Evolution of permeability and mesostructure of oil shale exposed to high-temperature water vapor

Abstract In the process of pyrolyzing oil shale using water vapor as a heat carrier fluid, the internal structure of rock mass is subject to complex changes. Studies on the permeability of oil shale pyrolysis by high-temperature water vapor are of great importance to the design of oil shale mining technology by convection heating. In this paper, the permeability of oil shale samples after water vapor pyrolysis was studied using permeability tests and flow field simulations. The permeability of oil shale was compared and analyzed in the direct mode of retorting. The results can be summarized as follows. First, when the pyrolysis temperature increased from 20 °C to 382 °C, the increasing rate of permeability was relatively low. At temperatures between 382 °C and 555 °C, the permeability increased significantly with increasing pyrolysis temperature. Second, when the water vapor temperature was 314 °C, there was no broad range of seepage pathways in the sample, and the flow field distribution was relatively limited. At water vapor temperatures of 534 °C and 555 °C, the streamline distribution in the sample was tight, and the permeability of oil shale was high. Third, the permeability of oil shale in the convection heating mode was significantly higher than that in the direct mode of retorting. At temperatures between 314 °C and 382 °C, the difference in permeability between the two modes was more prominent. Finally, the results of the permeability test and the results of the seepage field simulation were combined to explain the permeability evolution law and thermal cracking characteristics of oil shale heated by high-temperature water vapor.