Injected steam can be converted to the sub/supercritical state during the in situ exploitation of oil shale. Thus, the pyrolysis behavior and pyrolysate characteristic of Fushun oil shale during anhydrous pyrolysis and sub/supercritical water pyrolysis were fully compared. The results revealed that the discharged oil yields from sub/supercritical water pyrolysis were 5.44 and 14.33 times that from anhydrous pyrolysis at 360 °C and 450 °C, which was due to the extraction and driving effect of sub/supercritical water. Also, sub/supercritical water could facilitate the discharge and migration of shale oil from the pores and channels. The H₂ and CO₂ yields in sub/supercritical water pyrolysis were higher than that in anhydrous pyrolysis, resulting from the water–gas shift reaction. The component of shale oil was dominated by saturated hydrocarbons in anhydrous pyrolysis, which accounted for 50–65%. In contrast, a large amount of asphaltenes and resins was formed during pyrolysis in sub/supercritical water due to the solvent effect and weak thermal cracking. The shale oil from anhydrous pyrolysis was lighter than that from sub/supercritical water pyrolysis. Sub/supercritical water reduced the geochemical characteristic indices and lowered the hydrocarbon generation potential and maturity of solid residuals, which can be attributed to the fact that more organic matter was depolymerized and released. The pyrolysate characteristic of oil shale in sub/supercritical water pyrolysis was controlled by multiple mechanisms, including solvent and driving effect, chemical hydrogen-donation and acid–base catalysis.

The evolution of pore structure during in situ underground exploitation of oil shale directly affects the diffusion and permeability of pyrolysis products. In this study, on the basis of mineral analysis and thermogravimetric results, in combination with the low-pressure nitrogen adsorption (LPNA) and mercury intrusion porosimetry (MIP) technique, the evolution of pore structure from 23 to 650 °C is quantitatively analyzed by simulating in situ pyrolysis under pressure and temperature conditions. Furthermore, based on the experimental results, we analyze the mechanism of pore structure evolution. The results show the following: (1) The organic matter of Fushun oil shale has a degradation stage in the temperature range of 350–540 °C, and there is no obvious temperature gradient between decomposition of kerogen and the secondary decomposition of bitumen. The thermal response mechanisms of organic matter and minerals are different in each temperature stage, and influence the change of pore structure. (2) Significant changes occur in pore shape at 350 °C, where thermal decomposition of kerogen begins. The ink-bottle pores are dominant when the temperature is less than 350 °C, whereas slit pores dominate when the temperature is greater than 350 °C. (3) The change in pore structure of oil shale is much less significant from 23 to 350 °C. The pore volume, porosity, and specific surface area (SSA) of samples increase rapidly with temperature varying from 350 to 600 °C. The variation of each parameter is dissimilated from 600 to 650 °C: the porosity and pore volume increases with a small gradient from 600 to 650 °C, and SSA decreases significantly. (4) The lithostatic pressure does not cause change in the evolution discipline of the pore structure, but the inhibitory effect on the pore development is significant.

In this paper, the evolution of thermal conductivity, wave velocity and microscopic crack propagation both parallel and perpendicular to the bedding plane in anisotropic rock oil shale were studied at temperatures ranging from room temperature to 600 °C. The results show that the thermal conductivity of the perpendicular to bedding direction (K_PER) (PER: perpendicular to beeding direction), wave velocity of perpendicular to bedding diretion (V_PER), thermal conduction coefficient of parallel to beeding direction (K_PAR) and wave velocity of parallel to beeding direction (V_PAR) (PAR: parallel to bedding direction) decreased with the increase in temperature, but the rates are different. K_PER and V_PER linearly decreased with increasing temperature from room temperature to 350 °C, with an obvious decrease at 400 °C corresponding to a large number of cracks generated along the bedding direction. K_PER, V_PER, K_PAR and V_PAR generally maintained fixed values from 500 °C to 600 °C. 400 °C has been identified as the threshold temperature for anisotropic evolution of oil shale thermal physics. In addition, the relationship between the thermal conductivity and wave velocity based on the anisotropy of oil shale was fitted using linear regression. The research in this paper can provide reference for the efficient thermal recovery of oil shale, thermal recovery of heavy oil reservoirs and the thermodynamic engineering in other sedimentary rocks.

Numerical simulation and thermo-hydro-mechanical coupling model of in situ mining of low-mature organic-rich shale by convection heating

It is well known that underground in situ pyrolysis technology for oil shale production is a promising field. In the in situ modification mining process, the permeability property of a shale matrix has a great effect on the transport capacity of pyrolytic products. For oil shale undergoing pyrolysis, the changes of internal structure (fracture and pore space) have a considerable influence on the permeability network which further affects the migration of hydrocarbon products. In this study, based on an oil shale retorting experiment performed under different temperatures (20 °C, 100 °C, 200 °C, 300 °C, 325 °C, 350 °C, 375 °C, 400 °C, 425 °C, 450 °C, 475 °C, 500 °C, 525 °C, 550 °C, 575 °C, 600 °C), an investigation on the distribution characteristics of the fractures was conducted using micro-CT technology. Meanwhile, mercury injection porosimetry was used to characterize the pore structure of the oil shale samples under different temperatures. Finally, a fracture-pore dual medium model was constructed to calculate the percolation probability to quantitatively describe the permeability variation of oil shale with temperature. The test results indicated that the higher the temperature, the larger were the pore spaces. The increase in pore volume due to pyrolysis temperatures mainly affected the pores ranging from 10 nm to 100 nm and occurred in the specific temperature range (400 °C to 425 °C). Additionally, CT images show that the fracture morphology varied with increasing temperature and the number and length of fractures at different temperatures were in great accordance with the fractal law statistically. On the other hand, simulation of the percolation probabilities discovered that in a single pore media model over the whole range of tested temperatures they were too low to exceed the threshold. In contrast, in the dual medium model, the theoretical threshold of 31.16% was exceeded when the temperature reached 350 °C. Moreover, the results demonstrated that fractures dominated the seepage channel and had more significant effects on the permeability of oil shale. What has been done in this study will provide some guidance for the in situ fluidization mining of oil shale.