Stability and rheology of high internal phase CO₂-in-water foams and stability and transport of polymer grafted nanoparticles
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In an effort to reduce the water consumption in hydraulic fracturing by using foam based fracturing fluids, the stability and rheology of ultra-dry supercritical CO₂-in-water foams were investigated in terms of bulk phase viscosity, interfacial tension, and interfacial rheology. The foam morphology and long term stability was studied in situ with high pressure microscopy. Foams with up to 0.98 CO₂ volume fraction and apparent viscosities of hundreds of centipoise were stabilized for hours to days by increasing the viscoelasticity of the interface and the continuous phase. The interfacial shear viscosity and compression elasticity were enhanced with either mixture of oppositely charged surfactant and nanoparticles (with and without high molecular weight polyelectrolyte) or viscoelastic surfactant alone which formed wormlike micelles. The increased interfacial viscoelasticity leads to a rigid foam film as characterized by high Boussinesq number and Marangoni number. Thus, the drainage of the foam film was suppressed due to the immobile interface and the high continuous phase viscosity, as described by the Reynolds drainage equation. The resulting thick film as well as the elastic interface decreases the rate for coalescence and Ostwald ripening. Consequently, foams with fine texture of ~20 µm bubbles were produced and stabilized for hours. Nanoparticles that can be transported through porous rock at high salinity and high temperature are expected to have a large impact on the wellbore diagnostics, electromagnetic tomography and enhanced oil recovery. A series of stable anionic and zwitterionic polymers in high salinity brine at high tempeartures were identified and synthesized. Furthermore, when covalently tethered to the nanoparticles via "grafting to" or "grafting through" approach, the obtained polymer grafted nanoparticles exhibited colloidal stability in high salinity brine for over 1 month, and also low static adsorption to silica microspheres at 1mg/m². The silica microspheres were used to mimic mineral surfaces. The remarkable colloidal stability and low adsorption on mineral surfaces was attributed to electrosteric repulsion exerted by the charged and extended polyelectrolyte chains on the nanoparticle surface.