Evaluation of Shale Pore Wettability under Oil–Water Coexistence Conditions and Its Control on Fluid Mobility

Shale reservoirs exhibit complex wettability. Under in-situ formation conditions, oil and water coexist as two phases, and interfacial interactions are pronounced, thereby influencing fluid occurrence state and mobility. Therefore, developing methods to evaluate wettability and mobility under oil–water coexistence conditions, and elucidating the controlling mechanism of wettability on mobility, is crucial for assessing petroleum reserves, production performance, and economic benefits. To this end, this study selected representative terrestrial medium- to high-maturity shale samples from the Lianggaoshan Formation in the Sichuan Basin and conducted alternating oil–water imbibition–NMR–centrifugation experiments. The results indicate strong heterogeneity in pore wettability: water-wet pores are mainly associated with quartz–feldspar minerals, attributable to their intrinsically hydrophilic surfaces; oil-wet pores are related to clay minerals and organic matter rich in polar functional groups, where adherent oil films readily form; mixed-wet pores are associated with carbonate minerals, for which a more balanced interfacial energy state is conducive to oil–water coexistence. Mobility analysis further shows that better reservoir properties, favorable pore–throat configuration, higher proportions of NMR-identified large pores plus microfractures, and higher TOC content all promote shale-oil mobility. Moreover, wettability serves as an even more critical factor influencing mobility: the mobile fraction is positively correlated with the proportion of mixed-wet pores and negatively correlated with the proportion of oil-wet pores. Wettability differences can enhance fluid connectivity by regulating capillary effects and interfacial tension, thereby increasing the mobile fraction. This study highlights the necessity of investigating the controlling mechanism of wettability on mobility, providing an important theoretical basis for understanding pore-scale fluid behavior in shale and for optimizing unconventional oil and gas recovery.