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

No abstract

High-resolution X-ray computed tomography (Xray CT) is a noninvasive nondestructive method for studying the internal structure of source rocks such as oil shale. Here, to accurately identify the distribution of organic matter (OM), pores, and fractures in such rocks based on CT images, a three-phase segmentation method for OM recognition of source rocks via CT images was proposed and used to study the internal structure of Xinjiang Balikun oil shale heated with superheated steam. First, the oil shale samples were heated at different steam temperatures and CT images were acquired, to which the three-phase segmentation method was then applied to obtain the distribution of OM, pores, and fractures in the rock. The effectiveness of the method was verified by means of thermogravimetric analysis (TGA) measurements. The results showed that, within the spatial resolution limit of the CT images (pixel size 3.6 × 3.6 μm 2 ), a large number of ellipsoidal OM clusters were distributed in the oil shale at room temperature (25 °C), accounting for 4.29% of the total volume of the oil shale. As the steam temperature increased, the OM inside the oil shale gradually pyrolyzed, with the discharged pyrolysis products leaving behind a large number of ellipsoidal pores. Simultaneously, fractures gradually developed in the rock and connected with the pores, forming tadpole-like pore-fracture structures. TGA measurements showed that the temperature range of the oil shale's maximum weight loss was 400−550 °C and that the weight loss rate reached 10.4%, which is consistent with the changes in the OM obtained from the CT images.

The connectivity of the internal pores and fractures in oil shale is the critical factor in determining the success of the insitu pyrolysis of the oil shale with superheated steam. In this paper, using a self-developed superheated steam pyrolysis experimental system, oil shale samples were subjected to pyrolysis experiments at different steam temperatures. Then, the oil shale samples were scanned with high-precision micro-CT equipment to obtain the three-dimensional digital core of oil shale (DCOS). Based on the three-dimensional site percolation theory and renormalization group algorithm, the pore and fracture connectivity characteristics of the DCOSs were studied. The results show that when the steam temperature reached the pyrolysis temperature for oil shale, a series of pores was formed during the pyrolysis process. These pores gradually connected the adjacent fracture and subsequently formed a massive pore-fracture cluster. However, from room temperature to 555 °C, there were always parts with porosity less than 5% in the DCOSs perpendicular to the direction of the sedimentary bedding, forming the bottleneck of the seepage passage. This occurrence is the main reason that the permeability of the oil shale perpendicular to the direction of the sedimentary bedding is far lower than that parallel to the direction of the sedimentary bedding.

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.