Potential of nonionic polyether surfactant-assisted CO2 huff-n-puff for enhanced oil recovery and CO2 storage in ultra-low permeability unconventional reservoirs

Sequestering CO2 in oil reservoirs using CO2 huff-n-puff technology while maximizing oil recovery is critical for the efficient development of shale/tight reservoirs. In this work, intrinsic reasons for the limited EOR efficiency of CO2 huff-n-puff technology and the potential of a nonionic polyether surfactant C4(PO)3-assisted CO2 huff-n-puff process in further enhancing oil recovery and increasing CO2 storage capability in shale/tight reservoirs were investigated by physical modeling experiments combined with nuclear magnetic resonance techniques. The EOR performance, the mobilization of oil from different pore size ranges, and CO2 storage capability were analyzed in the CO2 huff-n-puff and CO2–C4(PO)3 mixed fluid huff-n-puff experiments. The results showed that the EOR efficiency of CO2 huff-n-puff was limited by the rapidly declining cyclic recovery and the meager recoveries from micropores and deep matrix regions. Applying CO2–C4(PO)3 mixed fluid with a surfactant concentration of 0.5 wt% resulted in a 9.7%–13.2% increase in the cumulative matrix oil recovery relative to CO2 under the same huff-n-puff cycles. Moreover, relative increases in oil recoveries from micropores, mesopores, and macropores reached 113.4%–142.2%, 14.2%–41.1%, and 0.8%–17.6%, respectively. When CO2 no longer enhanced production after the 4–5 huff-n-puff cycles, approximately 14%–19% of the residual oil was effectively recovered by applying the C4(PO)3-assisted CO2 huff-n-puff stimulation. Simultaneously, the ultimate CO2 storage capability in the examined cores increased by 22.3%–32.2% after the CO2–C4(PO)3 mixed fluid huff-n-puff process. The enhancement of CO2 injection and diffusion capabilities induced by the CO2–C4(PO)3 mixed fluid improved CO2 sweep and oil displacement efficiencies in shale/tight reservoirs, leading to a significant increase in the oil mobilization within micropores and an extended effective stimulation cycle, as well as achieving a higher CO2 storage capability. The increase of CO2 injection and diffusion amounts induced by nonionic polyether surfactants during the huff-n-puff process was attributed to the reduction in capillary resistance caused by the lowered CO2–oil interfacial tension and the increase of dissolved CO2 in oil resulting from the improved CO2–oil miscibility. This paper presents new strategies for advancing the sustainable and efficient development of shale/tight reservoirs with highly developed nanopores.