Temperature-pressure coupled experiments and response mechanisms of permeability evolution in deep continental shale

To address the unclear permeability evolution mechanisms during in-situ conversion of deep continental shale, this study employs a self-developed online THMC (thermo-hydro-mechanical-chemical)-CT coupled experimental system to investigate the permeability evolution, dynamic pore-fracture structural responses, and hydrocarbon production behavior under high-temperature and high-stress conditions. The results show that: (1) Under high stress constraints (axial/confining stresses of 50/25, 100/50 MPa), shale permeability exhibits a three-stage evolution with increasing temperature, including a low-permeability stage (25–350 °C), a rapid-increase stage (350–450 °C), and a significant-decrease stage (450–600 °C). (2) Under coupled in-situ stress (25/20 MPa axial/confining stress) and temperature, fractures undergo a dynamic “two-expansion and two-contraction” process, where the first expansion (25–300 °C) and first contraction (300–350 °C) correspond to the low-permeability stage, the second expansion (350–450 °C) corresponds to the rapid-increase stage, and the second contraction (450–550 °C) corresponds to the significant-decrease stage. (3) The controlling mechanisms at each stage are as follows: at temperatures up to 350 °C, the maximum yield of retained oil and the filling of heavy hydrocarbons in pores and fractures result in reduced permeability. Between 350 °C and 450 °C, thermal cracking and kerogen decomposition jointly enhance pore-fracture network development. Above 450 °C, illitization of clay minerals, matrix plastic deformation, and fracture closure under stress result in permeability reduction. These findings clarify the staged permeability behavior and associated mechanisms, providing essential theoretical and experimental support for the temperature-stress synergistic optimization of in-situ shale oil conversion processes.