Study on the Mechanism of Thermal Damage on Pore Structure and Permeability of Sandstone

The study of the mechanism by which thermal damage affects the pore structure and void permeability of sandstone holds significant theoretical and practical value for deep engineering applications. In this study, the changes in the pore structure and permeability of sandstone were investigated using low-field nuclear magnetic resonance spectroscopy (NMR) and a multifield coupled testing system, with samples subjected to various high-temperature pretreatments. The results of NMR showed that the T2 spectrum of sandstone exhibited an increase in peak area and a decrease in the number of peaks with rising temperature. Porosity decreased slightly in the low-temperature range (25°C to 200°C) but increased significantly in the high-temperature range (400°C to 800°C). Simultaneously, the fractal dimensions of medium to large pore segments decreased exponentially with increasing temperature, indicating an increase in the number of cracks and a gradual evolution of the pore network from a dense to a more porous structure. The permeability exhibited a decreasing trend followed by an increase, with 400°C serving as the critical temperature at which stress sensitivity peaked. Regression analysis revealed that the porosity, tortuosity, and fractal dimension significantly influenced permeability. Finally, the SDR model was enhanced by incorporating a temperature correction factor (ξT), extending its applicability to high-temperature conditions and enabling accurate predictions of the permeability of thermally damaged rocks. These findings have important implications for practical applications in fields such as geothermal development, oil and gas extraction, and underground storage.