Study on the mechanism of CO 2 displacement crude oil utilization in tight reservoirs

Tight oil reservoirs suffer from ultra-low permeability and complex pore-throat structures, leading to unsatisfactory waterflooding recovery below 20%. Meanwhile, the CO₂ minimum miscible pressure (MMP) of most tight reservoirs exceeds formation fracture pressure, restricting conventional miscible CO₂ flooding and limiting efficient reservoir development. This study adopts capillary tube experiments, cast thin section observations, dynamic nuclear magnetic resonance (NMR) testing, and pore-scale numerical simulation to systematically investigate the non-miscible CO₂ displacement mechanism in tight reservoirs. Experimental results show that the target reservoir has a CO₂ MMP of 33.28 MPa, far higher than its fracture pressure, confirming dominant non-miscible displacement. The average reservoir porosity is 6%, and non-miscible CO₂ flooding achieves an overall oil recovery of 31.5%, significantly higher than 21.8% of N₂ flooding. CO₂ exhibits superior crude oil mobilization performance in 0.01–0.1 μm small pores with a diffusion coverage of 68.7%, but shows limited effectiveness in ultra-small pores below 0.01 μm. This study clarifies the multi-scale oil displacement mechanism of non-miscible CO₂ flooding, providing reliable theoretical support for the industrial application of CO₂ enhanced oil recovery in tight reservoirs.