Advances in shale gas development: Resource assessment, production mechanisms, and CO₂ sequestration potential

Shale gas has received considerable attention as a clean and high-quality energy resource, while CO₂ injection into shale reservoirs is increasingly explored for its potential to enhance CH₄ recovery and reduce emissions. Shale exhibits a wide pore size distribution, with micropores (<2 nm) and mesopores (2-50 nm). This review explores three key aspects of shale gas research: gas-in-place (GIP), production mechanisms, and CO₂ storage potential with emphasis on significance of both micropores and mesopores. First, the pore morphology and topology characterization using techniques such as electron microscopy, probe gas adsorption, and connectivity analysis were reviewed. Reservoir conditions, shale characteristics, and gas compositions of six typical shale reservoirs were summarized. Second, the gas states in shale nanopores, including free, adsorbed, and absorbed phases, were evaluated due to their importance in estimating GIP. The sorption isotherms for the six representative shale reservoirs, adsorption models, and excess-to-absolute conversion methods were summarized. Molecular simulation models were also reviewed and compared to elucidate adsorption mechanisms at the nano scale, highlighting kerogen models with both micropores and mesopores that better represent experimental adsorption behavior. Third, production mechanisms and CO₂ storage potential were reviewed. Field tests, experiments, and molecular simulations on CO₂ huff-n-puff were summarized. CO₂ injection in shale can effectively enhance CH₄ recovery and store CO₂, due to its higher affinity compared to CH₄. Formation-specific and global CO₂ storage potential of shale were analyzed. Based on our previous molecular simulation and the assumption that technically recoverable CH₄ can be replaced by CO₂, we estimated the global shale CO₂ storage capacity to exceed 570 Gt. This review provides methodological guidance for simulations and experiments and supports future efforts in shale energy development and CO₂ sequestration.