Experimental study on enhanced oil recovery by advanced CO2 flooding in tight oil of ordos basin using nuclear magnetic resonance technology

The poor physical properties and low pressure coefficient of tight reservoirs hinder the direct application of advanced water injection technologies developed for low-permeability reservoirs. As an emerging enhanced oil recovery (EOR) technique, advanced CO 2 flooding has attracted increasing attention. In this study, core samples from the Long 8 reservoir in the West 331 block of the Ordos Basin were investigated through water flooding, CO 2 flooding at different pressures, and advanced CO 2 flooding experiments, combined with nuclear magnetic resonance (NMR) analysis. The results show that water flooding achieves an oil recovery of approximately 40%, mainly from macropores. The oil recovery of CO 2 flooding increases with injection pressure and reaches 76% under miscible conditions. Advanced CO 2 flooding further improves sweep efficiency, significantly enhancing the utilization of mesopores and micropores, and increases oil recovery to 87%. After immiscible CO 2 flooding, residual oil is mainly present as contiguous oil, with increasingly heterogeneous distribution and more isolated droplets at higher pressures. Following advanced CO 2 flooding, the remaining oil primarily exists as isolated droplets, and crude oil in some regions is completely dis-placed. The lower limit of pore utilization decreases with increasing miscibility, reaching approximately 50 nm for miscible flooding and 20 nm at 1.2 times the mini-mum miscibility pressure for advanced CO 2 flooding.