Stability mechanism and steady-state flow characteristics of oil-resistant foam in high-salinity reservoirs

High salinity and high oil content present major challenges to the effectiveness of foam in enhanced oil recovery (EOR). This study introduces RCS, a novel oil-resistant foam system designed for reservoirs with salinity levels reaching 2.1 × 10⁵ mg/L. RCS forms stable foams at oil–water ratios up to 60% and is effective across a wide crude oil viscosity range (10.8–7890 mPa·s). We investigated the film properties of oil-containing foam and the co-permeation behavior of the crude oil–N₂–foam system to elucidate the mechanisms underlying foam stability and steady-state flow. RCS emulsified high-viscosity crude oil into stable, large droplets that accumulated within the plateau borders, reducing drainage. Even at concentrations as low as 0.01 wt%, RCS formed stable pseudoemulsion films that prevented intrusion into the gas–water interface, allowing the foam half-life to be mainly controlled by the dilatational viscoelasticity of the interface. With increasing oil–water ratios, both drainage resistance and dilatational modulus increased, extending the drainage and foam half-lives. Coreflood experiments showed that co-injection of RCS with N₂ and crude oil produced stable foams and in-situ emulsions. At 5% oil fractional flow, the critical foam quality (f_g^*) remained unchanged compared to oil-free conditions, although the maximum apparent viscosity decreased by 29.8%. At 10% oil fractional flow, f_g^* shifted to a lower value, while the apparent viscosity in the low-quality regime increased markedly—exceeding that of the oil-free condition. These findings highlight that while crude oil more strongly impairs foam stability in porous media than in bulk, the formation of in-situ emulsions can partially offset or even enhance mobility control through a synergistic Jamin effect. Therefore, in-situ emulsification should be emphasized in foam applications within oil-containing environments.