Molecular Simulation Study Of The Stability Of Liquid Film In Foam Flooding

Because of the reduced increment of proven oil reserves and the increased demand for fuel per year, developing the enhanced oil recovery methods is necessary. Foam flooding is a promising enhanced oil recovery method after polymer flooding. Foam is a disperse system, consisting of gas bubbles, separated by continued liquid phase (film and plateau boarder). Its stability is maintained by the surfactant monolayer at the gas/liquid interface. Foam can be used as a displacing agent because the displacement process can be stabilized due the high viscosity of foam; On the other hand, foam can be used as a blocking agent, as to block high-permeable swept zones and divert the displacing fluid into the unswept zones, as a result, the oil recovery is increased. The key for increasing the oil recovery is the stability of foam. The temperature in the oil reservoir is higher than ambient temperature, and there are crude oil and brine in the reservoir, which may influence the stability of foam. In this thesis, classical molecular dynamics simulations are used to study the influences of temperature, oil and salt to the microscopic structure of foam film and the stability of foam. Due to the importance of stable Newton black films (with thickness less than 5 nm) to foam stability, the microscopic structure of Newton black film and the influence of salt to it are studied. The dynamics property of Newton black films is also analyzed.First, as the temperature in the reservoir is often over 50℃, the microscopic structure of foam film in the range from 25℃ to 75℃ is studied. As temperature increases over 55℃, sodium dodecyl sulfate monolayer transits to a gaseous phase. The adsorption of surfactants in the gaseous phase at the interface is weaker as compared to that in the liquid-expanded phase, unfavorable for the stability of foam. The looser structure of surfactants in the gaseous phase is hard to resist the fast evaporation of water and permeation of gas at high temperature, so that foam is easy to break off.Second, the diffusion of oil droplets into the foam film probably harms the stability of film. The diffusion of an oil droplet is possibly influenced by the hydrogen bond structure and dynamics of water next to the oil droplet surface. In this thesis, the hydrogen bond exchange mechanism next to the oil droplet surface is studied in detail. At the oil/water interface the retardation of water dynamics is small, and at the second hydration shell of the oil droplet water dynamics is almost the same as bulk water. So the influence of interfacial water dynamics to the diffusion of an oil droplet is small. However, hydrogen bonds at the oil/water interface can be broken easily, leading to the quick formation of free OH bonds. As a result, the interface is negatively charged, maintaining the stability of the oil droplet inside the foam film to some extent.Third, cations (Ca2+, Mg2+and Na+) in the reservoir may influence the structure of film and foam stability. With the existence of alkaline-earth-metal salt (MgCl2 and CaCl2), the interfacial bending modulus is decreased leading to the increased interfacial fluctuation. Large interfacial fluctuation helps stabilize the stability of thick film, but stable Newton black film with thickness less than 5 nm is hard to be formed. However, the existence of alkali-metal salt (NaCl) does not change the interfacial fluctuation. Because of the electrostatic screening effect, with the existence of alkali-metal salt the disjoining pressure of foam film is decreased, so that thinner Newton black film would be formed more easily. However, as the thickness of a Newton black film is less than certain value (more or less 1.5 nm), the thermodynamic stabilities of films with and without salt are almost the same. So the influence of salt to the stability of foam film not unique.In the end, because in kinetics the stability of foam film is largely influenced by the film drainage rate, water dynamics inside the film and its influencing factors are studied. The solvation of surfactants and counterions and the electrostatic field across the film concertedly retard the film drainage. The high temperature in the reservoir enhances water dynamics, beneficial to film drainage, unfavorable for the foam stability. Oil droplets have little influence on the dynamics of water in the foam film. Salt ions retard water dynamics, weakening film drainage, beneficial to the stability of foam. In practice, increasing the adsorption of surfactants at the interface can increase water solvation and the strength of the electrostatic field, so that the film drainage is retarded and the foam stability is strengthened.