CO2 enhanced gas recovery and sequestration in depleted gas reservoirs: A review

Increasing CO2 emissions and global warming side effects have prompted the researchers to look for safe and reliable storage sites that have high capacity. Among the available CO2 capturing sinks, depleted gas reservoirs have high potential to sequester CO2. Depleted conventional and unconventional gas reservoirs have large pore space after natural gas production and pressure reduction. Moreover, their ability to store hydrocarbons for many years inside the sealed reservoir with impermeable cap rocks provides safer options than saline aquifers or other geological traps. Incremental recovery of residual natural gas after injecting CO2 could decrease the cost of the process. This review highlights the efforts made to investigate the CO2 adsorption/desorption for EGR applications under typical reservoir conditions in conventional depleted sandstone and carbonate reservoirs. Moreover, it analyzes the advances in CO2-EGR in unconventional resources such as coal beds and shale to extract the knowledge from these reservoirs. In addition, various factors that control the displacement efficiency of natural gas by injecting CO2 and the consequent influence of CO2 on rock integrity are discussed. Nanoscale basis of CO2-EGR using multiscale molecular simulation that could improve the design and operational conditions for CO2-EGR operations is overviewed. Furthermore, this article assesses the ecological and economic impact of storing CO2 in different types of reservoirs. Field pilot tests, as well as challenges in the application of the CO2-EGR technique, are also covered.