Synthesis And Properties Of Hightemperature Controlled Release Microcapsule Breaker For Oilfield Fracturing

Hydraulic Fracturing technology is the main measure to improve the production of low permeability oil and gas wells.Fracturing fluids are important medium aiming to form and propagate fractures,transport and place proppants in the formation.The encapsulation technology of breaker is an important means to achieve high temperature application of breaker.With the increase of reservoir temperature,traditional persulfates have been difficult to meet the application requirements in high-temperature wells.It has become an urgent need to develop gel breakers suitable for high temperature fracturing.In this article,high-temperature controlled release microcapsule breaker was synthesized by the O/W mini-emulsion polymerization.Amphiphilic macro-RAFT agent poly(acrylic acid-b-butyl acrylate)(PAA_x-b-PBA_y)prepared by reversible addition-fragmentation chain transfer(RAFT)polymerization as emulsifier,methyl methacrylate and butyl acrylate copolymers which are degraded at high temperature were used as shells,microcapsule coated with CHP was prepared by the O/W mini-emulsion polymerization.The effects of comonomers,different types of initiators,dosage of emulsifier,and hydrophilic and hydrophobic chain lengths of emulsifiers on the particle size,morphology,and stability of the microcapsules were studied.The release performance of encapsulated CHP was tested by dialysis.the factors affecting the increase of CHP embedding rate and high temperature release rate,and the reduction of low temperature release rate were discussed.The optimal preparation conditions of microcapsules were determined through optimizing the experimental conditions.Experiment results showed that stable and uniformly dispersed microcapsule particles could be prepared under the condition that the emulsifier was dissolved in the O phase,the dosage of the emulsifier was 30 mg/m L and appropriate chain lengths of hydrophilic and hydrophobic segments of block copolymer PAA_x-b-PBA_y(x?10,y?40)were selected,respectively.A stable microencapsulated CHP dispersion was prepared by the external phase initiation method,with a particle size between 500 nm and 800 nm.The addition of Na Cl,the hydrophobic monomer butyl hexafluoro-acrylate and the solvent chlorobenzene during the mini-emulsion polymerization can greatly increase the embedding rate of CHP,reduce the release rate at room temperature,and improve the controlled release effect of encapsulated CHP.Under optimized conditions,the embedding rate of CHP reached 96.6%,and the release rate at room temperature at 6 hours was as low as 10.6%.When the temperature was increased to 100?,the release rate of the core material CHP reached45.6%after 12 h,which provided a basis for the subsequent gel breaking experiments.Then,the viscosity of partially hydrolyzed polyacrylamide(HPAM)was measured by rheometer to evaluate the gel breaking performance of microcapsules.Microencapsulated CHP prepared under optimized conditions was used to evaluate the breaking performance of actual fracturing fluids.At high temperature,viscosity retention rate of the fracturing fluid was more than 90%within 2 h under the action of microcapsule breaker(0.25%)/urea(0.0375%)system.It was expected to be able to maintain the early stability during the fracturing process.After 6 h,the viscosity retention rate was below 38.7%,showing a certain delayed breaking performance.However,the gel breaking ability was still insufficient,and the gel breaking accelerator needed to be further optimized to meet the needs of high temperature fracturing.This research provided basic data for the development of high-temperature breakers.