Research progress of in-situ pyrolysis technology for tar-rich coal

China has abundant resources of tar-rich coal, and the in-situ pyrolysis extraction of oil and gas can not only achieve efficient, clean, and low-carbon utilization of tar-rich coal, but also alleviate the external dependence of oil and gas, which is one of the important development directions of tar-rich coal exploitation and utilization in the future. In this paper, the thermal and physical properties and distribution characteristics of tar-rich coal, the technological characteristics of surface pyrolysis and in-situ pyrolysis are described briefly. It focuses on the research status and progress of in-situ pyrolysis of tar-rich coal (including well layout, heating method, integration of in-situ gasification and pyrolysis, waste heat utilization, reservoir fracturing, and underground sealing technology). Also, the future development direction of tar-rich coal in-situ pyrolysis technology is prospected. The findings are summarized as follows: ① the cascade utilization is an important way for the surface pyrolysis of tar-rich coal. Well drilling in-situ pyrolysis can achieve underground oil recovery, but it is necessary to solve the insulation problem of long-distance transportation pipelines. Drilling in-situ pyrolysis is carried out through high-temperature medium or electric heating, and can use distributed energy from the mining area to heat the heat carrier fluid. ② It is recommended to use a composite heating technology with convection heating as the main method and others as auxiliary heating methods, which can use the residual heat from in-situ pyrolysis to preheat the heat carrier fluid, and the heat from the local oxidation reaction of coal seam can also be used in the in-situ pyrolysis process of oil-rich coal to reduce the heat injection energy consumption of heat carrier fluid. ③ The integrated technology of in-situ gasification and pyrolysis can effectively improve the utilization rate of thermal energy, but the gasification cavity needs to be filled. ④ In order to enhance the heat conduction efficiency of coal seam and reduce the migration difficulty of oil and gas products in coal seam, the porosity change law of reservoir fracturing and in-situ pyrolysis of oil-rich coal is comprehensively considered to control the fracture formation path and form the fracture network. ⑤ Based on the characteristics of the pyrolysis process of tar-rich coal, selecting suitable underground system closure technology can promote efficient energy utilization and ecological environment protection during the in-situ pyrolysis process of tar-rich coal, among them, the CO₂ gas drive water sealing technology can not only realize the closure of the underground system, but also carry out oil and gas displacement and realize the geological storage of CO₂.