Mechanism Study of Enhanced Oil Recovery by CO ₂ Huff-n-Puff in Ultralow-Permeability Reservoirs Based on Nuclear Magnetic Resonance

Ultralow-permeability reservoirs, characterized by low porosity and permeability, narrow pore throats, and strong capillary forces, pose much greater development challenges compared to conventional reservoirs and urgently require efficient enhanced oil recovery (EOR) technologies. In recent years, CCUS technologies have brought new opportunities for the development of ultralow-permeability reservoirs. Among them, CO₂, owing to its swelling and expansion effects, viscosity reduction, and interfacial tension lowering properties, is considered a highly promising displacing agent. In this study, displacement performance of four mediawater, surfactant solution, N₂, and CO₂was systematically investigated through variable differential pressure core flooding experiments, microfluidic displacement tests, and simultaneous huff-n-puff physical simulations. The results show that oil displacement efficiency increases with higher displacement pressures for all media. Notably, CO₂ exhibits relatively high mobilization efficiency even at low pressures and achieves higher ultimate oil recovery (70-80%), which is higher than N₂ flooding (65-70%) and significantly higher than water flooding or surfactant flooding (60-65%). Microfluidic experiments indicate that CO₂ significantly reduces residual oil saturation and demonstrates superior displacement uniformity and pore-scale sweep efficiency compared to water and N₂ flooding. Simultaneous CO₂ huff-n-puff experiments reveal that, with increasing huff-n-puff cycles, both the swept volume and mobilization depth expand markedly; the mobilization depth increased from 12 cm (cycle 1) to 16 cm (cycle 3). NMR T₂ spectra show a decrease in long relaxation time components, indicating effective mobilization of residual oil within the pores. These results indicate that CO₂ has excellent oil displacement potential in laboratory-scale tests, suggesting promising application prospects in ultralow-permeability reservoirs, provided that field-scale conditions and capillary constraints are carefully considered. The findings provide significant experimental evidence and theoretical support for optimizing displacing media selection and huff-n-puff operational strategies in ultralow permeability reservoirs.