Monitoring CO2 storage and enhanced gas recovery in unconventional shale reservoirs: Results from the Morgan County, Tennessee injection test

Permanently sequestering carbon dioxide (CO2) in gas-bearing shale formations is beneficial in that it can mitigate greenhouse gas emissions as well as enhance gas recovery in production wells. This is possible due to the sorption properties of the organic material within shales and their greater affinity for CO2 over methane (CH4). The phenomenon of preferentially adsorbing CO2 while desorbing CH4 has been proven in unconventional coalbed methane reservoirs successfully, and is feasible for shale formations. The objective of this paper is to explore the potential for enhanced gas recovery from gas-bearing shale formations through a successful small-scale 'huff-and-puff' injection of CO2 into a targeted shale formation. Approximately 510 short tons of CO2 were successfully injected into a horizontal production well completed in the Chattanooga Shale formation in Morgan County, Tennessee. After the injection phase, the well was shut-in to allow for the CO2 to equilibrate within the target formation. After the soaking phase was completed, the well was flowed back and returned to normal production. During this flowback phase, gas composition and flow rate were frequently monitored. Results indicated that there was a significant increase in gas flow rate during the first five months of the flowback phase. There was also a significant increase in gas quality, such that the percent composition of NGLs (natural gas liquids), including ethane, propane, and butane, increased. The results from this injection test confirm the injectivity and storage potential of CO2 in organic shales formations while enhancing gas recovery.