Discussion on underground system sealing methods in in-situ pyrolysis exploitation of tar-rich coal

China has abundant tar-rich coal resource, in-situ underground pyrolysis is the main trend of green and efficient development of tar-rich coal resources. However, the underground in-situ pyrolysis of tar-rich coal is still in the stage of technology research and development, and it is necessary to focus on the key technology of tar-rich coal to carry out research . Underground system sealing is one of the key technologies to ensure the efficient use of energy and ecological protection of tar-rich coal in-situ pyrolysis mining, and there is an urgent need to develop and construct underground system closure technology suitable for the characteristics of tar-rich coal in-situ pyrolysis development. Based on the current status of research and application of underground system sealing technology, the principles, advantages and disadvantages of various types of underground system sealing technologies such as underground artificial freezing, grouting curtain, water-stopping by gas flooding and foam water-stopping, etc. were systematically sorted out and summarized, and the stratigraphic adaptability of various technologies weas analyzed in depth. The applicability of different system sealing technologies in the development of tar-rich coal pyrolysis is analyzed in the context of geological characteristics such as burial depth, layer thickness and regional tectonics of tar-rich coal seam, as well as the pyrolysis development process and scale. Further, the development trend of system sealing technology is discussed, i.e., innovating of existing technical principles and development of composite sealing technology. For the engineering implementation of underground system sealing technology, a systematic approach including design-monitoring-repair and environment restoration of underground system sealing is proposed around the fine design before development, strict monitoring during development and safe restoration after development. Finally, an integrated technology concept of underground in-situ pyrolysis exploitation of tar-rich coal and CO₂ geological storage was proposed by integrating the key technologies such as in-situ pyrolysis, underground system sealing, product displacement and CO₂ geological storage, which provided references for the development and industrial application of in-situ pyrolysis exploitation technology of tar-rich coal in China.