Multiscale Investigation of CO₂/Oil Competitive Adsorption and Interfacial Evolution in Shale Reservoirs

The efficiency of CO₂-enhanced oil recovery (CO₂-EOR) in shale reservoirs is governed by complex multiphase oil-water-rock interfacial interactions at the nanopore scale, where interfacial properties such as wettability, interfacial tension, and adhesion work collectively control the adsorption-desorption behavior of fluids on pore surfaces and dominate the competitive adsorption and displacement processes between CO₂ and crude oil. In this study, high-pressure and high-temperature contact angle and interfacial tension experiments were conducted on a CO₂-oil-water-shale system to systematically characterize the evolution of interfacial properties under different temperature and pressure conditions, with particular emphasis on the coupled variations of wettability, interfacial tension, and adhesion work and their impacts on oil mobility. The experimental results indicate that CO₂ injection can significantly promote the alteration of shale wettability from water-wet or oil-wet toward intermediate-wet conditions, accompanied by simultaneous reductions in oil-water interfacial tension and the adhesion work of oil droplets on shale surfaces. These changes weaken the oil-rock interfacial binding strength and enhance oil desorption and flowability. Hydrophilic shales exhibit higher sensitivity to pressure variations, whereas oil-wet shales are primarily controlled by temperature, demonstrating that the initial wettability state plays a critical role in regulating interfacial responses to CO₂ exposure. Furthermore, molecular dynamics simulations based on Illite-kerogen composite slit pores reveal that low-temperature and high-pressure conditions favor the formation of a dense and stable CO₂ adsorption layer near pore walls, thereby significantly enhancing its competitive advantage over crude oil and its displacement capability. During this process, CO₂ progressively occupies adsorption sites from the pore interior toward the wall surface and from weak to strong adsorption sites, achieving stepwise competitive adsorption and replacement of oil molecules. This microscopic competitive adsorption mechanism provides molecular-scale evidence for the experimentally observed macroscopic evolution of interfacial properties. Overall, this study elucidates the intrinsic multiscale linkage among interfacial properties, molecular-scale interactions, and adsorption behaviors during CO₂-oil competitive adsorption, providing a theoretical basis for understanding CO₂-EOR mechanisms in shale reservoirs and for optimizing CCUS engineering applications.