T2) of red sandstone treated at various temperatures were observed using nuclear magnetic resonance (NMR), and the porosity and the characteristics of the pore structure were analyzed. The results show that the variations in the pore structure of red sandstone occur primarily owing to the escape of pore water, thermal expansion, and the decomposition of some minerals. In the range of 200 to 500 °C, the escape of free water, bound water, and structural water in the sandstone resulted in a significant increase in the proportion of pores with a volume of r < 0.1 μm. At 573 °C, the phase transition of quartz and the high-temperature decomposition of calcite led to a marked increase in the proportion of mesopores (with a volume of 0.1 μm < r < 1 μm), and the connectivity between pores increased. At the same temperature, the number of mesopores and macropores increases with increases in the number of cycles. At 600 °C, the number of mesopores in sandstone increased significantly after five thermal cycles.