Research review on technical effectiveness of CO₂ geological storage and enhanced coalbed methane recovery

Technology of CO₂ geological storage in coal seams for enhancement of coalbed methane recovery exhibits a promising prospect for greenhouse gas emission reduction and new fossil energy development. Technical effectiveness of CO₂ enhanced coalbed methane recovery(CO₂-ECBM)is a foundation for the economical, long-term and safe CO₂ geological storage and CH₄ production. Therefore, it is an urgent mission to establish effectiveness theory and production technology of CO₂-ECBM in deep coal seams in China. Many investigations from China, US, UK, Australia, Japan and other countries have been conducted on CO₂-ECBM related experimental simulation, numerical modeling and engineering implementation. The results shows that CO₂ injectivity, CO₂ containment mechanism, CO₂ storage capacity and CH₄ yield-enhancing effect constitute core connotations in the technical effectiveness of CO₂-ECBM. Therein, CO₂ injectivity has been the focus. CO₂ injection into coal seams contributes to volumetric strain and geochemical reaction and further results in a rapid decline of permeability and CO₂ injectivity, which restrict the effectiveness of CO₂-ECBM. Competitive adsorption between CO₂ and CH₄ and their exchange could be considered as the major CO₂ containment mechanism and in fact they determine the main body of the effective storage capacity under in-situ reservoir environment. As for supercritical CO₂, the modified D-R model shows a best fitting with the experimental results. By means of injection parameter optimization, periodic injection, injection after fracturing, alternant injection using N₂ and other adaptive approaches, CO₂ injectivity could be significantly improved. After CO₂ injection, the CBM yield of well group increased up to 3.8 times. Technical effectiveness of CO₂-ECBM into deep anthracite in Qinshui basin has been preliminarily validated by laboratory investigations and engineering implementations. Chinese scholars devote their best to conduct and expand related research and technology on CO₂-ECBM, and whereby key points of its technical effectiveness are expected to be solved in China.