Construction Of CO₂ Sensitive Clean Fracturing Fluid System And Research On Drag Reduction Law

Unconventional oil and gas resources have become the strategic alternative energy of conventional energy.However,due to the strong heterogeneity and deep buried depth of the reservoir,large-scale volume fracturing is generally required to transform the reservoir,and fracturing fluid is an essential working medium in the process of transformation.Clean fracturing fluid has the advantages of no residue,easy flowback,low damage and simple process,but it has the problems of poor drag reduction in practical application.Therefore,it is urgent to develop a temperature resistant clean fracturing fluid system with high drag reduction,and preliminarily clarify its drag reduction mechanism,which is of great significance to the development of unconventional oil and gas reservoirs in China.In this paper,a CO₂ sensitive clean fracturing fluid system was developed.Taking water solubility,responsiveness,viscosity and temperature resistance as indexes,the main agents and additives of fracturing fluid were screened and optimized to construct CO₂ sensitive clean fracturing fluid system,which contains long-chain amidopropyl dimethylamine（OKO）,sodium p-toluenesulfonate（Na Pts）and anti swelling agent（DQ-2）.Rheometer,plate fracture simulator,friction tester and physical simulator are used to evaluate the rheological properties,temperature resistance and shear resistance,dynamic sand carrying performance,drag reduction performance,gel breaking performance,filtration performance,anti swelling performance and core damage performance of clean fracturing fluid.The effects of pipe diameter,concentration,shear time,temperature,water quality and the ratio of main additives on the drag reduction performance of the system were further studied.The viscosity drag reduction rate chart was obtained,and the drag reduction mechanism of clean fracturing fluid was clarified.The results show that 0.70 wt%OKO+1.0 wt%Na Pts+0.30 wt%DQ-2 is the best CO₂sensitive clean fracturing fluid.The system is a pseudoplastic fluid with good temperature and shear resistance（80℃,2 h,20 m Pa·s）,gel breaking（80℃,2 h,1 m Pa·s）and anti swelling（CST=1.38<1.5）,low filtration(<9×10^-3 m·min^-0.5)and core matrix permeability damage（<20%）.The results of plate fracture simulation experiment show that the system is easier to carry fine sand proppant（70-140 mesh）to deep fractures,which is better than viscous slippery water with the same viscosity,and has excellent sand carrying performance.The friction experiment results show that the system has a wide range of drag reduction,and the maximum drag reduction rate is 77.4%under experimental conditions,which has excellent drag reduction performance.The performance of the system fully meets the needs of unconventional oil and gas reservoir fracturing development.The experimental results show that the drag reduction rate of the system decreases with the increase of the pipe diameter at low Reynolds number,and the maximum drag reduction rate increases with the increase of the pipe diameter.In a certain concentration range,the effective drag reduction range and the maximum drag reduction rate increase with the increase of the total concentration.The drag reduction performance of the system has excellent shear resistance and salt resistance;With the increase of temperature（60℃,CO₂ escaping）,the drag reduction rate decreases（>60%）.The ratio of main agent to auxiliary agent has little effect on the drag reduction rate at the same viscosity.Considering the influence of concentration and temperature,the viscosity drag reduction rate chart is drawn to guide fracturing design and operation.The mechanism of drag reduction is preliminarily explored,in which the effective sliding drag reduction mechanism is dominant,and the change of flow layer thickness is closely related to the drag reduction rate.The adhesion of CO₂ gas to the pipe wall can reduce the friction with the fluid,and there is also a synergistic drag reduction mechanism.The spatial network structure and viscoelasticity of the system can reduce the energy dissipation,which provides a reference for further revealing the drag reduction mechanism.