The Self-assembly And Viscosified Mechanism Of Surfactants In Supercritical CO₂

The technology of supercritical carbon dioxide（sc CO₂）fracturing has the advantages of small formation damage,strong conductivity,and high oil and gas recovery.It has widespread concerned in the field of development of unconventional oil and gas reservoir.However,the low viscosity of sc CO₂ leads to the poor capacity of proppant carrying and high fluid loss in the reservoir.Adding surfactants and self-assembling into a rod-like or wormlike reverse micelle can effectively enhance the viscosity of sc CO₂.But,there are two problems that limit the use of this method.First,the solubility of surfactants is usually very low in sc CO₂.Then,the Morphologies of rod-like reverse micelle are difficult to generate.Therefore,in this work,we studied the self-assembly and viscosified mechanism of surfactants in supercritical CO₂ by using molecular dynamics and quantum chemistry.The fragments that control solubility and control self-assembly morphologies on surfactant have been distinguished.Further,the effects of functional group properties and packing behavior of surfactants on solubility were clarified.And the effects of inter-molecular interaction and structure of surfactants on self-assembly morphology were revealed.The potential method of design rod-like reverse micelles is proposed.The research results are shown as follows:（1）The behavior and structures of surfactant self-assembly were studied.We found that,in reverse micelles system,the self-assembly is driven by the hydrogen bonds of waters and water and hydrogen bonds of water and surfactants,which is different from the entropy-driven mechanism of micelles system.Moreover,the distribution of CO₂ illustrated that the terminal group of surfactant tails affected the surfactant CO₂-philicity,significantly.Further studies on the aggregation of surfactant behavior in reverse micelles system supposed a theoretical method that predict the aggregation number of surfactants.We can infer that the existed aggregation number is originated from the equipotential accumulation between head-group and tail terminal.A potential well appeared between the midpieces of two surfactants,which indicated that the midpiece has little effect on the stable of RMs.Consequently,we suggested that designing a tackified surfactant could be divided into two steps:（1）Designing the tail terminal ensure the solubility of surfactants.（2）Designing the midpiece control the self-assembly morphologies of surfactants.（2）The key factors of surfactant CO₂-philicity were studied.（1）The studies on interaction indicated that all of the oxidative functional group,fluorinated functional group and methyl group can enhance the interaction between surfactants and CO₂.However,they have different type of interaction.The structure of dimer between oxidative functional surfactant and CO₂ is disadvantageous to that stability of self-assembly reverse micelles.And the methyl group has little effect on enhancing the interaction between surfactants and CO₂.Only fluorination is beneficial to the stability of the reverse micelles formed by surfactants in sc CO₂.Furthermore,we found that the binding strength of dimer between surfactant and CO₂ depends on how it binding.The more complex binding is,the higher binding energy will be.（2）The studies on surfactant covering indicated that the conformations constrained by hydrogen bond between the fluorinated terminal and the water molecules are beneficial to the surfactant covering on RMs.The studies on the hybrid surfactants indicated that shortening hydrocarbon chain could reduce the volume of surfactants and thus decrease the ability of separation for water and CO₂.But,shortening hydrocarbon chains length also curve fluorocarbon chains conformation to increase the ability of separation for water and CO₂.（3）The relationship between midpiece and self-assembly morphologies of surfactants was studied.We found that enhancing the interaction among the midpieces of surfactants can transform the spherical reverse micelles to rod-like morphology.Further,we introduce the phenyl to midpiece,and then the self-assembly morphology appeared sphere-to-rod transformation.Further analysis indicated that,originated from theπ-πinteraction,the introduced phenyl facilitated the parallel arrangement of surfactants,thus resulting in the improved fusion ability of reverse micelles.This is the primary reason of the sphere-to-rod transformation.Moreover,we designed a rod-like reverse by using hydrocarbon surfactants.The studies were taken for investigating and designing the shape of hydrocarbon surfactants self-assembly with inducing the co-surfactants.Adjusting the affinity of co-surfactant,a wormlike reverse micelles with a beads-on-a-string structure was finally generated with a multiple structure.