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The formulation and mechanism of a hybrid ultralow interfacial tension spontaneous emulsification (SE) system consisting of alkyl polyglucoside surfactant and low-concentration alkali were investigated to enhance oil recovery for low-permeability reservoirs. Multiple-light scattering was used to characterize the stability and demulsification process of oil-in-water (O/W) emulsions. The emulsions were evaluated in terms of droplet size distribution, ζ potential, and three-phase rheological property. The average droplet size of the optimum SE O/W emulsion is about 0.2 μm, which is smaller than the average diameter of the low-permeability throats (0.2–4.0 μm). The coalescence and floatation of emulsion droplets are well-deferred. The oscillation–shear–oscillation rheological property could be further improved with the presence of the optimum system. This can be attributed to the synergistic effect of surfactants and alkali, which enables the formation of a more tight arrangement film of surfactants at the oil–water interface. Subsequently, diffusion of the surfactant from the interface to the bulk phase is inhibited by enhancing the interfacial viscosity. Moreover, the flooding results from the visual micromodel experiments demonstrate that the small residual oil droplets could be displaced easily in the porous media. The newly formulated SE system can be applied to enhance oil recovery for low-permeability reservoirs.

Some heavy oil reservoirs with deep burial depths, low permeabilities, high temperatures and high salinities that cannot be thermally recovered or chemically flooded are treated with the “CO2 + chemical agent” compound huff and puff method, and positive results are achieved. However, there are still problems, and a more efficient compound system needs to be developed. This study first determined the design principles for molecular optimization, selected six chemical systems for two crude oils with two viscosities, and then analyzed and evaluated the viscosity reduction performance, increased CO2solubility performance, and synergistic viscosity reduction performance to obtain the optimal solubilization synergist formula. The results show that the preferred system has a better effect on BN-1 crude oil. For the BN-1 crude oil, the ZR-1 system can increase the amount of dissolved CO2by more than 30% under 15 MPa, and the synergistic viscosity reduction rate with CO2can reach 98% at 10 MPa. Therefore, research on CO2compound systems can improve the synergistic viscosity reduction effect, reduce the dosage of agents, further reduce the cost and increase the efficiency of viscosity reduction technology.