Molecular Dynamics Simulation And Experimental Study On Nanoparticles Modified Viscoelastic Fracturing Fluids

Nanoparticle modified viscoelastic fracturing fluid can improve the proppant carrying capacity, temperature-resistant capacity of fracturing fluid, and reduce the filtration of the fracturing fluid. It is very important to study the micromechanism and engineering property of nano-particle modified viscoelastic fracturing fluid. In this paper, we used molecular dynamics simulation to study the effect of salts, the addiction of nanoparticle and temperature on the fracturing fluids. We also tested the engineering property of the nano-particle modified viscoelastic fracturing fluids. The main researching work and results are shown below:1. Molecular dynamics simulation of the effect of salts on the micellization of surfactantWe studied the effect of organic salt NaSal and mineral salt KC1 on the micellization of simple chain surfactant CTAB and Gemini surfactant 16-6-16, and also contrasted the micellization process of CTAB and 16-6-16. In a certain rang, the addition of NaSal can promote the transition from spheric micelle to rodlike micelle, and promote the rheological behavior of viscoelastic fracturing fluid. However, when the addition of NaSal exceed a center ratio, the surface that surfactant supplies for the Sal- to connect is limited, the micelle must divide into two or more micelles to supply enough surface, which will play a bad role for the micellization process. So there is an upper limit for the addition of NaSal, and Gemini surfactant can supply a more big geometric space for Sal- than single tailed surfactant.As the addition of KCl increasing, the radius of gyration of micelle fluctuates, and there is no obvious law. There is a peak distribution of Cl- on the surface of micelle, and the sum of Cl-and Br- is larger than the Br- when the addition of NaSal is zero. There is no obvious peak distribution of K+. And with the increase of the addition of KCl, the distribution of cation K+far from the micelle decreases. The addition of KCl can offset the repulsive force between the cation groups of surfactant, and shield the effect of water around the micelle. So the addition of KCl can make micelles move easily, and let micelles tangle with each other together to form net structure, which can promote the rheological behavior of viscoelastic fracturing fluid.At the same mass fraction, CTAB and 16-6-16 form rodlike micelles and spheric micelles, but the rodlike micelle of 16-6-16 is more rodlike than that of CTAB, which implies that Gemini surfactant can form micelle more easily than single tail surfactant. At the initial stage of micellization, monomer and oligomer collide to form small micelle, and later the growth of micelle comes from the collision of small micelles.2. Study on the impact factors of the formation of viscoelastic fracturing fluidWe used molecular dynamics simulation method to study the effect of temperature and the addition of nanoparticle on the viscoelastic fracturing fluid. The system, which contains mass fraction 6% CTAB,2%KCl and 2% NaSal, forms rodlike micelles and wormlike micelles. The addition of nanoparticle makes the system form net structure, and accelerates the micellization process. Nanoparticle connects with micelles through a half spheric face, and the hydrophobic tail chains of CTAB and hydrophobic groups of Sal- locate on the center section, and the outer-ring of the face are hydrophilic groups of CTAB and Sal-. And the results of cryo-TEM show that the micelles of Gemini surfactant-formed fracturing fluid range from 5nm to 40nm, and nano-particles can connect with wormlike micelles to form net structure, which will promote the rheological behavior of viscoelastic fracturing fluid.At the same mass fraction, the system which contains 6nm particle forms a big micelle, the length of which reaches about 40nm.For the system which contains nanoparticle, when temperature reaches 330K, the net structure disappears, micelles break away from nanoparticle to form a wormlike one. For the system without nanoparticle, two wormlike micelles and a spheric micelle become a wormlike micelle when temperature reaches 315K, the wormlike micelle forms net structure when temperature reaches 330K. We think that the addition of salts can restrain the increase of CMC.3. Test of filtration property and permeability damage rate of coreWe study the effect of addition of nano-particle on the filtration property and permeability damage rate. The fracturing fluid can form fictitious cake with the addition of nano-particle to reduce the filtration property. But the viscosity of filter liquor is still relatively large because the filter liquor also contains some nano-particles. The filter liquor of 15nm nano-partilces formed fracturing fluid damages core permeability badly. The diameter of shale hole ranges between 5-300nm, and the small hole takes up a great proportion, so the 15nm particles can enter into the hole and damage the core. The damage of filter liquor of 30nm nano-particles formed fracturing fluid and base fluid on core is almost same, but the filtration coefficient and rate of 30nm nano-particles formed fracturing fluid are lower, so the damage is small.