Study On Construction Of Anhydrous Carbon Dioxide Fracturing Fluid And Drainage Law For Tight Oil

China is rich in tight oil reserves,but restricted by its harsh reservoir conditions,it must be exploited by fracturing.As the blood of fracturing,fracturing fluid is directly related to the development effect of tight oil.Due to the huge water consumption of conventional water-based fracturing,it is easy to cause water-sensitive damage to the reservoir and affect the fracturing effect.The anhydrous carbon dioxide fracturing fluid can overcome the above problems effectively because of the non-aqueous components.And it can be miscible with crude oil,so as to reduce the viscosity of oil and improve the fluidity of oil.Anhydrous carbon dioxide fracturing fluids can also add energy to the formation.It is conducive to the geological storage of carbon dioxide,and helps the green development of oil fields.However,the current anhydrous carbon dioxide fracturing fluid has some defects such as low viscosity and poor sand-carrying performance.Therefore,it is imperative to construct an anhydrous carbon dioxide fracturing fluid with high viscosity and excellent performance.In this paper,through PVT phase equilibrium experiment,the thickener and cosolvent of anhydrous carbon dioxide fracturing fluid were screened,and their viscosity was tested by capillary viscometer.Then the formulation of the system was optimized to complete the construction of anhydrous carbon dioxide fracturing fluid.The thickening mechanism of anhydrous carbon dioxide fracturing fluid thickener was revealed by molecular simulation.The performance of fracturing fluid is evaluated systematically.The influence of various factors on the drainage efficiency of anhydrous carbon dioxide fracturing fluid is also studied.Finally,the rules of draining-driving of anhydrous carbon dioxide fracturing fluid to tight oil were analyzed by scanning electron microscopy and dynamic nuclear magnetic imaging.The results show that the formulation of anhydrous carbon dioxide fracturing fluid is: 7wt% Sipzl-3 + 5 wt% ethanol.At-15 °C and 30 MPa,the viscosity of the system can reach 6.52 m Pa·s,which is 37 times that of pure carbon dioxide.Molecular simulation showed that the polymer chain would intertwine with carbon dioxide and the polymer molecules to form a spatial network structure,thus improving the viscosity of carbon dioxide.The oxygen atom in the negative electric center of siloxane is attracted to the carbon dioxide which is obviously concentrated in the positive electric center,which promotes the dissolution of thickener in carbon dioxide.The performance evaluation experiment shows that the viscosity retention rate of anhydrous carbon dioxide fracturing fluid reaches 47.92% when the temperature rises 50 °C.When the pressure rises 25 MPa,the viscosity increases by 2.6 times.The viscosity of the system decreases obviously with the increase of shear rate.In addition,the anti-swelling rate of anhydrous carbon dioxide fracturing fluid reached 90.91%.Compared with pure carbon dioxide,the sand carrying capacity increased by 61.98% and the filtration coefficient is reduced by69.20%.For tight sandstone,the damage rate of core matrix permeability is 18.80%.Because of the acid rock reaction,the core permeability of water-saturated tight sandstone increases by39.23% after aging in anhydrous carbon dioxide fracturing fluid for 30 h.And the wettability of rock changes from hydrophilic to neutral.The anhydrous carbon dioxide fracturing fluid does not need to break glue and does not have solid residue after fracturing,which reduces the damage to the reservoir.The drainage experiments show that the drainage effect of anhydrous carbon dioxide fracturing fluid increases with the increase of the time,temperature,injection pressure,injection volume and fracture complexity,and increases first and then decreases with the increase of fracturing fluid viscosity.Scanning electron microscopy and dynamic nuclear magnetic imaging experiments show that the oil adsorbed on the surface of the core hole is washed off after drainage.And most of the crude oil in the core pores can be used by anhydrous carbon dioxide fracturing fluid during drainage,the drainage process is random.