Air-Assisted In Situ Conversion of Oil Shale: Dual-Atmosphere Analysis of the Pore Network and Permeability Evolution

The in situ air-assisted conversion of oil shale has attracted considerable attention because of its low cost, abundant gas supply, rapid heating rate, and high energy efficiency. However, an insufficient understanding of pore evolution, connectivity, and permeability changes during this process has hindered its further application. In this study, low-temperature nitrogen adsorption was used to characterize pore-structure evolution, and fractal analysis was applied to quantify pore complexity and irregularity. Low-field nuclear magnetic resonance (NMR) was employed to evaluate pore-fluid occurrence and connectivity, while the pulse-decay method was used to determine the porosity and permeability changes during pyrolysis. The results show that pore-structure evolution is controlled by the combined effects of chemical reactions and physical structural adjustment and exhibits clear stage-dependent characteristics. Temperature plays a decisive role in the pore-network connectivity and seepage capacity. In particular, at the high-temperature stage, the increases in porosity and permeability under a nitrogen atmosphere are significantly greater than those under an air atmosphere, indicating that oxygen participation results in a pore-network remodeling pattern different from that under nitrogen and limits further improvement of permeability. On this basis, a stage-specific mechanistic framework, namely, “kerogen conversion and phase change → pore network remodeling → permeability response”, is established by integrating fractal parameters, NMR connectivity, and pore–permeability data. This framework clarifies the intrinsic link between atmosphere-controlled pore evolution and seepage behavior and provides a theoretical basis for optimizing air-assisted in situ oil shale conversion.