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Real-time high-temperature CT scanning and a rock-mechanics test system were employed to investigate the mechanical properties of oil shale at temperatures from 20 to 600 °C. The results reveal that up to 400 °C, the aperture of fractures initially decreases and then increases when loading is perpendicular to the bedding. However, the number and aperture continuously increase when loading is parallel to the bedding. Beyond 400 °C, the number of pores increases and the aperture of the fractures becomes larger with rising temperature. The changes in microstructures significantly impact the mechanical properties. Between 20 and 600 °C, the compressive strength, elastic modulus, and Poisson's ratio initially decrease and then increase under perpendicular and parallel bedding loadings. The compressive strength and elastic modulus reach minimum values at 400 °C. However, for Poisson's ratio, the minimum occurs at 500 °C and 200 °C under perpendicular and parallel bedding loadings, respectively. Simultaneously, while the crack damage stress during perpendicular bedding loading, σcd-per, initially exhibits an upward trend followed by a decline and subsequently increases again with temperature increasing, the initial stress during perpendicular bedding loading, σci-per, parallel bedding loading, σci-par, and damage stress, σcd-par, decrease initially and then increase, reaching minimum values at 400 °C. These research findings provide essential data for reservoir reconstruction and cementing technology in the in situ mining of oil shale. [ABSTRACT FROM AUTHOR]

In situ mining is a practical and feasible technology for extracting oil shale. However, the extracted oil shale is subject to formation stress. This study systematically investigates the pyrolysis–mechanics–seepage problems of oil shale exploitation, which are subject to thermomechanical coupling using a thermal simulation experimental device representing a closed system, high-temperature rock mechanics testing system, and high-temperature triaxial permeability testing device. The results reveal the following. (i) The yield of gaseous hydrocarbon in the closed system increases throughout the pyrolysis reaction. Due to secondary cracking, the production of light and heavy hydrocarbon components first increases, and then decreases during the pyrolysis reaction. The parallel first-order reaction kinetic model shows a good fit with the pyrolysis and hydrocarbon generation processes of oil shale. With increasing temperature, the hydrocarbon generation conversion rate gradually increases, and the uniaxial compressive strength of oil shale was found to initially decrease and then increase. The compressive strength was the lowest at 400 °C, and the conversion rate of hydrocarbon formation gradually increased. The transformation of kaolinite into metakaolinite at high temperatures is the primary reason for the increase in compressive strength of oil shale at 400–600 °C. (ii) When the temperature is between 20 and 400 °C, the magnitude of oil shale permeability under stress is small (~10−2 md). When the temperature exceeds 400 °C, the permeability of the oil shale is large, and it decreases approximately linearly with increasing pore pressure, which is attributed to the joint action of the gas slippage effect, adsorption effect, and effective stress. The results of this research provide a basis for high efficiency in situ exploitation of oil shale. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]