Carbon Dioxide-in-Brine Foams at High Temperatures and Extreme Salinities Stabilized with Silica Nanoparticles

The stabilization of carbon dioxide-in-water (C–W) foams with nanoparticles (NPs) becomes highly challenging as the temperature and salinity increase, particularly for divalent ions, as the nanoparticles often aggregate in the brine phase. For silica nanoparticles with a medium coverage (MC) and high coverage (HC) of organic ligands, the hydrophilic–CO₂-philic balance (HCB) was found to be in the appropriate range to produce a large reduction in the C–W interfacial tension (IFT). Furthermore, the nanoparticles were colloidally stable in concentrated brine (15% total dissolved solids, TDS) up to 80 °C. With these interfacially active nanoparticles, C–W foams were stabilized with apparent foam viscosities up to 35 cP and foam textures with bubble sizes on the order of 40 μm at various gas fractional flows (foam qualities) in beadpack experiments. At the foam quality where the apparent viscosity was a maximum (transition quality) in the beadpack, we also produced CO₂ foams in Boise and Berea cores versus temperature with apparent viscosities up to 26 cP at 70 °C and 15% TDS and hysteresis in the apparent viscosity versus the interstitial velocity. The reductions in the IFT and foam strength at elevated temperature were modestly larger for the HC nanoparticles than for the MC nanoparticles but were low for the low-coverage case. Given that the interfacial adsorption increased with salinity up to 15% TDS, the screening of the charge helped drive the particles from the brine phase to the interface, which was necessary to stabilize the foams