Impact of soaking time on shale oil recovery during CO 2 huff-n-puff: regularity and mechanisms

To clarify the effect of soaking time on shale oil recovery during CO2 huff-n-puff and the role of CO2-induced reservoir modification, CO2 huff-n-puff and static soaking experiments were conducted on continental shale samples at 75 °C and 20 MPa from 6 h to 40 d. Nuclear magnetic resonance, X-ray diffraction, scanning electron microscopy, and contact angle measurements were used to characterize recovery behavior and reservoir modification. The oil recovery factor (ORF) ranged from 15.43% to 51.06%, with the 20 d case producing the highest ORF under the tested conditions. ORF was jointly affected by soaking time and initial petrophysical properties: permeability and fractures controlled early recovery, whereas appropriately extended soaking promoted long-term recovery, although the enhancement remained constrained by initial properties and may exhibit diminishing marginal returns. Mesopores were the main contributors to oil occurrence and production, while micropores showed limited mobilization and macropores showed oil redistribution and retention. Static soaking results suggest that CO2-induced calcite dissolution, pore enlargement, improved connectivity, and wettability alteration may contribute to long-term recovery enhancement. By examining long-term ORF evolution and its relationship with CO2-induced reservoir modification, this study provides experimental support for optimizing soaking time in CO2 huff-n-puff development of continental shale oil.