Mechanism and potential evaluation method of CO₂ geological storage in depleted shale gas reservoirs

Large-scale commercial development of shale gas has been achieved in China, and in the future, a significant number of depleted shale gas horizontal wells could provide substantial CO₂ sequestration capacity. However, the geological characteristics of shale gas reservoirs significantly differ from those of conventional sequestration geological bodies, such as deep saline aquifers, conventional oil and gas reservoirs. The mechanisms of CO₂ sealing, trapping, and the methods for assessing sequestration potential in shale formations require further study. Based on the geological characteristics of shale gas reservoirs, this paper analyzes the mechanisms of CO₂ sealing and trapping within these reservoirs. CO₂ geo-sequestration can be regarded as the reverse process of shale gas production, with the main storage space being the pore and fracture space originally occupied by the shale gas produced. Based on the premise of stimulated reservoir volume (SRV) as the CO₂ storage space, combined with the effective volume method and “free-adsorption-dissolution” multiple storage mechanism, and with cap rock safety as the constraint condition, a shale gas reservoir CO₂ storage potential evaluation model was established. According the current state of shale gas development in China, the CO₂ sequestration potential of China’s future depleted shale gas reservoirs’ was preliminarily evaluated. The results indicate that, vertically, capillary sealing is the primary mechanism of shale caprock. Horizontally, the SRV is surrounded by unfractured dense shale, forming an effective barrier to lateral CO₂ migration. Based on these mechanisms and general experiences from shale gas exploration and development in China, the maximum CO₂ sequestration potential within the SRV controlled by a single horizontal well is estimated to be 712,000 tons. The proportion of free, adsorption, and dissolution storage mechanism is 41.82%, 56.79%, and 1.39%, respectively. The CO₂ sequestration potential of existing shale gas wells in China is preliminarily estimated at 1.502 billion tons, with an additional potential of 12.104 billion tons from future shale gas wells.