The Study On The Rheological Characteristics Of Viscoelastic Fracturing Fluid Improved By Nanoparticles

Nanofluid can be formed with nanoparticles dispersed in fluids, which is a stable two-phase mixture with specific characteristics. This paper will use the nanoparticles to modify the property of a viscoelastic surfactant fracturing fluid. This water-based fracturing fluid which consists of small surfactant molecules can form similar cross-linked structure of polymer fracturing fluid after thicken by salts. Because of the unique composition, this viscoelastic surfactant fluid is much faster and more complete at the gel breaking stage. This novel fluid could be an effective solution to engineering problems of traditional polymer fracturing fluid, such as complex fluid preparation, too much water insoluble residues, poor permeability of rock, etc. However, the performance of this viscoelastic surfactant fracturing fluid at a high temperature needs to be improved, especially for shale gas exploitation in deep shale strata. This paper focuses on the properties changing of viscoelastic surfactant fracturing fluid by adding nanoparticles with high surface area, such as improving the stability of micelle at high temperature, simplify the fracturing fluid preparation, increase the applicability.In this paper, a series of research for the preparation, stability, rheological characteristics and other engineering properties have been done to this kind of viscoelastic surfactant fracturing fluid mixed with nanoparticles. Some nanoparticles were found and selected, which could modified the viscosity and sand carrying properties of. The main content and results of the research work are as following.1. Study on the preparation method and the stability of viscoelastic surfactant fracturing fluid mixed with nanoparticlesIn the paper, a fluid system made by two-steps preparation method with Microfludizer high shear fluid processor has good size distribution, good static stability and good shear stability after experimental research of its microstructure, size distribution, static sedimentation and long-time shear process.2. Study on the rheological characteristics of viscoelastic surfactant fracturing fluid mixed with nanoparticlesThe influence of factors of nanoparticles on the viscosity, zero-shear viscosity and the parameters of viscosity and elasticity are tested and discussed, by using a nonlinear viscoelastic model.The apparent viscosity is mainly affected by the shear rate, zero shear viscosity, relaxation time. Zero shear viscosity and relaxation time is mainly affected by temperature, material and adding percentage of nanoparticles.Some rules could be found by analyzing these factors and fitting formula. In the testing range, zero shear viscosity with greater percentage was reduced, even less than that of the base fluid viscosity. At30℃, high shear rate viscosity was increased by adding0.228%bv BaTiO3or SiO2or graphite. BaTiO3nanoparticle which is pyroelectric could raise the high shear rate viscosity at30-90℃, especially at45℃. There was a approperate range of adding ratio of there nano materials where the zero shear rate viscosity was increased at30℃. Zero shear rate viscosity was increased by adding graphite nanopaticles from30℃to80℃. In addition, with the same percentage of silica particles added, storage modulus and loss modulus were increased or decreased at the same trend. As a result, relaxation time mainly depended on the influence of temperature, less affected by the adding percentage. And with a rise in temperature, the relaxation time shows a decline.Two main reasons of the viscosity is increased or decreased is explained in the paper:1) Nanopaticles could strengthen the cross-linking structure through its high specific surface area attracting sufactant micelle;2) The micelle culsters are attracted by the nanopartiles too closed and overlaped, breaking the cross-linking structure.3. Test on other technical properties of viscoelastic surfactant fracturing fluid mixed with nanoparticlesAccording to the industry standard, sand carrying properties are tested by the traditional static sedimentation experiment. And gel breaking characteristics testing are carried out by gel breaking experiments using different gel breakers. Sand carrying properties are mainly affected by temperature, silica or BaTiO3nanoparticle could slow down the free settling ratio. Compared with guar fracturing fluid, the viscosity of the tested fluid at the same settling rate is lower. Viscoelastic surfactant fracturing fluid has a good gel breaking performance with water and ethanol and the optimum adding percentage of gel breaker has been gotten so that the viscosity of gel-broken fluid could reach the industry standard.