Molecular Design And Performance Study Of Foam Flooding Systems With Ultralow Interfacial Tension Formed From Multiple Surfactant Mixtures

Enhanced oil recovery (EOR) includs thermal flooding, miscible flooding, chemical flooding, and other micro-organisms flooding. Foam flooding, as a chemical flooding technique, could enhance the oil recovery by improving the sweep efficiency and reducing oil/water interfacial tension, has high potential in technology and also in aplpication. Besides, using the decarbonization flue gas after CO2 being captured from flue gas as foaming gas in foam flooding, not only has important economic significance, but also has important environmental and social implication. The performance of mono surfactant system is proved to be not good enough to meet the application requirement, while the application performance of mixed surfactant systems could be improved in a great degree. Research works focused on the mixed surfactants systems instead of new types of surfactants might be more meaningful in terms of economic and environmental significance. In this thesis, mixed surfactants foam flooding system possessing high foam stabilities using decarbonization flue gas as foaming gas which ensured low interfacial tension was designed successfully, and the mechanism was revealed by combining the experiments and molecular dynamics simulation results. The interaction and interface assembly rule of mixed surfactant molecules at oil/water interface and the gas/water interface was investigated from molecular level, which could contribute a lot to the theoretical basis for the guidance of the formula design.This thesis is divided into four parts:In the first part, the interfacial microscopic molecular properties and synergistic mechanism of mixed surfactants system were studied. By comparing the molecular behaviors of the mixed system and unitary system, it was found that the anionic surfactant have synergistic effect with cationic surfactant and amphoteric surfactant on water/oil and gas/water interfaces. The molecules arrayed more uniform and more dense at the interface, which is conducive to the achievement of high foam stability and low oil/water interfacial tension. The simulation and experimental results revealed-that using the synergistic effect between the surfactant molecules at the interface to reach uniform and dense arrangement is the key approach to complete the low tension foam system.In the second part, the effect of different agent gases (N2, O2, and CO2) on foam stabilities were investigated by combining molecular dynamics simulation and experimental approaches. The experimental results showed that the stability of foam formed from CO2 was very poor, and the stability of foam formed from O2 was also worse than N2 foam stability. The array behaviors of surfactants and the properties of foam films effected by different agent gas (N2, O2, and CO2) was determined by experiments and simulations, and the dissolution and diffusion behavior of different gases in the foam film were studied. The simulation results showed that there is obvious interaction not only between CO2 molecules and the tails groups of SDS but also between CO2 molecules and the head groups of SDS, which made the hydrophobicity of surfactants enhanced, some channels were formed in the interface layer, which enhanced the chance of CO2 molecules dispersing through the foam films. The experimental results showed that the coalescence process of CO2 bubbles was very serious, which corresponded well with the simulation results. For O2 system, the quicker movement of O2 molecules through the foam films was the main reason led to the slightly worse stability of O2 foam.In the third part, the mixed system based on anionic/cationic surfactant mixtures was used to increase the surfactant molecular density on the interface, and the design of low tension foam formulations was completed. The experimental results showed that the anionic/cationic/zwitterionic/nomonic surfactant multiple mixtures not only have better capability to stabilize foam, but also could reduce the oil/water interfacial tension to ultra-low (＜10-4mN/m). Diverse research methods were used to analyze the molecular array behaviors and interfacial density, for example, FT-IR and molecular dynamics simulation. FT-IR was used to detect the gas permeability of foam film, and the simulation method was used to provide detail information in molecular level, for example, the MSD, IFE (interfacial formation energy), and the thickness of interfacial layer etc..In the fourth part, the capability of the low tension foam formulations including the oil displacement efficiency was detected. The static foam stability, the dynamic foam stability, the salt resistance and oil resistance ability of the foam system, the adsorption losses were all detected in this part. It was found that the designed multiple system got ideal properties and high oil displacement efficiency.To summarize, the interfacial microscopic molecular properties and synergistic mechanism of mixed surfactants system were studied in this thesis, the mechanism about how N2, O2, and CO2 influenced the foam properties and the molecular behaviors were investigated, and how to enhance the capability of the mixed surfactant systems stabilizing the decarbonization flue gas foam under high temperature and high salinity was explored. The low interfacial tension foam system with the reduction of the absorption loss at solid/water interface was designed successfully at the end. The mechanism about how ultra-low interfacial tension and high foam stability being achieved at the same time was revealed, which is not noly theoretically meaningful, but also has high potential in the practical application.