Y. Akkutlu, K. Bui, J.A. Silas, A. Zelenev, H. Saboowala
In enhanced oil and gas recovery, the production of hydrocarbons is influenced by the ability of injected surfactants to adsorb and to modify interfacial tension at different interfaces. It has been demonstrated that surfactant and microemulsion additives enhance oil and gas production to a different extent but the reasons and mechanisms behind the differences in their action are not fully understood. Molecular dynamics simulation of adsorption from surfactant micellar solutions and from microemulsions helps to demonstrate the difference. The simulation shows that the rate of adsorption at a solid surface differs for these two systems and is higher in the case of microemulsion. Furthermore, it shows that the solvent solubilized in microemulsion is transported to the solid surface together with the surfactant, and hence the kinetics and effectiveness of adsorption could be controlled by designing the chemistry and composition of the self-assembled structure. The simulation involves an aqueous solution in the presence of a gas, oil (heptane), and an organic solid wall. The solution consists of the nonionic surfactant dodecylhepta(oxy-ethylene)ether, or briefly C12E7, and a solubilized terpene solvent. We found that the presence of solvent inside the micelles causes the trends in adsorption behavior to deviate from those expected for adsorption from micellar surfactant solutions. In the case of solubilized terpene the swollen micelles adsorb on the surface as one entity. The delivery of a surfactant to the interface and the associated reduction in the interfacial tension takes place at much faster rates controlled by the solvent concentration. Molecular dynamics simulation also reveals the complex distribution of fluids at the interface with the organic wall. The terpene swollen micelle merges with the thin film of hydrocarbon on the organic wall (Fig. 2, middle). The surfactant deposits on the interface between the aqueous and hydrophobic liquid, thereby reducing its interfacial tension. The solvent originally solubilized in microemulsion droplet penetrates the thin film of hydrocarbon (Fig. 2, right). The resulting mixture of hydrocarbon and solvent is expected to have different properties from the hydrocarbon alone, indicating a primary difference between the mechanism of action between surfactant and a combination of surfactant and solvent. The presence of surfactant at the oil-water interface and solvent in the hydrocarbon layer will change the properties of the interfacial hydrocarbon layer and hence may impact the effectiveness of crude oil recovery from the reservoir.