Mechanisms and patterns of crude oil mobilization in microscopic pores under gravity-assisted CO2 miscible displacement: A case study of tight conglomerate reservoirs in the Mahu Sag, Xinjiang

To address the unclear understanding of the microscopic oil displacement mechanisms, patterns of oil mobilization in pore throats, and optimal displacement methods for gravity-assisted CO2 miscible displacement in tight conglomerate reservoirs, this study utilized tight conglomerate cores from the Mahu Sag as the research subject. Experiments on gravity-assisted CO2 miscible displacement were designed for cores with varying physical properties under different displacement conditions. NMR technology was employed for real-time monitoring. The study elucidated the microscopic oil mobilization patterns and displacement mechanisms of gravity-assisted CO2 miscible displacement and established a recovery factor prediction model. The results demonstrate that gravity assistance enhances the recovery factor of cores with different physical properties to varying degrees, with the most pronounced effect observed in low-permeability cores. The presence of locked pores invalidates the method for determining sweep efficiency based on the lower limit of mobilization; instead, it can be quantitatively assessed through recovery factor and displacement efficiency. The incorporation of gravity assistance helps the miscible system overcome the constraints of some locked pores, thereby improving core recovery. Gravity assistance effectively enhances the degree of oil mobilization in pore throats of different scales and influences the frequency distribution of remaining oil, although it does not alter the distribution pattern. Remaining oil is still predominantly concentrated in pores with radii <1 μm. The mechanism by which gravity-assisted CO2 miscible displacement improves recovery is not merely a result of simple pressure superposition but arises from complex multi-scale physical field coupling.