Study on the construction of 3D complex pore networks and the dynamic evolution of seepage pathways in oil shale during in-situ pyrolysis by convective heating

Based on CT scan data of oil shale subjected to in-situ steam pyrolysis at different temperatures, an equivalent pore network model (EPNM) was constructed. By introducing concepts from social network theory into the analysis of oil shale seepage pathways, a 3D complex pore network was developed in MATLAB to perform topological characterization and connectivity analysis of the pore structure under varying pyrolysis conditions.The main research findings are as follows: Firstly, from 20 °C to 300 °C, micropores gradually develop and become interconnected due to thermal fracturing. At 350 °C, a large number of pores close, leaving primarily mesopores and macropores. Between 350 °C and 550 °C, intense pyrolysis of organic matter leads to a significant increase in the number of all pore types, with the growth of mesopores and macropores being particularly pronounced at 500 °C to 550 °C.Secondly, the in-situ high-temperature steam pyrolysis process of oil shale can be divided into four stages: the effective pore development stage, the ineffective pore closure stage, the pyrolysis-induced pore proliferation stage, and the high-temperature pore coalescence stage.Thirdly, a critical transition occurs at 450 °C, where the dominant flow paths change from these pre-fractured cracks to the entire oil shale matrix.Furthermore, as the pyrolysis temperature increases, the shortest seepage path evolves from a single dominant flow channel into a component of a more complex seepage network. Finally, compared with traditional CT-based analysis methods, the 3D complex pore network perspective demonstrates clear advantages in analyzing the evolution of seepage characteristics.This study provides a new perspective for the dynamic analysis of rock microstructures and seepage characteristics.