Molecular Dynamics Simulation Of Foaming And Liquid Film Drainage In Multiphase Medium

Tertiary oil recovery has become an important method to greatly improve crude oil recovery and ensure the sustainable development of old oilfields.Clear mechanism and high recovery efficiency are the advantages of chemical flooding technology represented by alkali/surfactant/polymer(ASP)flooding.However,it is easy to form a large number of foams with different stability in the gathering and transportation system,due to the flow disturbance of the produced fluid,the precipitation of dissolved gas and the return of the oil displacement agent,which directly affects the accurate measurement of crude oil and the standardization of oil and water treatment,as well as brings new challenges to the safe and stable operation of the oilfield system.More importantly,not only the complexity and diversity of the liquid phase and gas phase,but also the existence of oil in oilfield produced multiphase medium,make the formation and stability mechanism of foam more complex,which directly affects the development and application of defoaming and anti-foaming technology.Therefore,in this paper,the treatment of ASP-produced fluid is taken as the engineering background,and different types of aqueous and oil-based foam systems are considered respectively.Considering the distribution of oil displacement agent molecules on the film surface,the molecular dynamics simulation method was used to reveal the influence mechanism of component type and interaction on the stability of foam film in liquid phase.The microscopic parameters such as radial distribution function,diffusion coefficient,interfacial formation energy and molecular tail-chain angle were used to characterize the film discharge and Ostwald ripening.Overall,considering the mechanism of molecular copolymerization and inter-component synergy,the stability of foam film under different factors was studied.The results showed that compared with monovalent cations,divalent cations were more likely to combine with the SDBS head group,reducing the aggregation degree of monovalent cations at the gas-liquid interface of film,and enhancing the stability of film in aqueous foam system.Although the presence of HPAM molecules can reduce the formation ability of foam film,the electrostatic interaction between HPAM and cations enhances the interfacial activity of the co-adsorption layer.HPAM promotes the filling of'holes'on the surface of the foam film,which weakens the permeation of gas molecules into the film.The ability of film co-adsorption layer to bind'bound water'molecules and the stability of foam film are enhanced due to the electrostatic attraction between OH^-and cations in film of aqueous foam system.SDBS,HPAM and OH^-can play a synergistic effect,weaken the electrostatic shielding of the polar head group of SDBS,enhance the interfacial activity of SDBS molecules,and reduce the electrostatic shielding of-COO^-groups on the molecular chain of HPAM,so that the film drainage slowed down due to the stretching of HPAM adsorption configuration.Otherwise,with the increase of SDBS concentration,HPAM concentration and p H value,the stability of foam film is enhanced.At the same time,compared with the co-adsorbed polymer molecules,surfactant molecules not only play a synergistic aggregation role but also dominate the overlapping staggered arrangement of adsorbed layer molecules on the film surface.Compared with CH₄ and C₂H₆,the aggregation of gas molecules at the gas-liquid interface of the C₃H₈ foam system is the strongest,and the Ostwald ripening caused by the aggregation of gas molecules is still the key to the stability of foam film.The carbon number of chain alkane molecules in crude oil increases,the permeability of gas molecules to oil-based foam film decreases,and the influence of aromatic molecules on the foam film drainage rate is greater.The study results further enrich the understanding of foam film stability mechanism,and provide a theoretical basis for the targeted development of defoaming and anti-foaming technology in the process of ASP flooding production fluid gathering and transportation in the oilfield.At the same time,it provides a useful reference for the comprehensive application of foam in related fields.