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.

During the in-situ exploitation of oil shale, the evolution of pore structure will affect the heat transfer medium and the transport capacity of products. In this study, using the oil shale from Jimsar region of Xinjiang, in combination with the low-temperature nitrogen adsorption experiment (LNTA) and high-pressure mercury injection experiment (MIP), the effect of temperature on evolution law of its pores during pyrolysis were analyzed according to the LNTA and MIP joint characterization results. The results show as follows: the change curve of oil shale porosity with pyrolysis final temperature in Xinjiang shows an “S” shape. The pore volume increases slightly before 300 °C, and it shows an increasing trend in the subsequent temperature range, with the mesopore volume change being the most noticeable. However, the increase slows down after 600 °C. The evolution of full-scale pores in the pyrolysis process of oil shale is reflected by the joint characterization of LNTA and MIP, which indicates that the temperature has a controlling effect on the evolution of pore, and provides basic theoretical support for how to improve the oil recovery rate of exploitation of oil shale.

The study of the pyrolysis characteristics of oil shale, an important strategic resource, is of great significance for oil shale refining and in situ underground mining. In this study, using the oil shale from Jimsar region of Xinjiang, in combination with thermogravimetric analyzer (TG), Fourier transform infrared spectroscopy (FTIR), and mass spectrometry (MS) techniques, the pyrolysis characteristics and mechanism of the oil shale are analyzed according to the experimental results and explored the effect of its pyrolysis on the development of the pore. The results, the pyrolysis with three stages, show the following: (1) The first stage is 23-390C. The precipitation of adsorbed water within the oil shale and the dehydration of gypsum are mainly at 100C or so, appearing some microcapillary pores and other temperature sections change little. (2) The second stage is 390-527C. The organic matter pyrolysis of Jimsar oil shale accounts for 71.1 of the total weight loss of oil shale. This stage has the greatest impact on the evolution of pore structure, and there are two weight loss peaks at 458C and 506C, respectively, in the thermogravimetric curve. Combined with FTIR-MS, the main products of its pyrolysis gas are H₂, H₂O, CO₂, CH₄, and C_nH_m and will change the original pore surface of oil shale, create new pore volume, and produce complex and irregular pore structure. (3) In third stage (527-800C), the formation of H₂, CO₂, and hydrocarbon gases suggests that this stage includes not only the decomposition of major inorganic components like carbonate and clay but also the degradation process of some organic matter. At this stage, the CO₂ generated in the precipitation process will lead to a large number of pores in oil shale. Crack grids will appear, due to the melting and recrystallization of some clay minerals. Once the interaction between the two is particularly intense, the mineral skeleton may rupture and collapse. The pyrolysis characteristics above provide a basis for the analysis of the pore structure evolution of oil shale and are of implications and practical application value to the exploitation of shale oil.