Effect Of Inorganic Salts On Foam Stability Of Surfactant And Its Composite Systems

Foam flooding is a promising tertiary recovery technique due to its contribution to EOR.Foam system can substantially expand the swept volume and increase the oil displacement efficiency due to its ability of water plugging and profile control.Foam is a thermodynamic instable system,its stability is a key factor that restrict the development and industrial application of foam flooding.A large number of pilot tests and laboratory studies have shown that inorganic salts are one of the most important factors that affect foam stability.During the flooding process,a combination of different kinds of surfactants or surfactants and polymers is often used to improve foam stability.Effect of inorganic salts on foam stability of surfactant composite system is under the way of research.However,studies of inorganic salts on foam stability of surfactant polymer composite system have rarely been reported.Further more,most of the researches have been focused on the regularity of foam stability,and the understanding of the microscopic mechanism is still insufficient.Therefore,the microscopic mechanism of the effect of inorganic salts on the stability of the composite foam system was studied by experiments and molecular simulations.Firstly,several kinds of surfactants were selected as foaming agents,inorganic salts with high content in the formation are used as additives,and foam is produced by venting method.The influence of the type and concentration of inorganic salt ions on stability of the composite foam systems was studied by investigating the half-life time of foam volume and the surface tension of foam liquid.The results show that the addition of sodium salt had a negative impact on the stability of surfactant foam,and a higher concentration lead a lower foam stability.Low concentration of calcium and magnesium ions can improve the stability of foam in a certain extent,while high concentration will reduce the foam stability.The effect of inorganic salts on the composite foam stability at a high ions concentration declines in the order Na⁺>Ca²⁺>Mg²⁺.Secondly,a series of foam film models contain pairwise surfactants of different proportion and inorganic salts were constructed.After the dynamics calculation,we investigated the microscopic mechanism of the influence of inorganic salts on the composite foam stability at the molecular level by analyzing the adsorption configurations of surfactant,the interaction between surfactants and water molecules and the diffusion behavior of the bound water molecules.The results show that Mg²⁺can enhance the interaction of surfactant headgroups with water,restrict the spread of bound water,centralize the distribution of surfactants at the gas-liquid interface and reduce the surface tension,which is conducive to the stability of composite foam.Na⁺and Ca²⁺reduce the interaction strength of surfactant and water,lower the ability to bound water,and increase the surface tension of liquid film.Therefore,it is harmful to the stability of composite foam.Thirdly,in order to investigate the influence of inorganic salts on the stability of composite surfactant polymer foam,a series of foam systems were also generated by venting method.Foam volume decay process,liquid drainage process and the surface tension were studied.The results show that inorganic salts have great influence on the stability of ionic surfactant polymer foam system.At a low concentration,salt ions tend to surround the surfactant headgroups and weaken the electrostatic repulsion between them,which is conducive to enhance the foam stability.At a high concentration,the salt ions have a negative impact on the stability of foam.However,inorganic salts have little influence on the stability of nonionic surfactant polymer foam system.At a low concentration,the weak interaction between salt ions and surfactants or polymers has little effect on foam stability.At a high concentration,the interaction of salt ions and polymer charged groups decreases the number of bound water molecules,which can reducethe viscosity of foam liquid and is not detrimental to the stability of foam.Finally,we constructed the surfactant polymer composite foam film models by adding the polymer molecules on the basis of the single surfactant foam film models.After the dynamics calculation,we investigated the microscopic mechanism of the influence of inorganic salts on the surfactant polymer composite foam stability at the molecular level by analyzing the adsorption configurations of surfactant,the interaction between surfactants,polymers,water molecules and salt ions.The results show that the interaction of divalent ions(Ca²⁺and Mg²⁺)with surfactant headgroup is stronger than that of monovalent salt ion?Na⁺?,the order of influence on foam stability is CaCl₂>MgCl₂>NaCl.With the increase of inorganic salts concentration,the tilt angle of the tail chain and the Z axis of surfactant decreases gradually,which leads to the decrease of the surfactant monolayer stability.At the same time,the number of water molecules around the surfactant and the polymer decreases gradually,which leads to the decrease of viscosity and foam stability.