Design of nanoparticles for generation and stabilization of CO2-in-brine foams with or without added surfactants.

Whereas many studies have examined stabilization of emulsions and foams in low salinity aqueous phases with nanoparticles (NPs) with and without added surfactants, interest has grown recently in much higher salinities relevant to subsurface oil and gas applications. It is shown for the first time that NPs grafted with well-defined low molecular weight ligands colloidally stable in concentrated brine (in particular, API brine, 8% NaCl + 2% CaCl2) and are interfacially active at the brine-air interface. These properties were achieved for three types of ligands: a nonionic diol calle GLYMO and two short poly(ethylene glycol) (PEG) oligomers with 6-12 EO repeat units. Carbon dioxide-in-water (C/W) foams could be formed only with modified NPs with higher surface pressures at the A/W interface. Furthermore, these ligands were sufficiently CO2-philic that the hydrophilic/CO2-philic balance of silica NPs was low enough for stabilization of CO2-in-water (C/W) foam with API brine. Additionally, NPs with these three ligands formed stable dispersions with various free molecular surfactants in DI water and even API brine (8% NaCl + 2% CaCl2) at room temperature. A wide variety of mixtures of NPs plus anionic, nonionic, or cationic mixtures that formed stable dispersions were also found to stabilize C/W foams in porous media at high salinity. These results provide a basis for future studies of the mechanism of foam stabilization with NPs and NP/surfactant mixtures at high salinity.