Pore-scale dynamic interfacial transport and mobilization of shale oil with different occurrence states using CO2 and CO2–polyether composite fluids

Shale oil hosted in complex pore structures exhibits markedly different mobility depending on its occurrence state, posing challenges for understanding the pore-scale hydrocarbon mobilization processes. In this work, a nuclear magnetic resonance-based characterization method was employed to differentiate free and ad/absorbed shale oil and to investigate the dynamic adsorption behavior of polyether surfactants in shale nanopores, as well as the interfacial transport behavior of shale oil under CO2 and CO2–polyether composite fluids. The mechanisms governing the enhanced mobilization of shale oil by CO2–polyether composite fluids were elucidated. The results showed that CO2 could mobilize ad/absorbed shale oil through countercurrent diffusion, yet its mobilization remained significantly more difficult than that of free oil. Even when the CO2 extraction pressure exceeded the minimum miscibility pressure, the mobilization efficiency of free oil surpassed 80%, whereas that of ad/absorbed oil remained below 40%. Incorporating low-molecular-weight polyether surfactants into CO2 fundamentally altered the interfacial force balance and substantially enhanced the mobilization of ad/absorbed oil in shale nanopores. Among the tested surfactants, C4(PO)3 exhibited the strongest affinity for kerogen and yielded the greatest improvement in ad/absorbed oil mobilization by 17.5% at 1 wt. %. Mechanistic analysis demonstrated that the accumulation of polyether molecules on kerogen surfaces enhanced the CO2-philicity of the organic matter, thereby promoting CO2 dissolution and diffusion within organic nanopores, resulting in a substantial enhancement of ad/absorbed oil mobilization. These findings deepen the understanding of multiphase flow and interfacial transport behavior of surfactant-modified CO2 fluids in confined porous media.