Molecular Dynamics Studies On Self-Assembly And Enhencing Viscosity Of Flouranited Surfactants/SC-CO₂ System

With the increasing demand for oil and gas consumption,the unconventional oil and gas resources in China are abundant and have great potential for exploitation.Recent studies showed that the supercritical carbon dioxide（SC-CO₂）has a great advantage in improving oil reservoir conditions and enhancing oil recovery.SC-CO₂ fracturing has a serious of advantages such as the low threshold pressure of rock breaking,rock breaking efficiency,the low damage to the reservoir,can effectively improve the oil recovery efficiency.But,the low viscosity of SC-CO₂ also makes the sand carrying capacity poor and the filtration is high,which greatly limits the application of SC-CO₂.Surfactant in SC-CO₂ can be self-assembled into rod-like reverse micelles,which can improve the viscosity of SC-CO₂ effectively.However,the current research in this area is still less,the self-assembly and morphology control mechanism is unclear.It is difficult to guide the design of SC-CO₂ tackifier.Fluorinated succinate sulfonate surfactant is the best tackifier in SC-CO₂ in rencent research.Therefore,the molecular dynamics simulation method was performed to study the self-assembly mechanism of surfactants in supercritical carbon dioxide and the mechanism of reverse micelle morphology.This work can be help to design tackifier of SC-CO₂ fracturing fluid,and is significant to the exploitation of unconvential oil and gas reservoir.The surfactant/SC-CO₂ self-assembly reverse micelles results showed that:the self-assembly process can be conclude to"three steps assembly mechanism";reverse micelles with core-shell structure is stable,the surfactant monolayer can effectively isolate water and SC-CO₂phase;hydrogen bond network between surfactant headgroups and polar water can weaken the electrostatic repulsion of headgroups;there is a strong electrostatic attraction between the ions and the headgroups,based on witch the saltbridge can effectively shield the electrostatic repulsion between the head groups.The hydrogen bond and salt bridge can make the surfactant molecules impact.Which can enhance the stability of reverse micelles.The surfactant relative converage is the important parameter to evaluate the property of a given surfactants.In this article,the surfactant converage was calculated using molecular dyanmics simulation to calldage the favorable property of fluorinated surfactants.Besides,the stretching of the long tailchain and the bend of the short tailchain can also improve the stability of reverse micelles.The simulation results of shearing system showed that:the self-assembly process can be slowing down with shear condition.The strong shear can destroy the structure of the reverse micelles and hinder the formation of reverse micelles.And the sight shear can make the reverse micelles from ball-like to rod-like.The relationship between the viscosity of the system and the morphology of the reverse micelles showed that the viscosity of spherical micelles was poor.Under the condition of weak shear,the rod like micelles can be cross-linked to form a spatial network structure,and the reverse micelle network can bind the molecular motion in the system,thus thicken the surfactants/SC-CO₂ system.