Nuclear Magnetic Resonance Investigation of Imbibition and CO2 Huff-and-Puff Enhanced Oil Recovery in Shale Porous Media

Efficient recovery of shale oil remains a major technical challenge due to the ultralow permeability, complex pore-fracture structure, diverse lithofacies, and inherently limited natural productivity. Conventional enhanced oil recovery (EOR) techniques often yield limited effectiveness when they are applied independently. The present study investigates a hybrid EOR strategy that combines acidic slickwater imbibition with CO2 huff-and-puff, aiming to enhance oil recovery through synergistic physical and chemical mechanisms. Core samples from four representative shale reservoirs were tested under reservoir-representative temperature and pressure conditions. A suite of experiments, including 2D nuclear magnetic resonance (NMR), rheology tests, and minimum miscibility pressure (MMP) evaluations, were conducted to assess fluid behavior and recovery. 2D NMR results revealed that CO2 huff-and-puff significantly enhanced oil displacement, especially from micropores and mesopores that were less responsive to water imbibition. Interfacial tension measurements confirmed that CO2 exhibited the lowest values (∼1 mN/m), while acidified slickwater outperformed base slickwater. CO2 dissolution further reduces the oil viscosity, enhancing fluid mobility. MMP tests showed that miscibility between CO2 and oil was achieved only in the DMT reservoir (37.61 MPa), partially explaining the superior recovery performance. Overall, the hybrid acidic slickwater + CO2 approach is proven effective, achieving average oil recoveries of about 50% and up to nearly 70% in most cores, representing an improvement of 10–30% over single-method treatments. These findings support the design of EOR strategies in shale reservoirs.