Underground coal gasification: Overview of field tests and suggestions for industrialization

Driven by the dual demands for achieving the goals of peak carbon dioxide emissions and carbon neutrality and for ensuring national energy security, underground coal gasification (UCG) in China is embracing a new historical development opportunity. To scientifically formulate a route for making technological breakthroughs in UCG and accelerate its industrialization, this study analyzed the history of UCG field tests, dividing UCG into three stages: UCG based on coal mines, vertical/directional wells, and horizontal wells. By exploring the underlying logic that propelled innovations in UCG technologies in different stages, this study delved into the technical and non-technical reasons for the failure of UCG industrialization and finally proposed suggestions. Key findings are as follows: (1) The UCG technology combining horizontal wells with the controlled retracting injection point (CRIP) process can effectively avoid the risks of surface subsidence and freshwater pollution caused by shallow gasification. Furthermore, this technology enjoys advantages in terms of expanding the vertical coal mining range, increasing single-well-controlled coals, improving the quality of raw gas, and ensuring continuous gasification. Hence, this technology serves as a mainstream technical route currently and in the future. (2) After the longest field test period, China has remained long in the stage of UGS based on coal mines. Although China is still in the initial stage of tests for breakthroughs in moderately deep UCG, it roughly keeps the same pace with other major coal-rich countries in terms of technical research and development due to the challenging technology research and low technological maturity. Therefore, China is expected to overtake these countries in the technology for UCG based on drilled wells. (3) Regarding technical reasons, limited technology applicability poses challenges in the industrialization of UCG based on coal mines and vertical wells, while low technological maturity predominantly restricts the industrialization of UGS based on horizontal wells. Consequently, long-term stable and high gas production is yet to be achieved. (4) Concerning non-technical reasons, the termination of UCG tests abroad is primarily caused by low-cost production of conventional natural gas, the influence of the shale gas revolution, public concerns about environmental pollution caused by shallow gasification, and governments' UCG policy shift. In contrast, China's UCG industrialization is principally hindered by a prolonged gap in development planning, relatively limited entities engaging in scientific research and tests, insufficient investment in scientific research, a lack of industrial support policies, and the absence of joint innovation mechanisms. This study proposed suggestions for China's UCG industrialization. In the new era, it is necessary to thoroughly identify the complexity and challenges of UCG technology. Then, great efforts should made to achieve long-term stable production with considerably high quality. It is recommended that the technological maturity should be constantly improved through synchronous scientific research and field tests. The cascade production mode featuring physical gas extraction followed by chemical gasification should be employed to avoid competition with coalbed methane production. Meanwhile, it is necessary to actively explore the utilization mode that integrates oil and gas, new energy, and coal chemical industry for elevated economic benefits. As a revolutionary production technology for artificially created gas reservoirs, UCG, after successful achievement, can provide a technical reference for the fluidization exploitation of other mineral resources and push China's unconventional technology for fossil energy development to a new level.