The Study Of Engineering Characteristics And Molecular Dynamics Simulation On Viscoelastic (VES) Fracturing Fluids Modified By Nanoparticles

Viscoelastic surfactant(VES)fluids have been widely used as fracturing fluids due to the excellent rheological properties and low-formation damage characteristics compared with the polymer fracturing fluids.However,the viscosity of VES fluids would decrease dramatically at high temperature,which limit the applications.In this paper,nanoparticle,especially pyroelectric nanoparticle was added into VES fluids to enhance the viscosity at high temperature.Further,molecular dynamics simulation was performed to analyze the interaction between nanoparticles and micelle,and study the molecular mechanisms.The main researching results are shown as follows:1.The effect of nanoparticle materials,nanoparticle concentrations and temperature on rheological properties including zero-shear viscosity and viscoelasticity of VES fluids were studied.The adding of nanoparticles can effectively enhance the viscosity at 30?60?,because that moderate nanoparticles can attract surfactant micelle to strengthen the cross-linking structure.However,excessive nanoparticles would break the cross-linking structure causing the decrease of viscosity.And as temperature rises,viscosity decreased rapidly.However,the adding of nanoparticles did not enhance the viscosity at high temperature.2.To solve the low viscosity of VES fluids at high temperature,pyroelectric nanoparticles were added.The pyroelectric nanoparticle can generate charges on the surface when it was heated.The interaction between nanoparticle and micelle was enhanced due to the electrostatic adsorption.As a result,the net structure was strengthened and viscosity was enhanced.The experimental results showed that compared with normal nanoparticles,pyroelectric nanoparticles can effectively enhance the viscosity of VES fluids at high temperature.3.The molecular dynamics simulation was performed to illustrate the mechanism of viscosity which was enhanced by adding nanoparticles.The effects of nanoparticle concentration,temperature and pyroelectric nanoparticle on VES fluids were simulated.The simulation illustrated that nanoparticle can attract worm-like micelle to form cross-link structure to enhance the viscosity.However,excessive nanoparticles would destroy the cross-linking structure,as a result,viscosity decreased rapidly.And when temperature reach 400K,some of micelles separate from nanoparticle,causing the destroy of cross-link structure,as a result,the viscosity decreased.However,when it was replaced by pyroelectric nanoparticles,micelles were still attracted by nanoparticles.As a conclusion,compared with normal nanoparticles,pyroelectric nanoparticles can strengthen the interaction between nanoparticle and micelle,as a result,strengthen the stability of cross-link structure and enhance the viscosity of VES fluids at high temperature.