Preparation And Properies Of Silicone Thickener For Supercritical CO₂ Thickening System

With the continuous discovery of oil and gas resources in low permeability reservoirs,supercritical CO₂ fracturing technology is gradually receiving attention due to the low cost,wide source and environmental protection.However,a low viscosity of pure CO₂ caused the poor properties such as poor sand-carrying and significant filtration loss,which make it the bottleneck of supercritical CO₂ fracturing,and it is imperative to develop a new thickener with the low-cost and high-thickness.This paper synthesizes a branched ladder siloxane thickener that can significantly thicken supercritical CO₂,and the effect of siloxane on rock adsorption,rheology of fracturing fluid,fracturing property,sand carrying and settling are also investigated.Finally,thickening mechanism of siloxane to supercritical CO₂ is studied by the molecular simulation,and the molecular design of siloxane that can thicken CO₂ thickener was proposed.First,Many silicones with different structure are prepared in this study to evaluate the effect of configurations of these silicones on the CO₂ viscosity at 12 MPa,170 s^-1 and 35 ^oC.The results showed that a silicone named highly branched ladder siloxane（HBPMS）has the best thickening.According to experimental results and molecular simulation,branched siloxanes showed a bigger CO₂ viscosity than straight siloxanes,and CO₂-philic group and excessively branches showed a big improvment of CO₂ viscosity but Aromatic group are not conducive to the CO₂ thickening.Avoiding excessively long branches（C<5）and increasing the number of branches contribute to the improvment of CO₂ thickening.Effects of factors on reaction conversion rate and reaction rate are studied to optimize the synthesis conditions by the Gravimetric and Attenuated Total Reflection Infrared Spectroscopy in the HBPMS preparation.The best preparation process of HBPMS with an maximum conversion is:Ring-opening polymerization is the 90 ^oC of temperature,0.02 g of catalyst,0.6g of Hexamethyldisiloxane and 8 h of reaction time.Hydrosilylation is the 70 ^oC of temperature,50 ppm of chloroplatinic acid,0.4 mol of Tetramethyldisiloxane and 7 h of reaction time.An increasing yield of HBPMS after optimization was 87.5%,which was 15.1%higher than that before optimization.The results of performance investigation of supercritical CO₂ thickening system showed that an excellent shear resistance of the supercritical CO₂ thickened by HBPMS is showed because the CO₂ viscosity thickened by HBPMS decreased by only 0.74 m Pa·s compared with dimethyl silicone oil with increasing the shear rate from 170 s^-1 to 290 s^-1.Viscosity and consistency coefficient increase with increasing thickener content and pressure,and decrease with increasing temperature and shear rate,but an opposite trend of rheological is showed.Langmuir monolayer with an adsorption amount of 3.45 mg·g^-1 is showed to the adsorption of HBPMS on the surface of kaolinite.An increasing sand-carrying performance is showed with the increase of HBPMS content,and the proppant sedimentation amount was significantly reduced with the particle size of proppants below 2 mm.Fracturing simulation based on ABAQUS showed that the fracture half-slit length increased from 36 m of pure carbon dioxide to 73 m with 6 wt.%HBPMS,and the fracture width increased from 4.6 mm to 10.1 mm,which significantly improved the fracturing effect.Molecular simulations have revealed the thickening mechanism of siloxanes for bridging the intermolecular bridges in a supercritical CO₂ system.Compared to 1.80 m Pa·s of dimethyl silicone oil PDMS and 4.03 m Pa·s of branched methyl methacrylate terminated silicone BMMT at 6 wt.%,12 MPa,35°C and 170 s^-1,the CO₂ viscosity thickened by HBPMS can be increased to 7.16 m Pa·s,and the supercritical CO₂ system thickened by HBPMS can be easier to form a denser microgrid structure and showed a better thickening ability than that of PDMS and BMMT.The viscosity and grid density of the thickened supercritical CO₂ increases with the increase of the thickener HBPMS content and the system pressure.But the incresing system temperature and shear rate shows a decrasing viscosity and grid density.The change of shear rate shows the smallest effect on the viscosity and the grid density of the thickened supercritical CO₂ system.