CO2 miscible flooding is a crucial technology for enhancing oil recovery in ultra-low permeability reservoirs. However, the relatively high minimum miscibility pressure (MMP) has restricted the application of this technology. To reduce the MMP of the CO2/crude oil system, gas-soluble surfactants have been widely adopted in CO2 miscible flooding technology. In this paper, gas-soluble surfactants were taken as the research object. Firstly, a cloud point pressure test system with high-pressure visual autoclave and an illuminance sensor as the core equipment was established. The cloud point pressure method was used to determine the solubility of three surfactants, namely Span 80, AOT and TXIB, in CO2 under reservoir temperature. Then, slim-tube experiments were conducted to evaluate the effect of different surfactants on reducing the MMP of the CO2/crude oil system, analyze the differences in MMP reduction efficiency between non-ionic and ionic surfactants, and screen out the optimal gas-soluble surfactant suitable for the target reservoir. The results show that TXIB has the highest solubility at all mass fractions, while AOT has the lowest. Meanwhile, the MMP reduction rate of the non-ionic surfactant TXIB reaches as high as 16.35%, which is much higher than that of the anionic surfactant AOT. Compared with anionic surfactants, non-ionic surfactants can form weak interactions with CO2 through ester groups and long-chain hydrophobic groups, thus achieving miscibility at lower pressure and being more suitable for the target reservoir.
Jing et al. (Thu,) studied this question.
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