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CO 2 -soluble surfactant foam systems have gained significant attention for their potential to enhance oil recovery, particularly in tight oil reservoirs where conventional water-soluble surfactants face challenges such as poor injectability and high reservoir sensitivity. This review provides a comprehensive explanation of the basic theory of CO 2 -soluble surfactant foam, its mechanism in enhanced oil recovery (EOR), and the classification and application of various CO 2 -soluble surfactants. The application of these surfactants in tight oil reservoirs, where low permeability and high water sensitivity limit traditional methods, is highlighted as a promising solution to improve CO 2 mobility control and increase oil recovery. The mechanism of enhanced oil recovery by CO 2 -soluble surfactant foam involves the effective reduction of CO 2 fluidity, the decrease in oil-gas flow ratio, and the stabilization of the displacement front. Foam plays a vital role in mitigating the issues of channeling and gravity separation often caused by simple CO 2 injection. The reduction in gas fluidity can be attributed to the increase in apparent viscosity and trapped gas fraction. Future research should prioritize the development of more efficient and environmentally friendly CO 2 -soluble surfactants. It is essential to further explore the advantages and challenges associated with their practical applications in order to maximize their potential impact.

CO 2 foam injection has been shown to be effective under reservoir conditions for enhanced oil recovery. However, its application requires a certain stability and surfactant absorbability on rock surface, and it is also associated with borehole corrosion in the presence of water. Adding surfactants to CO 2 can enhance the interaction between CO 2 and crude oil and control the CO 2 mobility, thereby improving the performance of CO 2 flooding. This paper presents a review of the research of CO 2 -soluble surfactants and their applications. Molecular dynamics simulation is introduced as a tool for analyzing the behavior of the surfactants in supercritical CO 2 (scCO 2 ). The applications of CO 2 -soluble surfactants, including CO 2 thickening, reducing miscibility pressure, and generating supercritical CO 2 foam, are discussed in detail. Moreover, some opportunities for the research and development of CO 2 -soluble surfactants are proposed.

Since the first outbreak of COVID-19, various interventions have been implemented to prevent the global spread of the virus. Using an agent-based model that describes the attributes and mobility of the Japanese population, the present research evaluates the effectiveness of mobility control, shortening of restaurants' opening hours, and working from home. Results show that early and severe mobility control that restricts 90% of domestic travel decreases the peak cases by 40%, compared to no intervention implementation. Mobility control that only limits movement to and from highly populated regions is as effective as nationwide travel restrictions. Furthermore, shortening of restaurants' opening hours is the most effective intervention in a state of emergency; it should be utilized even after the emergency. However, working from home has comparatively limited effects., (Copyright © 2021. Published by Elsevier Ltd.)

During CO 2 flooding, serious gas channeling occurs in ultra-low permeability reservoirs due to the high mobility of CO 2 . The chief end of this work was to research the application of responsive nanoparticles for mobility control to enhance oil recovery. Responsive nanoparticles were developed based on the modification of nano-silica (SiO 2 ) by 3-aminopropyltrimethoxysilane (KH540) via the Eschweiler-Clark reaction. The proof of concept for responsive nanoparticles was investigated by FT-IR, 1 H-NMR, TEM, DLS, CO 2 /N 2 response, wettability, plugging performance, and core flooding experiments. The results indicated that responsive nanoparticles exhibited a good response to control nanoparticle dispersity due to electrostatic interaction. Subsequently, responsive nanoparticles showed a better plugging capacity of 93.3% to control CO 2 mobility, and more than 26% of the original oil was recovered. Moreover, the proposed responsive nanoparticles could revert oil-wet surfaces to water-wet, depending on surface adsorption to remove the oil from the surface of the rocks. The results of this work indicated that responsive nanoparticles might have potential applications for improved oil recovery in ultra-low permeability reservoirs., (Copyright © 2020 Lai, Zhu, Qiao, Chen, Wang, Tang and Chen.)

A novel functionalized biochar was prepared using corn stalks for efficiently removal and mobility control of Cr(VI). Compared to single modified biochar, iron/zinc biochar (Fe@Zn@HBC) displayed a better removal property and the maximum adsorption capacity of Fe@Zn@HBC reached 138.89 mg·g -1 at pH = 2 and 260 ± 10 Lux illumination. The positive charge on biochar might be the main reason promoting the rapid adsorption of Cr(VI) by electrostatic interaction. Reduction functional groups and photocatalysis (derived from ZnFe 2 O 4 and B-H functional group) provided electrons for Cr(VI) reduction. And the SiO 2 could enhance the reduction process by improving electrons transfer. Finally, FeCr 2 O 4 and ZnFeCrO 4 were formed by coprecipitation and ion exchange. Meanwhile, Fe@Zn@HBC showed a high solid-liquid magnetic separation potential due to the Fe 3 O 4 and ZnFe 2 O 4 . In soil column experiment, Fe@Zn@HBC displayed a superior mobility control on biochar particles and Cr(VI) solution under the external magnetic field., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

In order to improve the CO 2 foam stability at high temperature and salinity, hydrophilic silica nanoparticles (NPs) were added into a dilute zwitterionic surfactant solution to stabilize supercritical CO 2 (SC-CO 2 ) foam. In the present paper, the foaming capacity and stability of SC-CO 2 foam were investigated as a function of NP concentration at elevated temperatures and pressures. It was observed that the drainage rate of SC-CO 2 foam was initially fast and then became slower with NPs adsorption at the gas-liquid interface. The improved foam stability at high temperature was attributed to the enhanced disjoining pressure with addition of NPs. Furthermore, an obvious increase in the foam stability was noticed with the increasing salinity due to the screening of NP charges at the interface. The rheological characteristics including apparent viscosity and surface elasticity, resistance factor, and microstructures of SC-CO 2 foam were also analyzed at high temperature and pressure. With addition of 0.7% NPs, SC-CO 2 foam was stabilized with apparent viscosity increased up to 80 mPa·s and resistance factor up to 200. Based on the stochastic bubble population (SBP) model, the resistance factor of SC-CO 2 foam was simulated by considering the foam generation rate and maximum bubble density. The microstructural characteristics of SC-CO 2 foam were detected by optical microscopy. It was found that the effluent bubble size ranged between 20 and 30 μm and the coalescence rate of SC-CO 2 foam became slow with the increasing NP concentration. Oscillation measurements revealed that the NPs enhanced surface elasticity between CO 2 and foam agents for resisting external disturbances, thus resulting in enhanced film stability and excellent rheological properties., (Copyright © 2019 Li, Wei, Xiong, Ouyang, Shao, Dai, Liu and Du.)

Graphene oxide, nanographene oxide and partially reduced graphene oxide have been studied as possible foam stabilizing agents for CO₂ based enhanced oil recovery. Graphene oxide was able to stabilize CO₂/synthetic sea water foams, while nanographene oxide and partially reduced graphene oxide were not able to stabilize foams. The inability of nanographene oxide for stabilizing foams was explained by the increase of hydrophilicity due to size decrease, while for partially reduced graphene oxide, the high degree of reduction of the material was considered to be the reason. Graphene oxide brine dispersions showed immediate gel formation, which improved foam stability. Particle growth due to layer stacking was also observed. This mechanism was detrimental for foam stabilization. Gel formation and particle growth caused these particles to block pores and not being filterable. The work indicates that the particles studied are not suitable for CO₂ enhanced oil recovery purposes.

The ability of surfactant foam to enhance mobility control and LNAPL recovery in a heterogeneous porous media was investigated through sandbox experiments with p-xylene used as LNAPL. A layered heterogeneous porous media was represented in a 2D sandbox filled with two layers of coarse and medium silica sand. Based on previous tests, the surfactant solution used was Ammonyx Lo at a concentration of 0.1% (w/w). The sandbox experimental program included tracer tests done under both uncontaminated and contaminated conditions, foam injection under uncontaminated conditions and surfactant solution and foam injection under contaminated conditions. Tracer tests indicated that the permeability contrast between sand layers was increased by LNAPL contamination. Foam injection under uncontaminated conditions presented a S-shaped front that indicated a better mobility control than the piston-shaped front obtained during tracer tests. During foam injection, complete sweep of the sandbox was achieved with 1.8pore volume (PV) compared to 2.8PV during tracer injection, thus indicating better mobility control with foam. Pre-flush of the contaminated sandbox with surfactant solution initiated p-xylene mobilization but no free phase was recovered at the effluent. A negligible p-xylene residual saturation was reached following foam injection in the contaminated sandbox. However, mass balance indicated a total recovery of only 48% of the initial p-xylene, thus indicating an underestimation of the recovery by volatilization. Recovery by volatilization was corrected, which gave the following proportions of LNAPL recovery mechanisms: 19% by mobilization, 16% by dissolution and 65% by volatilization. Results show the potential efficiency of foam remediation of LNAPL source zones in heterogeneous porous media. Still, further developments are needed prior to field scale application, which could benefit from in situ foam production that would simplify on-site operations., (Copyright © 2016 Elsevier B.V. All rights reserved.)

In this study, the effect of monopotassium phosphate (MKP) on the reduction in mobility and bioavailability of 2,4,6-trinitrotoluene (TNT) was tested. In the test soil, collected from an active firing range, of which cation binding sites were mostly exchanged with H(+) or Al(3+), potassium ions in MKP exchanged the existing cations and hence significantly increased TNT sorption. In addition, a competitive sorption experiment with hexafluorobenzene and 2,4-dinitrotoluene suggests that TNT was specifically sorbed through cation-polar interaction in the test soil. The unit-equivalent Freundlich sorption coefficient of TNT in MKP-amended soil (1370.96 mg-TNT/kg-soil) was about 13 times higher than that in untreated soil (106.23 mg-TNT/kg-soil). Finally, modified synthetic precipitation leaching procedure and hydroxypropyl-β-cyclodextrin extraction result revealed that MKP application could reduce both the leachability and bioavailability of soil TNT. The leachable and extractable fraction of TNT in untreated soil were 87.63% and 94.47% of the initial TNT, respectively, whereas these fractions decreased to 49.15% and 54.85% of the initial TNT in the presence of MKP, respectively. MKP application can be a benign technology which can reduce both mobility and bioavailability of TNT in soil., (Copyright © 2014. Published by Elsevier B.V.)