Effects of High-Pressure Air Injection on Longmaxi Shale: Physical Property and Seepage Capacity Enhancement

High-pressure air injection (HPAI) emerges as a pivotal enhanced oil recovery (EOR) technique thanks to the heat from oxidation, which reduces the crude oil viscosity and modifies the shale matrix. This study examines the changes in composition, microstructure, and mechanical properties of shale from room temperature to 800 °C. Equivalent permeability models were developed to assess the shale’s seepage capability following exposure to elevated temperatures. The findings demonstrate that organic matter decomposes between 300 and 500 °C, resulting in almost 80% loss of total organic carbon. Mineral composition changes occur above 500 °C, with quartz undergoing a β-phase transition at 573 °C and clay minerals decreasing after 700 °C. Dolomite and calcite nearly completely decompose at 800 °C. Composition failure facilitates the formation of organic pores in shale. The internal stress of shale becomes disordered with grains expanding irregularly and slipping, leading to the formation of numerous intergranular fractures. Mechanical property tests elucidate these mineralogical changes. Elevated temperatures result in a 2.45-fold increase in shale strain while simultaneously reducing its compressive strength and elastic modulus by one-third and diminishing its tensile strength by more than half. The heating process induces pyrolysis of organic matter, modifications in mineral composition, and a decline in mechanical properties, all of which collectively facilitate the development of fractures and enhance physical properties. Physical experiments suggest that an increasing temperature raises porosity more than 3-fold and permeability by over 300 times. Simulation results indicate that the equivalent permeability of shale can increase significantly with heating by a factor of at least 100 000. This study elucidates the mechanism by which high temperatures affect the shale matrix and highlights the potential of HPAI.