Molecular Simulation Study On CO₂-philicity Of Surfactants

In the development of oil and gas reservoirs,fracturing is a reservoir modification technology,which can realize the stimulation of oil and gas reservoirs;supercritical carbon dioxide（sc CO₂）fracturing has the advantages of easy flowback,low reservoir damage and low rock breaking threshold pressure,which has gradually become a hot spot in the field of fracturing research;however,due to the extremely low viscosity and weak sand carrying capacity of sc CO₂,its application and development as a fracturing fluid are limited.The research shows that surfactant can be used as an effective viscosity enhancer of sc CO₂fracturing fluid.However,how to modify the surfactant to improve its solubility in sc CO₂,so as to improve the viscosity of sc CO₂ system,is the key problem to be solved in sc CO₂ fracturing.In view of this,the micro behavior of interaction between surfactant and sc CO₂ was studied by using the method of quantum chemistry and molecular dynamics.The influence mechanism of surfactant modified functional groups on CO₂ affinity was clarified.The relationship between surfactant structure and CO₂ affinity was clarified.The research work has a certain theoretical significance for the design of surfactant for sc CO₂ tackifier On the guiding significance.The specific research contents are as follows（1）The key factors affecting the CO₂ affinity of surfactants were revealed.The model of tail chain with different functional groups and CO₂ was constructed.The interaction between tail chain of surfactant and CO₂ was studied by quantum chemical calculation method.The results show that the interaction strength of surfactant with CO₂ mainly depends on its mode of action.Before the addition of functional groups,there is only the interaction between terminal atoms and carbon atoms in CO₂.With the addition of functional groups,the properties of hydrogen atoms adjacent to functional groups change significantly,resulting in a strong positive charge.Therefore,these hydrogen atoms will interact with charged and negative oxygen atoms in CO₂ molecules There is a strong van der Waals interaction between atoms,forming hydrogen bond auxiliary structure,and it is the synergistic effect of these two interaction modes that leads to the enhanced interaction between tail chain and CO₂;it is also found that the binding structure of oxygen-containing functional groups with CO₂ is too close to the tail chain of surfactant,which leads to the poor stability of reverse micelles formed by surfactant self-assembly The oxygen-containing surfactant did not show good properties in sc CO₂.（2）The relationship between the structure of surfactant and its CO₂ affinity was clarified.AOT surfactants with different functional groups were designed by molecular dynamics simulation.Based on the structure and effect of self-assembly,the CO₂ affinity of the surfactants was theoretically analyzed and evaluated.It is found that the type and position of the functional groups modified by the surfactant tail chain will affect the binding structure of molecules with CO₂;the study of the distribution of water and CO₂ shows that tail chain fluorination can improve the ability of reverse micelles to separate water and CO₂;further investigation of the coating behavior shows that both fluorination and oxidation at the end of the tail chain will make the carbon chain conformation more curved,and this conformation will make the reverse micelles more stable However,the conformation of carbon oxide chain is too bent and the thickness of reverse micelles is too low,so the separation ability can not reach the effect of fluorinated surfactants.