| In recent years,the rapid growth of the number of high-temperature deep wells in China is not only a challenge to development technologies such as drilling and completion,but also a higher requirement for fracturing technology.The cross-linking agent is an important additive that directly affects the temperature resistance of the fracturing fluid,so the choice of crosslinking system is particularly important.Since the organic boron crosslinking agent could not withstand the high temperature of the formation,it is generally suitable for medium and low temperature reservoirs below 120?.Therefore,it is of great significance to develop organic titanium and zirconium crosslinking agents with better temperature resistance.Nano-titanium dioxide was synthesized by isopropyl titanate,benzyl alcohol using sol-gel method.A small amount of water and hydrolysis inhibitor were added.The synthesis conditions were optimized by single factor experiment:benzyl alcohol:deionized water:hydrochloric acid(37%(w/w))=100:3:3(volume ratio),the concentration of isopropyl titanate was 0.13 g/mL,the reaction temperature was 80?,and the reaction time was 60 minutes.The synthesized products were characterized by X-ray diffraction,infrared spectroscopy,UV-visible diffuse reflectance spectroscopy and zeta potential.The results showed that the anatase phase titanium dioxide was successfully synthesized.The particle size distribution and scanning electron microscopy showed that the nano-titanium dioxide was spherical and granules.The diameter distribution is relatively uniform.The nano titanium dioxide is crosslinked with hydroxypropyl guar gum under acidic conditions,and the optimum cross-linking pH is 2,the optimum crosslinking amount is 4%(v/v),the viscosity of the gel decreased to 50 mPa·s after the shear of 170 s-1 and 60? for 50 minutes.The microstructure of the gel was observed by scanning electron microscopy,the network of the gel was loose,the crosslinking strength was weak.Based on the synthesis of nano-titanium dioxide,two modified nano-titanium dioxide cross-linking agents were prepared by organic ligands.Based on 5%(w/w)nano-titanium dioxide,the synthesis conditions of cross-linking agent LM were optimized:10 mmol PT-1 was added,the reaction was at 60? for 60 minutes;the synthesis conditions of the cross-linking agent YS were optimized:10 mL of the ligand PT-2 was added,and the reaction was carried out at 90? for 6 hours.The two modified nano-titanium dioxide cross-linking agents are crosslinked with HPG under alkaline conditions,the optimum cross-linking pH is 10,and the optimum dosage of cross-linking agent LM is 8%(v/v),the optimum dosage of the crosslinking agent YS was 4%(v/v).The effect of different cross-linking pH values on the viscoelasticity of the gel was tested and analyzed by HAAKE rotary rheometer.The results showed that the gel viscoelasticity was the best at pH 10.Scanning electron microscopy was used to analyze the network structure and cross-linking strength of cross-linked gel of different cross-linking agents.The micro-morphology of inorganic titanium gel and cross-linking agent LM gel was compared by atomic force microscopy.According to the oil and gas industry standard "SY/T 5107-200 water-based fracturing fluid performance evaluation method",the performance of two fracturing fluid system of two modified nano-titanium dioxide cross-linking agent was evaluated.The HAAKE rheometer was used to test the resistance of temperature and shear of the fracturing fluid system.The viscosity of the cross-linking agent LM fracturing fluid system maintained at 50 mPa·s after the shear of 170 s-1 and 140? for 2 hours.The YS fracturing fluid system maintained at 80 mPa·s after the shear of 170 s-1 and 170? for 2 hours.Ammonium persulfate was used as a breaker,the fracturing fluid system can be broken at a certain temperature,the sand carrying performance of the system is good,and the core damage rate of the two cross-linking agent fracturing fluid systems were evaluated. |
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