With a constant upsurge in energy demand, production from depleted and harsh reservoirs through enhanced oil recovery techniques (EOR) has significantly increased. Among many EOR techniques, chemical EOR (cEOR) is one of the most widely used methods of oil extraction. Surfactants used in cEOR are instrumental in reducing interfacial tension (IFT) and altering the wettability of rock, which leads to additional oil recovery. This review draws attention to detail on surfactants from fundamentals to field scale. Properties of surfactants like phase behaviors, critical micelle concentration (CMC), hydrophilic–lipophilic balance and deviation, zeta potential, and their importance are discussed in depth. The presence of a saline environment, polymer, cosurfactant, and other factors affecting the performance of surfactant during the cEOR process are also elaborated. Key findings on surfactant adsorption on reservoir rock with other influencing aspects have also been reported in this study. Types of surfactants, from basic to the likes of polymeric, viscoelastic, Gemini, natural, and their effects on oil recoveries have been analyzed and compared. Special emphasis on emerging aids for surfactant flooding such as applications of nanotechnology, use amphoteric Janus particles, and synergies of surfactant–low salinity water flooding, along with their mechanisms and recent advances have been thoroughly duscussed. Lastly, the review delineates discerning criteria for the selection of surfactants, reviews recent field applications, and outlines the challenges that the industry faces while implementing surfactant cEOR. It has been found that exhaustive studies have been conducted on sandstones with success. However, extreme temperature and saline conditions in the case of carbonate reservoirs limit the applicability of surfactants, and the pursuit to accomplish its efficacy continues.

Sequestration of CO2 in geologic formations such as depleted oil reservoirs has emerged as one of the lead solutions to tackle greenhouse gas emissions to reduce pollution and global warming. Supercritical CO2 (sc-CO2) injection in oil reservoirs has proven to be useful as an enhanced oil recovery (EOR) technique along with the benefits of CO2 sequestration. In this study, a tortuous microscopic pore scale model was used to study and investigate the phenomena of water-alternating gas (WAG) and surfactant-alternating gas (SAG) with sc-CO2. The study scrutinizes the dynamics of the pore-level phenomenon in the multiphase WAG and SAG flows at the pore level in detail. Transient computational fluid dynamics (CFD) analysis was used to study the fluid flow characteristics of oil, water, and sc-CO2 at different reservoir pressure and temperature conditions in oil-wet conditions. Governing equations were coupled with EOS (Helmholtz free energy equation) to capture the viscous and intrinsic properties of sc-CO2 due to variations in pressure and temperature conditions. It was found that higher oil recovery does not necessarily indicate higher sc-CO2 sequestration and that temperature harms the displacement mechanism due to unfavorable mobility ratios. Comparing WAG and SAG for the first injection cycle, SAG showed a more diffused interface between displaced and displacing fluid. The additional oil recovery produced in patches was a result of pressure oscillations near the blind pores. Moreover, high vorticity promotes greater intermixing between the displacing and displaced fluid by increasing the rate of interface length. In SAG cases, faster sc-CO2 breakthroughs were observed due to reduced shear stress along the fluid interfaces, which resulted in higher sequestration values in a given time frame. The CO2 sequestration volume in SAG cases was found to be approximately 40% more than in WAG experiments. The study confirms that lower values of oil–water interfacial tension aids in faster and more efficient sequestration of sc-CO2 along with additional oil gain from a given reservoir.

Climate change has been linked to industrial and commercial activities caused by exploitation of fossil fuels for energy needs for over a century now. A significant rise in greenhouse gas (GHG) emissions, majorly CO 2 , has been reported in the last few decades. Global climate change necessitates mitigation of atmospheric CO 2 levels to suitable margins using CO 2 capture mechanisms. Direct air capture (DAC) technology is the most efficient way to mitigate or reduce greenhouse emissions by capturing carbon dioxide directly from the atmospheric air. It is imperative that DAC technology be ramped up quickly in order to meet the climate goal to reduce global temperature rise below 2 °C in the next 30 years. This review focuses on the specifics of various techniques, pilot projects, and commercial facilities for DAC technology deployment. DAC has seen substantial technical improvement in recent years, with commercial firms already functioning in the industry with significant upmarket potential. There are about 19 DAC plants in operation across the world, absorbing around 0.01 Mt CO 2 /year. This review article discusses the various DAC technologies used by the companies, future potential, life cycle assessment, as well as the economic viability of worldwide installation of these facilities. The paper provides details on the application of various sorbents including nanomaterials for current advances in DAC. Lastly, the outlook and perspectives are also presented with the concluding remarks.