Pore evolution mechanisms and fractal characteristics of sandstone after high-temperature treatment using NMR

As the dominant rock type in the roof and floor strata of coal seams, sandstone experiences considerable changes in its internal structure and mechanical properties when subjected to high temperatures, which directly compromise the sealing integrity of oil and gas reservoirs. This study examines the pore evolution behavior of sandstone subjected to extended high-temperature treatment, using the sandstone overlying tar-rich coal seams in northern Shaanxi as a case study. Nuclear magnetic resonance (NMR) and scanning electron microscopy were utilized to analyze the evolution of the pore structure across a temperature range of 25–600 °C and treatment durations of 2–8 h. The results indicate that, with increasing heat treatment temperature, both the total porosity and effective porosity of the rock samples exhibit a general increasing trend, with absolute increases of 0.67–1.39, and relative growth rates of 7.77% and 57.11%, respectively. Pore connectivity is substantially enhanced. Simultaneously, the proportion of micropores increases, while the proportions of mesopores and macropores gradually decrease. A remarkable exception occurs at 400 °C, where the total porosity decreases by 29.24%. Correlation analysis between pore size, temperature, total porosity, and fractal dimension reveals that the NMR-derived fractal dimension ( D NMR ) is positively correlated with the content of micropores but negatively correlated with the content of small, medium, and large pores. Among these factors, the content of large pores and the treatment temperature exhibit the strongest correlations with D NMR , suggesting that they are the key factors influencing pore structure complexity.