Design,Synthesis And Properties Of Polyethers As Carbon Dioxide Thickening Agent

Enhancing Oil Recovery（EOR）is an urgent requirement for our country to enhance the resource support capability,to improve the level of energy security and to meet the needs of social and economic development.After years of development,CO₂-EOR has become one of the most promising tertiary oil recovery technologys to improve oil recovery.However,in the CO₂-EOR process,due to the low viscosity of CO₂ fluid,CO₂ tents to “finger” toward production wells without more contact with the crude oil in the reservoir,thus decreasing the sweep efficiency.As a result,the displacement efficiency of CO₂ is limited.Increasing the viscosity of CO₂ fluid can reduce the mobility ratio of CO₂-crude oil,and then weaken or even eliminate the viscous fingering phenomenon,thus improving the CO₂ displacement efficiency.Therefore,it is worth exploring and necessary to develop a thickening agent which can increase the viscosity of CO₂ and improve the recovery efficiency.Fluoropolymers and polysiloxanes with large amount of co-solvent are the most effective two types of CO₂ thickening agents that can significantly thicken CO₂.Unfortunately,the practical applications of the two kinds of polymers are limited by their environmental and cost issues.Due to the low solubility in CO₂,other polymers cannot effectively thicken CO₂.The objective of this thesis is to design and synthesize a low-cost,environmentally friendly non-fluoropolymer which is soluble in CO₂ and can effectively thicken CO₂,and this provides theoretical and technical support for the practical application of CO₂ thickening agent.In this paper,polyoxypropylene（PPO）exhibited highest CO₂ solubility among polyether was used as the CO₂-philic basic polymer and the thickening group was introduced into PPO by copolymerization and the thickening performance of the thickening agent to CO₂ were studied.After confirming that the polyether copolymer can thicken CO₂,we have analyzed and discussed the modification method of polyether from the view of thermodynamics.Incorporating a silicon unit into a polyether backbone enhanced its solubility in CO₂.Finally,further thickening modification of the CO₂-philic modified-polyether was performed,so as to achieve better CO₂ thickening effect.We first designed and synthesized polyether copolymer as CO₂ thickening agent based on CO₂-philic propylene oxide oligomer（PPO）.The propylene oxide-phenylglycidyl ether copolymer and the propylene oxide-styrene oxide copolymer were designed and synthesized with poly（propylene oxide）as the soluble fragment and phenyl as the thickening group.In order to promote the polymer dissolved in CO₂,we use acetyl chloride to modify the hydroxyl end-group with philic-CO₂ acetate ester group insteading of phobic-CO₂ hydroxyl group.The effects of benzene on the intermolecular force and chain flexibility of polymer were investigated by surface tension and glass transition temperature.The phase behavior of polyether thickening agent in CO₂ was systematically studied by the cloud point pressure measurement.The effects of the content of thickening group,spacer between thickening group and polymer backbone,polymer molecular weight,polymer concentration and temperature on the solubility of thickening agent in CO₂ were studied.On the basis of the dissolution,the effects of thickening group content,spacer and polymer molecular weight on CO₂ thickening capability.The experimental results show that the designed two kinds of polyether copolymers both can thicken CO₂ at a certain extent.The difficulty of the development of CO₂ thickening agent is that CO₂ is a feeble solvent,and the common high molecular weight materials exhibit low solubility in CO₂.After confirming that the polyether copolymer can thicken CO₂,in order to improve the solubility of the polyether in the CO₂ and provide a better dissolution base for the thickening modification,we have analyzed and discussed the modification method of polyether from the view of thermodynamics.The main factors that limit the solubility of polyethers in CO₂ are the unfavorable enthalpy of mixing,and it is expected that decreasing the polymer-polymer interaction of polyether can improve the solubility of polyether in CO₂.We designed and synthesized the silicon-modified polyether based on the idea that introducing a silicon unit into a polyether backbone.The results of surface tension and glass transition temperature tests show that the silicon unit significantly reduced polyether self-interactions and improved the chain flexibility of polyether,which is beneficial to improve the enthalpy of mixing and mixing entropy.The phase behavior of silicon-modified polyether in CO₂ indicates that the silicon unit significantly improved the solubility of the polymer in CO₂.After comprehensive analysis,we believe that the high solubility of silicon-modified polyether in CO₂ is mainly attributed to the lower polymer-polymer interactions.We designed and synthesized silicon-modified polyether thickening agent to achieve better thickening effect based on polyether modifying method had explored previously.We use bulky heptamethyltrisiloxane as the side group to modify polyether,which is expected to reduce the polymer-polymer interaction and increase the free volume of the polymer to improve the chain flexibility;this is helpful for polyether to dissolving in CO₂.Due to the propylene oxide-phenylglycidyl ether copolymer and the propylene oxide-styrene oxide copolymer both can thicken CO₂,We still select phenyl group as a thickening group into the silicon-modified polyether.First,we designed and synthesized heptamethyltrisiloxane-modified polyether as a thickening agent for CO₂.Second,we studied the effect of heptamethyltrisiloxane on the physical and chemical properties of thickening agent,such as surface tension and glass transition temperature.Then,the phase behavior of silicon-modified polyether thickening agent in CO₂ was studied.Finally,the effect of the silicon-modified polyether on the viscosity of CO₂ was evaluated.Viscosity measurements show that silicon-modified polyether thickening agent has more influence on the viscosity of CO₂ than polyether thickening agent.