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Efects of high-temperature thermal treatment on the porosity of red sandstone: an NMR analysis

Zhang He, Sun Qiang, Jia Hailiang, Dong Zhihao, Luo Tao

Acta Geophysica

2021

Vol. 69, no. 1

113--124

High temperatures afect the physical properties of red sandstone seriously, especially the pores. Understanding its mechanism is of great signifcance in coal mining following underground gasifcation, geothermal energy utilization, and the deep burial of nuclear waste. Nuclear magnetic resonance (NMR) was used to detect pore structure characteristics, and scanning electron microscopy (SEM) and polarizing light microscopy (PLM) were used to mechanism of change. The transverse relaxation time (T2) and signal strengths of red sandstone treated at various temperatures were observed by NMR, and then, the pore situation can be obtained, and fnally, the infuence of temperature on the pore structure of red sandstone can be obtained. Microscopic photographs of the pores of red sandstone were obtained by SEM and PLM to assist in explaining the process of microstructural change, especially the infuences of temperature on pore characteristics and grain morphology and distribution. The researches indicate that after the heat treatment of red sandstone at 25–1300 °C, the pore and strength characteristics change in well-defned stages. Before 500 °C, the pore diameters and distribution range increase, but the porosity and internal grain structure do not change signifcantly. At 500–1000 °C, red sandstone micropores contract, mesopores and macropores develop, and strength decreases. After 1000 °C, the grains that comprise sandstone melt and fll many of the pores, decreasing porosity. The proportion of micropores decreases, while mesopores and macropores increase. In addition, a large number of bubbly holes appear in and on the sandstone, presumably caused by gases such as CO2, and water vapor from dehydrating grains. The changes in pore and cementation states with temperature are the main factors afecting the tensile strength of red sandstone.