Geological problems in underground coal gasification

Underground coal gasification (UCG) is one of the diversified ways to fill the gap between the supply and demand of natural gas in China. However, after more than 80 years of field tests, the technology has not yet been industrialized. The carrier of UCG gasifier is geological body and the geological endowment of coal deposit restricts the feasibility of specific UCG project. To promote this process, it is necessary to overcome the bottleneck of geological risk. Based on this strategic need and basic understanding, the authors in this paper briefly review the history of UCG, put forward relevant problems, review the understanding and progress on the geological control of UCG at home and abroad from three aspects of resource endowment, site selection and evaluation, and environmental safety, and put forward some suggestions for geological work to promote the UCG industrialization. It is considered that the geological work of UCG runs through the whole process of planning layout, site selection, gasification production and post-combustion treatment. The lack of systematic geological work is an important reason for the long-term failure to realize the UCG industrialization. It is necessary to fully understand the adaptability of China's coal resource endowment to the current UCG technology from a strategic perspective, and the strict site selection standards abroad enlightens us to strengthen the research and exploration of in-situ large-scale gasification technology for difficult-to-extract, inferior and scattered coal resources. The potential environmental safety caused by the superimposition of high temperature factors and static geological bodies needs to be faced up to and actively dealt with. Based on the geological considerations above, it is suggested that the attitude toward the UCG technology should be cautiously optimistic, not conservative, not blindly, and the selection of appropriate geological conditions for industrial UCG test should be encouraged. The UCG should be regarded as a strategic measure to ensure the long-term energy security of the country. It should focus on five aspects: the evaluation and area selection of UCG resources and strategic planning, the exploration and site evaluation technology of gasifier, the technology of UCG with high safety and wide applicability, the technology of dynamic monitoring and early warning of production geology, and the series of specifications for the UCG geological guarantee. The implementation of the Key Technologies for Geological Risk Assessment and Prediction of UCG plan should be promoted. Among them, the UCG resources evaluation, site selection and strategic planning are the key to break the current difficult industrialization deadlock. It is necessary to, based on the characteristics and occurrence of coal resources in China, summarize the industrial test experience of UCG at home and abroad, establish a national unified technical criterion for the UCG resources evaluation, and evaluate the adaptability of national or regional coal resources on current UCG technology. Then, the suggestions of national pilot test areas with typical geological conditions which can be implemented step by step should be put forward. © 2019, Editorial Office of Journal of China Coal Society. All right reserved.