Abstract
CO(2) injection in shale reservoirs is more suitable than the conventional recovering methods due to its easier injectivity and higher sweep efficiency. In this work, Grand Canonical Monte Carlo (GCMC) simulation is employed to investigate the adsorption/desorption behavior of CH(4)-C(4)H(10) and CH(4)-C(4)H(10)-CO(2) mixtures in organic and inorganic nanopores during pressure drawdown and CO(2) huff and puff processes. The huff and puff process involves injecting CO(2) into the micro- and mesopores, where the system pressure is increased during the huffing process and decreased during the puffing process. The fundamental mechanism of shale gas recovery using the CO(2) injection method is thereby revealed from the nanopore-scale perspective. During primary gas production, CH(4) is more likely to be produced as the reservoir pressure drops. On the contrary, C(4)H(10) tends to be trapped in these organic nanopores and is hard to extract, especially from micropores and inorganic pores. During the CO(2) huffing period, the adsorbed CH(4) and C(4)H(10) are recovered efficiently from the inorganic mesopores. On the contrary, the adsorbed C(4)H(10) is slightly extracted from the inorganic micropores during the CO(2) puffing period. During the CO(2) puff process, the adsorbed CH(4) desorbs from the pore surface and is thus heavily recovered, while the adsorbed C(4)H(10) cannot be readily produced. During CO(2) huff and puff, the recovery efficiency of CH(4) is higher in the organic pores than that in the inorganic pores. More importantly, the recovery efficiency of C(4)H(10) reaches the highest levels in both the inorganic and organic pores during the CO(2) huff and puff process, suggesting that the CO(2) huff and puff method is more advanced for heavier hydrocarbon recovery compared to the pressure drawdown method. In addition to CO(2) storage, CO(2) sequestration in the adsorbed state is safer than that in the free state. In our work, it was found that the high content of organic matter, high pressure, and small pores are beneficial factors for CO(2) sequestration transforming into adsorbed state storage.