RIASSUNTO
Abstract
Viscoelastic surfactants can provide viscoelastic properties in brines at high temperatures (higher than 230°F). Recent studies have shown the advantages of use nanoparticles in VES fluid systems. The nanoparticles have large surface area as well as unique surface morphology and surface reactivity. They can strengthen the micelle-micelle interactions. A small concentration of these particles can maintain viscosity at higher temperatures (up to 300°F) and decrease the rate of VES fluid leakoff (Huang, Crews and Willingham, 2008).
The viscoelasticity of nanoparticle-networked VES fluid systems was analyzed with a HP/HT viscometer. A series of rheology experiments have been performed by using 4 vol% amidoamine oxide surfactant in KCl, NaCl and CaBr2 brines at temperatures up to 275°F and a shear rate of 10 s-1. The nanoparticles evaluated were MgO and ZnO at the concentration of 6 pptg. In addition, the effect of different nanoparticle concentrations (0.5 to 10 pptg) on the viscosity of VES fluid was investigated. The oscillatory shear rate sweep (100 to 1 s-1) was performed from 100 to 200°F. The effect of an internal breaker on the viscosity of VES micelles was examined.
Introduction
Wormlike micelles are elongated and semi-flexible aggregates resulting from the self-assembly of surfactant molecules in aqueous solutions. The growth and stability of micellar aggregates depend on the packing of the surfactant molecules, or the curvature of the water/hydrocarbon interface (Israelachvili, 1992). The change in the packing parameter is governed by the addition of an inorganic/organic salt (Van Zanten, 2011). Above the critical micellar concentration (cmc), spherical micelles form spontaneously, and their size is related to that of the amphiphilic molecules.
The nanoparticles are inorganic crystals that do not dissolve in water, oil, or solvent (Huang and Crews, 2008a). Some unique nanoparticles have been applied to viscoelastic surfactant fluid systems to improve their performance as fracturing and fracture-packing fluids (Yu et al., 2010; Pourafshary et al., 2009). Adding nanoparticles to VES fluid may improve the fluids' viscosity stability at high temperature, improve solids suspension, and prevent surfactant phase separation by improving its solubility at high temperatures. These additives could reduce the amount of VES surfactant required to achieve the stable viscosity of the fluid (Huang and Crews, 2008b). Nanoparticles are smaller than the pores and pore-throat passages within a hydrocarbon reservoir, so VES fluids with nanoparticles can be more easily removed and cause less damage to the reservoir permeability compared with polymers (Huang and Crews, 2008c).
Pyroelectric and piezoelectric crystals are good viscosity enhancers because they are small and may stay within the VES fluid that flows into the target formation. The surface charges of pyroelectric crystals, such as ZnO nanoparticles, are generated by heating or pressing. These surface charges may improve nanoparticle/VES micelle interactions that result in increased fluid viscosity (Crews and Huang, 2011; Huang et al., 2011). MgO micron and nano-sized particles have been used as VES fluid stabilizers at temperatures from about 180 to 300°F. MgO and ZnO nanoparticles have unique particle charges that promote pseudocrosslink with VES micelles. They may be added to VES fluids before pumping downhole (Huang and Crews, 2008b). The MgO in powder form easily reacts with water to form an outer layer of Mg(OH)2. Therefore, water is not an appropriate carrier fluid for them. Propylene glycol, miscible in the water, was found as a suitable carrier fluid. It generates a microemulsion that improves suspension of highly concentrated particles (Huang and Crews, 2008c).