Gas permeability characteristics and pore structure evolution of sandstones with high-temperature damage effects

This study investigates the gas permeability characteristics and pore structure evolution of fine-grained sandstone under high-temperature conditions. Sandstone specimens were subjected to temperatures ranging from 20 °C to 800 °C, followed by gas permeability, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and X-ray diffraction (XRD) tests. The results indicate that gas permeability demonstrates a three-stage variation with temperature: a gradual increase up to 400 °C due to the expansion of mineral grains and dehydration, a reduction from 400 °C to 600 °C associated with pore closure and microstructural collapse, and a dramatic increase beyond 600 °C driven by thermal cracking and mineral decomposition. The confining pressure significantly influenced gas permeability, exhibiting a reduction of over 90 % at pressures above 50 MPa across all temperatures. The Klinkenberg slip effect, characterized by enhanced gas flow in low-permeability pores, was prominent at low gas pressures but diminished with increasing gas pressure, suggesting critical thresholds for slip behavior under thermal effects. Porosity increased exponentially with temperature, particularly after 500 °C, as new pores and microcracks formed. Microstructural analysis revealed that high temperatures led to the transformation of kaolinite into mixed-layer illite, which played a pivotal role in altering pore structures and gas permeability. These findings provide a systematic understanding of the coupled thermal-mechanical effects on sandstone and have implications for geothermal energy extraction, underground coal gasification, and other subsurface engineering applications.