Investigation On The Self-assembly And Enhancing Viscosity Mechanism Of Quaternary Ammonium Gemini Surfactants

In the fracture stimulation of low permeability reservoirs,a clean fracturing fluid system formed by surfactant self-assembly has become a focusing topic due to its unique advantagerous of easy-back,no residue and low harmness to reservoirs,etc.In order to adapt the local demanding of reservoir mining in China,developing of new surfactant used in fracturing fluid with specialized features of shear-tolerate,low concentration for forming viscosic micelle and high economic value will be the main issue.However,the prevalent route to develop such surfactant is based on abundant ‘trial-and-error',a long period and giant cost is demanded.Thereby,a theory is urgently needed to develop for researching and designing the new clean fracturing fluid with high efficiency.In this thesis,adopting molecular simulation combined with experimental method,the behavior of self-assembly and viscoelasticity of Gemini surfactants solution were investigated.The self-assembly and enhancing viscosity mechanism of Gemini surfactants were uncovered via investigating the structure-activity relationship between molecular structure and self-assembled morphology,and the regulatory mechanisms of wormlike micelles by organic salts.Based on above research,the theoretical evaluation method for the viscoelasticity of surfactant solutions was proposed.These studies have provided an important theoretical guidance for researching and designing the thickening agent which used as fracturing fluid.The main research results in our studies were list as following.The assembly morphologies and viscoelasticity of surfactant solution are largely determined by the self-assembly structure of the surfactants.The coarse-grained molecular dynamics(CG MD)simulations approach was implemented to investigate the self-assembly of Gemini surfactants with different spacer group lengths.Both the microscopic dynamics processes and the driving force for the formation of different morphologies were systematically studied,and the influence mechanism of the spacer group lengths on Gemini surfactant aggregate morphologies was revealed.In addition,based on the microscopic analysis,an effective and practical strategy was proposed to predict the self-assembled morphology of surfactant-systems based on simulations.Organic salt has an important effect on viscoelasticity of surfactant solution.Both the experimental and simulated analysis were made to investigate the effect of the organic salt on the viscoelasticity of Gemini surfactants solution and its effect mechanism,meanwhile the conventional single chain surfactants was also employed for comparison.As the increase of organic salt concentration,the binding energy of micelles fusion and the binding energy of branch formation were decreased gradually.It caused the self-aggregates varies from spherical micelle to worm-like micelle,to branch micelle and then to net structure.Eventually,this led to the viscosity of surfactant solution increased at first,decreased later,and then increased.Compared with single chain surfactant,wormlike micelles formed by Gemini surfactant were more difficult to break in the uniaxial tension process owing to the higher scission energy of single wormlike micelles,so the average length of wormlike micelle and zero shear viscosity of Gemini systems were bigger.Based on above research,the theoretical evaluation method for evaluating the branched micelles forming ability and the wormlike micelles length were proposed.The formation of branched structures will greatly reduce the viscosity of surfactant solution.Adopting CG MD method,the formation micro-process,the formation mechanism of branched micelles were studied.A gross conformation diagram of micelle at different surfactant concentrations and molar ratios of surfactant/NaSal indicated that the branched structure was formed at low zeta potential.The dynamic process in branched micelles formation displayed four sequential stages,approaching,forming salt bridge,yielding stalk,and coalescing together.Our analysis found that the forming salt bridge is a key to the formation of branched micelles.From this,a strategy was proposed to reduce the formation probability of branched micelles,viz.by increasing the adsorption intensity of the organic salt on the micelle surface,the formation probability of branch micelles was reduced.By adopting the above prediction methods,viz.the prediction method of self-assembly morphology,the prediction method of the wormlike micelle length and evaluation method of the branched micelles forming ability,the influence of mixing ratio on viscosity of mixed cationic trioctylmethylammonium bromide(OTAB)and anionic 2-(N-erucacyl-N-methyl amido acetate(EMAA)surfactants was predicted.The prediction results were validated by experiments.Furthermore,the changing mechanism of viscosity with different mole ratios of OTAB/EMAA was uncovered by CG MD simullations,and it was found that dual effects of cationic surfactant on the self-assembly of catanionic surfactants mixtures.