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.

Due to the high energy density, large potential reserves and only release CO₂ and water after combustion, natural gas hydrate (NGH) is considered as the most likely new clean energy source to replace traditional fossil energy (crude oil, natural gas, etc.). However, unlike the exploitation of traditional fossil energy, the essence of natural gas hydrate exploitation is to induce the production of methane by artificially decompose the natural gas hydrate and to simultaneously collect the generated methane. Because of the uncontrollable decomposition, the methane percolation and the gas collection efficiency, methane emission is inevitably occurred during natural gas hydrate exploitation, which could significantly affect the environmental friendliness of natural gas hydrate. In this review, the methane emission detection was divided into three interfaces: Seafloor and sediment, seawater, atmosphere. Meanwhile, according the summary and analysis of existing methane emission detection technologies and devices, it was concluded that the existing detection technologies can identify and quantify the methane emission and amount in the three interfaces, although the accuracy is different. For natural gas hydrate exploitation, quantifying the environmental impact of methane emission and predicting the diffusion path of methane, especially the methane diffusion in strata and seawater, should be the focus of subsequent research.

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.

CO2 storage efficiency refers to the amount of CO2 storage in a certain volume of underground space, which is directly affected by CO2 sweeping volume. Besides, the balance between vertical and horizontal migration of CO2 is the key to increase the sweeping volume. This study focused on the influence of foam fluid on the fluidity and percolation characteristics of CO2 in porous media. The rheological properties, percolation characteristics and maximum injection volume of pure CO2 and CO2 foam were investigated by rheometer, percolation performance test and CO2 storage simulation experiment, respectively.As the experimental results shown, the apparent viscosity of CO2 foam reached 6000 mPa·s at 85℃, and the viscosity of pure CO2 was below 0.1 mPa·s at the same temperature; the resistance factor (the ratio of the pressure difference between the two ends of the core during foam injection and the pressure difference between the two ends of the core during water injection) of foam was over 500 times that of pure CO2 in 10mD core, and the difference in resistance factors was more significant in cores with lower permeability; in a core with a pore volume of 127 ml, the CO2 storage amount of foam injection was 136% that of pure CO2 injection. Meanwhile, the impact of foam`s property, such as diameter distribution, gas-liquid ratio, on the storage efficiency was investigated by a series experiments. Firstly, the resistance factor and residual resistance factor of CO2 foam reached the highest in the cores with permeability of 110 mD class, and the second in the cores with 1 mD class. Secondly, under the condition of the same permeability, the larger the gas-liquid ratio is, the better the blocking effect is. Thirdly, under all three permeability conditions, the residual resistance factor showed a decreasing and then increasing trend at the beginning of injection.According the results and analysis, foam injection can effectively improve the CO2 storage efficiency. The key parameters affecting the effectiveness of storage efficiency improvement are as follows. Firstly, matching of foam particle size to formation pore size. Bubbles shown a higher probability of entering narrow pore channels with pore diameters smaller than their particle sizes, resulting in a more frequent occurrence of the Jarman effect, which manifested in the increase of sweeping volume and fluidity control capacity in macroscale. Secondly, the larger the gas ratio, the more frequently the foam system is generated, and the greater the density of bubbles in the system, giving the foam system a higher chance of blocking when passing through pores and pore throats.Although the global CO2 storage potential is more than 4 trillion tonnes, if geological sequestration becomes a routine method to reduce CO2 emissions, underground space will be used up. Therefore, improving the CO2 storage efficiency is a key choice to enhance the CO2 storage potential and extend the life of CCUS technology. This study proposed a method to improve the CO2 storage potential by changing the fluid form, which can provide a new idea for the better utilization of underground space.