Study On Thickening Agents For Supercritical Carbon Dioxide And Their Action Mechanisms

Developing shale gas using SC-CO₂ fracturing has a series of advantages that hydraulic fracturing cannot match,and has become an important direction for shale gas development technology.As fracturing fluid,one of the main functions of CO₂ is to transport the proppants into the fracture and prevent the fractures closure.One of the core technical issues of SC-CO₂fracturing is to increase its proppant carrying capability by increasing the viscosity of CO₂.So studying the action mechanisms of thickening agents in SC-CO₂ and developing the thickening agents successfully is the key to breakthrough SC-CO₂ sand fracturing technology,which has important theoretical significance and application value.The relationship between the molecular structure and properties of polymer was analyzed,and a series of copolymers containing CO₂-philic fluorine monomers and CO₂-phobic monomers were designed.The molecular models of the copolymer-CO₂ systems were established and the molecular dynamics simulations?MD?were performed by Material Studio7.0.The simulation results,such as Intermolecular interaction of copolymer-CO₂ system,cohesive energy density?CED?,solubility parameter,copolymer equilibrium conformation and radial distribution function?RDF?,were analyzed to select copolymers with potential from the alternative thickening agents.The copolymers were synthesized by free radical polymerization,and their composition,molecular weight,surface tension and glass transition temperature were characterized.The“supercritical carbon dioxide phase equilibrium and viscosity measurement experiment”developed independently was used to measure the cloud point and relative viscosity to evaluate its solubility and thickening capability.The experimental results and MD resluts were combined to study the action mechanism,and analyze the relationships between the structure and the performance of the copolymers in SC-CO₂.Compared with the copolymer,Poly?1H,1H,2H,2H-Heptadecafluorodecyl acrylate??PHFDA?lacks inter-chain association due to the weak polymer-polymer interaction induced by fluorine repulsion,and its thickening capability is poor.Studies of the HFDA-linear ester copolymers show that the thickening performance of the copolymer increases with the increase of the CO₂-phobic group content,although its solubility decreases.The CO₂-group in the copolymers facilitate the increase of the polymer-polymer interaction and enhance the CO₂viscosity through inter-chain association.At 5 wt%concentration,P(HFDA_0.26-co-EAL_0.74),P(HFDA_0.28-co-MMA_0.72)and P(HFDA_0.49-co-VAc_0.51)could enhance the viscosity of SC-CO₂by 87,51 and 62 times,respectively.The thickening performance of the branched P(HFDA_0.49-co-VPi_0.51)could only increase the viscosity of SC-CO₂ by 24 times at most.This could be attributed to the high free volume and steric hindrance parameters caused by the tert-butyl group,which reduces the polymer-polymer interaction and leads to the weak intermolecular association.Studies of the HFDA-VBe?Vinyl benzoate?and HFDA-St?Styreen?copolymers show that both copolymers could increase the viscosity of SC-CO₂ significantly.At concentration of 5wt%,P(HFDA_0.70-co-St_0.29)and P(HFDA_0.67-co-VBe_0.33)increase the viscosity of SC-CO₂ by352 and 438 times,respectively.The?-?stacking between phenyl groups plays a key role.The?-?stacking provides the driving force for the inter-chain association of the copolymers,thereby enhancing the viscosity of the CO₂.The thickening performance of the copolymers do not change with the increase of the phenyl content monotonically,and there is an optimal value.The copolymers with lower phenyl groups exhibit less inter-chain association,while the copolymers with too high phenyl content show more intra-chain?-?stacking and less inter-chain?-?stacking.According to the research results above,we further carry out the design and optimization of the copolymer thickeners,and synthesize a fluorinated acrylate-methyl acrylate-styrene copolymer.The results show that the P(HFDA_0.32-co-MA_0.28-co-St_0.40)could increase the viscosity of SC-CO₂ by 846 times at the concentration of 5 wt%.It is much higher than the 411times of P(HFDA_0.68-co-St_0.32)and the 31.5 times of P(HFDA_0.69-co-MA_0.31).According to MD simulation results,the strong thickening capability of P(HFDA_0.32-co-MA_0.28-co-St_0.40)strong thickening capability originate from the?-?stacking between the phenyl groups and the decrease of intermolecular repulsion caused by the reduction of the HFDA group content.Compared to the thickening agent with the best performance reported abroad—PHFDA-0.299PSt,the P(HFDA_0.32-co-MA_0.28-co-St_0.40)increases the thickening effect by 111.5%and the molar content of fluorinated group decreased by 54.3%.The associative groups in the copolymers make it possible to increase the viscosity of SC-CO₂ effectively through intermolecular aggregation.The CO₂-philic group contributes to the dissolution of the copolymers in SC-CO₂.But the repulsion between HFDA groups is not conducive to the thickening capabilities of the copolymers.The optimization the content of different groups in the copolymers is benefit to increase their thickening capabilities.The solubility and thickening capability of PDMS in CO₂ modified by toluene are evaluated and the action mechanisms of toluence as co-solvent are studied.The results show that the solubility parameter of CO₂ increase significantly and become close to the PDMS with the addition of toluene.Toluence makes CO₂ have better dissoving capability.The cloud point pressure of the systems do not change with the increase of the concentration of the polymer when the mass concentration of toluene reaches 5wt%and above 5 wt%.The co-solvent improves the capability of CO₂ as solvent,so that the PDMS could dissolve in SC-CO₂ and form cross-linking system better.The PDMS with molar mass of 139000 could increase the SC-CO₂ viscosity by 53 times at 5 wt%.