| In hydraulic fracturing technology,fracturing fluid is mainly used to carry proppant,generate artificial fractures and enhance reservoir conductivity,so its performance is crucial.Polymer fluids such as polyacrylamide and guar gum are commonly used,but they would cause serious formation damage.The clean fracturing fluid with viscoelastic surfactant as thickener has gained great attention due to its advantages of easy preparation,less additives and less formation damage.The traditional VES fracturing fluid mainly uses a single-chain surfactant as a thickener,but its temperature resistance is poor,which is difficult to meet the development requirements of high temperature oil and gas reservoirs.In recent years,Gemini surfactant,which is composed of two single-chain surfactants linked by a spacer,is developed.This method has effectively improved the temperature resistance of clean fracturing fluids,but the temperature resistance of the most Gemini surfactant fracturing fluids are still less than 150?.Since the temperature resistance of the Gemini surfactant combined by the two monomers is significantly higher than that of the single-chain surfactant,we have further innovatively proposed the development of tri-cationic surfactant based on laboratory research,which has a stronger temperature resistance.Erucic acid or oleic acid,N,N'-Dimethyl-1,3-propanediamine and epichlorohydrin are used as raw materials to synthesize two kinds of tri-cationic surfactants(named as T-18 and T-22).The molecular structures of them are characterized by infrared spectroscopy and nuclear magnetic resonance spectroscopy.The results show that the synthesized products are consistent with the target products.The research results of physical and chemical properties show that both T-18 and T-22 have good solubility(the Krafft points are less than 0?),and the critical micelle concentration(CMC)are 1.20×10-4mol/L and 1.44×10-4mol/L,respectively.Both of them are smaller than that of conventional surfactants.In the formulation optimization of the VES fracturing fluid system,potassium chloride and sodium salicylate are preferably used as additives,and the optimal formulation of the tricationic surfactants and additives is determined.It is observed by SEM that there are different degrees of network micelle structures in the VES fracturing fluids of each formula.In the viscoelasticity test,the storage modulus G' of each formula of cleaning fracturing fluid is greater than the loss modulus G",which shows strong elasticity of the VES fracturing fluid.In the rheological study,the applicable temperature of the T-18 system is 140?,while the temperature resistance of the T-22 system is stronger.Specifically,the viscosity of the system with 5%T-22+1.2%sodium salicylate remains at about 42 mPa·s after shearing at 180? and 170 s-1 for 80 min,which satisfies the requirements of the hydraulic fracturing construction.It is determined that 5%T-22+1.2%sodium salicylate is the final high temperature resistant VES fracturing fluid formulation.There is no obvious settlement observed in the preferred VES fracturing fluid after two hours in the static suspension test.In addition,the gel breaking performance of the system is studied.After adding different kinds of(hydrogen,diesel and kerosene)hydrocarbon fluids,the fracturing fluid is quickly broken and there is no residue.In the core damage experiment,the permeability damage rate of the breaking fluid to matrix is only 4.21-6.31%,which is much smaller than the guar fracturing fluid under the same conditions.Therefore,the VES fracturing fluid developed in this paper can meet the fracturing construction requirements of the 180?formation.Until now,this is the highest temperature that the VES fracturing fluid could bear. |
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