Insight into pore-fracture structure and permeability of oil shale: Significance of water vapor temperature

In-situ oil shale mining technology faces critical challenges in optimizing heat transfer efficiency and seepage channels. This study conducts high-temperature water vapor pyrolysis experiments on oil shale, integrating permeability tests, mercury intrusion porosimetry, and micro-CT techniques to systematically investigate the evolution of permeability, pore structures, and fracture networks under different water vapor temperatures. In the low-temperature stage (room temperature to 350 degrees C), permeability follows a trend of "gradual increase-slight decline," reaching a low-temperature peak at 300 degrees C. In the high-temperature stage (350 degrees C-600 degrees C), under the high-temperature pyrolysis conditions used in this study, the permeability can increase by up to five orders of magnitude. With rising temperature, the bulk porosity of oil shale surges from 2.94 % to 22.22 %, while the pore size distribution shifts from a "micropore-macropore dominated" (inverse S-shape) pattern to a "mesopore-dominated" (S-shape) pattern, leading to a significant enhancement in pore connectivity. For the multi-scale fracture structure, the low-temperature stage (<350 degrees C) is dominated by the accumulation of micro-fractures, which tend to close due to effective stress and thermal mismatch coupling effects. In the high-temperature stage (>350 degrees C), macro-fractures expand and may form partially connected networks. Approximately 350 degrees C serves as the transition temperature for pore and fracture structure evolution in tested samples, while the 450 degrees C-600 degrees C range represents the high-efficiency pyrolysis zone. The study reveals the temperature-regulated mechanism governing permeability evolution during heat injection pyrolysis of oil shale.