The liquid phase of foam systems plays a major role in improving the fluidity of oil, by reducing oil viscosity and stripping oil from rock surfaces during foam-flooding processes. Improving the oil displacement capacity of the foam’s liquid phase could lead to significant improvement in foam-flooding effects. Oil-liquid interfacial tension (IFT) is an important indicator of the oil displacement capacity of a liquid. In this study, several surfactants were used as foaming agents, and polymers were used as foam stabilizers. Foaming was induced using a Waring blender stirring method. Foam with an oil-liquid IFT of less than 10–3 mN/m was prepared after a series of adjustments to the liquid composition. This study verified the possibility of a foam system with both an ultra-low oil-liquid IFT and high foaming properties. Our results provide insight into a means of optimizing foam fluids for enhanced oil recovery.

CO2 foam flooding technology can be used for geological storage of greenhouse gas CO2 while enhancing oil recovery. In this paper, we performed molecular dynamics simulations for the CO2 foam film systems stabilized with an anionic surfactant of sodium dodecyl sulfate (SDS). Compared to other factors of a foam system, interfacial tension was used as the primary indicator to evaluate the stability of the SDS foam film. The effects of the concentration of the SDS surfactant, temperature, and pressure on the interfacial tension were studied. Based on the calculated results, the stability of the CO2 foam film was discussed. It is found that under the high-concentration condition, SDS molecules can form a dense and thick molecular layer at the interface, blocking the contact of CO2 molecules and water molecules and reducing the interfacial tension. Consequently, the stability of the foam liquid film can be improved. Low temperature and high pressure lead to high density of the CO2 phase, which enables strong interactions between CO2 and the hydrophobic tails of SDS molecules. The interfacial structure thus formed can reduce the contact probability between CO2 and water molecules, generating lower interfacial tension. Therefore, high SDS concentration, low temperature, and high pressure are beneficial to the stability of the CO2 foam. We show that the adsorption of CO2 molecules at the interface, the interfacial thickness, and solvent accessible surface area of the surfactant to the CO2 phase are affected by the density of CO2 bulk phase, which are important interfacial properties affecting the CO2/water interfacial tension and the stability of the CO2 foam.

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Summary As one of the largest emitters in the world, the oil and gas industry needs to apply more effort to greenhouse gas (GHG) reduction. Methane, as a potent GHG, could largely determine whether natural gas could serve as a bridging energy toward a sustainable future. In the past decade, oil and gas companies in China have significantly enhanced casing gas recovery and reduced large volume flaring (>2×104 m3/d). However, the remaining low- to mid-volume flaring gas was left for further recovery. Shale gas production in China has met a surge in the number of drilling wells. Those new wells were characterized by a relatively low gas production rate (<1×106 m3/d) in comparison with shale gas wells in the US. As a result, flaring gas during well completion needs to be recycled or used to enhance the gas recovery rate. In this study, we carried out a pilot demonstration project of flaring gas recovery to reduce GHG emissions in the Weiyuan shale gas region in Sichuan Province, China. We adopted the technical route of dehydration and natural gas compression. The recycled natural gas was transformed into compressed natural gas (CNG) and transported to the nearest CNG station for further use. The inlet gas pressure was between 2.85 and 5.82 MPa, and the outlet pressure was kept stable at around 20 MPa to meet the standard of CNG. The manufactured device also showed sound flexibility with the recovery rate between 523.22 and 1224.38 m3/h, which was 28–157% of the designed capacity. The combination of the molecular sieve with high capacity, post low-pressure dehydration, and the application of hydraulic piston in the compression system have guaranteed the equipment to meet the designed performance. The equipment applied in the pilot demonstration has well matched with the local transportation, gas composition, and surface engineering of the well completion. It has the potential of popularization and application in the shale gas tight gas regions in China. Other technical routes, such as small-scale gas to chemicals or natural gas hydrate, should be considered for industrial application for gas flowing rate less than 2×104 m3/d to ensure a further drive down of methane emission along the value chain.