Macroscopic seepage and microstructural behavior of oil shale using water vapor injection during mining

In the context of convection-heating-based in situ oil shale retorting, fractures serve as primary pathways for fluid migration and product extraction. This study investigates the permeability and microstructural evolution of oil shale during water vapor injection in single-fracture and no-fracture scenarios. Three types of oil shale are investigated: intact oil shale, oil shale with a single straight crack, and oil shale with a single hydraulic crack. With increasing water vapor temperature, the permeabilities of the intact oil shale and oil shale with a fractured crack exhibit a trend of initial increase, followed by a decrease, and then a subsequent increase. However, the permeability of oil shale with a single straight crack consistently increases and exceeds that of oil shale with a fractured crack. The temperature-dependent permeability changes in fractured oil shale – a slight decrease in fracture cracks and a gradual increase in straight cracks – mainly occur in the range of 300 °C-350 °C. The permeability of oil shale with a straight crack is approximately three times that of oil shale with a fractured crack. This is attributed to the retention of viscous asphaltene and the frictional resistance caused by the rough fracture structure. For the oil shale with a single crack, the crack permeability has a dominant influence on the overall permeability of the rock. The contribution of the permeability of the straight crack exceeds 94.6%, while that of the permeability of the fractured crack is greater than 86.1%. The disparity in the contribution of these two crack structures is evident at 350 °C-550 °C.