CO₂ SEQUESTRATION IN UCG CAVITIES: RESEARCH PROGRESS AND FUTURE DEVELOPMENT TRENDS

Carbon capture and storage (CCS) could help a lot to achieve carbon peaking and carbon neutrality goals and is an effective way to deal with the Greenhouse effect. Among the geologic sequestration formations, the cavities resulting from deep underground coal gasification (UCG) become a hot topic in the research area of geologic CO₂ sequestration. However, compared with conventional sequestration methods, the related work is still in the theoretical exploration stage and lack of trial tests. To promote the development of UCG cavity sequestration, we have done the work as follows. First, we introduce the research progress of UCG and post UCG cavity sequestration, and divide the development of the latter one into three stages including the early stage of conception, stage of quantitative assessment and feasibility analysis, and stage of mechanism analysis. Currently, it is still in a stage of theory exploration. Second, we compare the UCG cavity sequestration with the conventional sequestration options in detail from the perspective of injectivity, sealing capacity, economy, storage capacity, and trapping mechanism. The results show that the UCG cavity sequestration holds an excellent injectivity, has a similar sealing capacity to the unmined coal seams but more complex, is capable to reduce transport cost a lot, has a great potential in storage capacity, and has complex trapping mechanisms, owing to the additional effects of cavity morphology, wall properties, and interactions between supercritical CO₂ and in-situ fluid on the injection and storage processes. Third, we point out the key scientific and engineering issues, and basic future development trends of the UCG cavity sequestration. Based on the above work, we suggest that the government introduces some policies to encourage and support the development of UCG and post cavity sequestration which could enrich the CCS family and promote the clean and low-carbon utilization of coal resources.