Optimization Of Polymer Nano-Composite Fracturing System And Preparation Of Microcapsule Gel Breaker

At present,most of the reservoirs in China are tight and low permeability,and the fracturing technology has been used more and more widely.The synthetic polymer thickener has some disadvantages,such as poor solubility and poor shear stability.Additionally,during the fracture process of oil production,the preliminary rapid break of hydrogel fracturing fluid will affect the fracturing construction.To solve these problems,polyacrylamide nanocomposite thickener was synthesized on the basis of previous work in our lab and mixed with sodium carboxymethyl cellulose in different proportions as fracturing fluid thickener,with which the drawback of poor solubility and poor shearing property of the thickener was overcome.Secondly,the microcapsule breaker with a acrylonitrile-butadiene-styrene copolymer?ABS?shell was designed and prepared to solve the problem of gel breaking in advance with its controllable sustained-release performance.The formula of the fracturing fluid system obtained by the optimization experiment is?mass percent?:0.3%sodium carboxymethyl cellulose conditioner,0.7%polyacrylamide nano-composite thickener,their cross-linking ratio is 100:1.5,the pH value of the base liquid is 6-7,the dosage of cleanup additive,clay stabilizer and foaming agent all are 0.5%.The experimental results showed that the fracturing fluid system was homogeneous with a high viscosity,the properties of optimized fracturing fluid system were characterized as follow.The average salt resistance was 83.42%.After cross-linking,the pick-up and sand suspension performances were good.At a shear rate of 170 s^-1,the fracturing gel was continuously sheared for 2 hours,its viscosity was 63.2 mPa·s when the temperature rose to 113.1°C.The static filtration factor and the initial filtration loss were 5.73×10^-4 m/min^1/2 and 0.62×10^-3 m³/m²,respectively.The addition amount of gel breaking agent is 0.15%,and the viscosity of gel breaking liquid was less than 5 mPa·s,and the residue content is 68 mg/L.In addition,the gel breaking liquid can realize secondary cross-linking with good picking property.When the compound of Na₂SO₃with ammonium persulfate or NaHSO₃ with ammonium persulfate was used respectively as breakers,the particle size distribution of the gel breaking liquid was about 10 nm which was beneficial to the formation of nanocomposite micelles.A microcapsule breaker with the controllable sustained-release performance was prepared.Ammonium persulfate?APS?was microencapsulated with the shell of acrylonitrile-butadiene-styrene copolymer?ABS?via coacervation process induced by polydimethylsiloxane?PDMS?.The composition structure,micro-morphology,particle size distribution,release performance of product,and factors effect on the products were investigated.The experimental results showed that the APS microcapsules coated with ABS?APS/ABS microcapsules?were micron-grade spheres with a core-shell structure,a controllable sustained-release property and benign breaking performance for the hydraulic gel fracturing system.There were some factors such as the ratio of nuclear to shell,the viscosity of PDMS and stirring rate affecting the particle size distribution,release property of microcapsules and the embedding rate of ammonium persulfate in microcapsules.Once the viscosity of PDMS surpassed 1000 cSt,the embedding rate of ammonium persulfate in microcapsule breaker could reach above 70%.It is mentionable that,all the reagents used in the preparation were recyclable,so,it is an economical and feasible method to solve the problem of gel breaking in advance with the APS/ABS microcapsule gel breaker.