Objective Extracting coal-based oil and gas from tar-rich coal through in situ pyrolysis plays a significant role in counteracting the shortage of China's oil and gas resources and enhancing the utilization efficiency of coal resources, representing a new trend for the efficient exploitation of tar-rich coals. During the in situ pyrolysis of tar-rich coals, the internal heat transfer, as well as the migration and production of pyrolysis products, is directly influenced by the oil and gas production characteristics and the characteristics of pores and fractures. Hence, it is particularly critical to identify the pyrolysis characteristics and pore-fracture evolutionary patterns of tar-rich coals.
Methods Using a self-developed pyrolysis test device, alongside a specific surface area and pore size analyzer, a three-dimensional microscope, and a simultaneous thermal analyzer, this study recorded the gas production regularities of tar-rich coals under different pyrolysis temperatures, determined the microscopic pore structures and pore size distributions of tar-rich coals after pyrolysis, and analyzed the fracture evolutionary patterns and surface fracture parameters, ultimately revealing the mechanisms behind the responses of coal structures and pyrolysis products to the pyrolysis temperature.
Results and Conclusions The results show that the total gas production and the maximum gas production rate of tar-rich coals through pyrolysis increased with temperature. When the temperature exceeded 300 ℃, the pyrolysis reactions became violent, accompanied by significantly increased gas production. The comparison of the pore and fracture structures under different pyrolysis temperatures revealed that the pore structure types changed with 300 ℃ as a threshold. Below 300 ℃, the pore types after pyrolysis were dominated by small bottlenecks and fractures/wedge-shaped pores, which were primarily mesopores. With an increase in the temperature, the predominant pore types transitioned to parallel slits, micropores increased, and mesopores and macropores further propagated and interconnected, forming microfractures. Compared to coal samples subjected to low temperatures, coal samples at high temperatures manifested elevated fracture aperture, fracture network density, and surface fracture ratio, as well as macroscopically damaged structures and reduced integrity. The evolution of pore-fracture structures during the pyrolysis of tar-rich coals was attributed to the combined effects of water evaporation, the desorption and precipitation of adsorbed gas, uneven thermal expansion, the pyrolysis of organic matter, and the escape of pyrolysis gas. The findings of this study serve as a useful reference for selecting parameters for in situ pyrolysis of tar-rich coals.
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