Preparation And Mechanism Of Supercritical Carbon Dioxide Viscosifiers Based On Host-guest Interaction

Supercritical carbon dioxide（SC-CO₂）is environmentally friendly,easy to diffuse,and has high density,and can be used in technologies such as continuous tubing drilling,which is expected to solve the problems of slow drilling speed and easy contamination of reservoirs in the development of unconventional oil and gas reservoirs.However,the low viscosity leads to poor rock-carrying ability and low swap efficiency of SC-CO₂,and the existing hydrocarbon,perfluorinated and silicon-containing SC-CO₂ viscosifiers have poor viscosifying ability.Therefore,it is of great theoretical significance and application value to prepare high-efficiency SC-CO₂ viscosifiers.Based on the semi-fluorinated structure,SF-ST and TF-ST,fluorinated SC-CO₂ visco-sifiers with different fluorinated structures,were synthesized in this paper using the heptadecfluorodecyl acrylate SF,terrafluorooctyl acrylate TF and styrene ST.Infrared spectra and NMR hydrogen spectra showed that SF-ST and TF-ST are random copolymers with both fluorinated and benzene ring structures.The SC-CO₂phase behavior test experiment showed that both SF-ST and TF-ST had a solubility of 5.0 wt%in SC-CO₂,and the cloud point pressure was below 20 MPa,and the solubility was good.The viscosifying ability of SF-ST and TF-ST at 23 MPa was evaluated by applying a high-pressure capillary viscometer,and the highest viscosity of SF-ST to SC-CO₂ was 3.60 m Pa·s at a temperature of 35°C,which was about 90times.SF-ST and TF-ST overcame the disadvantage of poor viscosity enhancement effect of perfluorinated viscosifiers.In response to the problems of high dosage,high cost and poor environmental friendliness of SF-ST and TF-ST,this paper innovatively proposed the preparetion of SC-CO₂ viscosifier based on the host-guest interaction for the first time.ACD-PPA was synthesized based on the interaction between the host peracetylated-β-cyclodextrin ACD and polyether compound PPA,and the infrared spectra and X-ray diffraction results showed that ACD-PPA was a host-guest inclusion.Through the compatibility experiment,10 wt%ethanol was selected as the co-solvent,and the viscosity of SC-CO₂ was increased 83 times by ACD-PPA at the temperature of 35℃and pressure of 23 MPa,which was much larger than that of single ACD by 9 times and PPA by 5 times,which proved the feasibility of increasing viscosity of SC-CO₂ by the host-guest inclusions.The solubility of viscosifiers in SC-CO₂ is influenced by concentration,temperature and molecular structure.The solubility of fluorinated viscosifiers was better than ACD-PPA,and the viscosity enhancement effect was affected by concentration,temperature,shear rate and molecular structure.At the temperature of 55℃and concentration of 2.0 wt%,ACD-PPA increased the viscosity of SC-CO₂ by 109 times,which was much higher than that of SF-ST by27 times and comparable to that of 5 wt%SF-ST（102 times）.ACD-PPA has more advantages in viscosifying ability,environmental protection and technical economy.The intermolecular interaction forces of the viscosifiers were investigated by contact angle measurement and differential scanning calorimetry,and the molecular structure of the viscosifiers were observed by scanning electron microscopy and transmission electron microscopy,and the mechanism of the viscosifiers was analyzed from multiple angles.The results showed that compared with fluorinated viscosifiers,ACD-PPA had higher surface tension and glass transition temperature,the rigid molecular chains were not easy to move,and the cloud point pressure was higher;meanwhile,the stronger intermolecular forces were favorable for ACD-PPA to form a continuous irregular structure and effectively increased the viscosity of SC-CO₂.