Experimental Study on the Effect of CO2 Flooding on Irreducible Water Saturation in Low-Permeability Reservoirs

Abstract Irreducible water saturation is a critical parameter for CO 2 flooding performance in low-permeability reservoirs, yet its dynamic variation and underlying microscopic mechanisms remain unclear. In this study, a series of core flooding experiments were conducted on low-permeability natural cores to systematically investigate the effects of displacement pressure differential, temperature, and gas type on irreducible water saturation. The main innovations and results are as follows: (1) The dual competitive mechanism of CO 2 flooding in reducing irreducible water saturation is quantitatively revealed for the first time. As the displacement pressure differential increased from 2 MPa to 10 MPa, irreducible water saturation decreased from 64.45% to 42.53% (a reduction of 34.01%), while water film thickness decreased from 347.5 nm to 302.0 nm, with the square of water film thickness reduced by 24.48%. The difference of 9.53 percentage points between the actual reduction (34.01%) and the reduction in the square of water film thickness (24.48%) demonstrates that the produced irreducible water originated not only from water film thinning but also from micro-capillary pores, with 9.53% mobilized from micro-capillary pores. The competition between “water film release” and “micro-pore recapture” under high pressure determines the net reduction. (2) The direct correlation between half-peak width reduction and hydrogen bond breaking ratio is established for the first time. Elevated temperatures led to decreased irreducible water saturation, with low-permeability samples exhibiting a two-stage decreasing pattern (reduction of 5.95 percentage points in the 30–60 ℃ range and 0.64 percentage points in the 60–80 ℃ range), while higher-permeability samples showed approximately linear decreases (overall reduction of 4.94 percentage points). 17 O -NMR analysis revealed that the half-peak width decreased from 100.2 Hz at 40 ℃ to 76.0 Hz at 80 ℃ (a reduction of 24.16%), corresponding to a hydrogen bond breaking ratio of 3.304%. The dissociation of water clusters enhanced water fluidity, which is the microscopic mechanism of temperature-induced irreducible water saturation reduction. (3) The differences in the effects of CO 2 and N 2 on irreducible water saturation in low-permeability reservoirs are systematically compared for the first time. CO 2 demonstrated significantly higher efficiency in reducing irreducible water saturation during the initial stage (within 30 min), with a reduction of 14.98% compared to 11.65% for N 2 , and required a shorter time to reach stability (30 min vs. 40 min). After stabilization, the reduction by CO 2 (16.02%) was slightly higher than that by N 2 (14.83%), attributed to the high solubility and supercritical properties of CO 2 , providing experimental evidence for the use of CO 2 /N 2 combined injection strategies when CO 2 sources are limited. This study provides new theoretical insights and quantitative evidence for the integrated development of CO 2 -EOR and storage in low-permeability reservoirs.