CO2 is an effective solvent for enhanced oil recovery (EOR) because of its high displacement efficiency and potential for commercial-scale carbon sequestration. CO2-brine foams improve the volumetric efficiency of CO2-EOR via increased viscosity and conformance control, often using stabilizing surfactants that reduce CO2-brine interfacial tension. Surfactants are susceptible to insolubility at high salinity and temperature, however, and foams stabilized solely with nanoparticles (i.e., Pickering foams) are a less-studied but promising alternative. Nanoparticles can be designed to be stable at reservoir conditions and partition preferentially to the CO2-brine interface, physically preventing the collapse of lamellae. This can be accomplished by covalently grafting ligands to the particle surface, achieving the necessary balance of wettability between brine and CO2 for a particle to occupy the interface. Moderate concentrations of ligand - having sufficient coverage to prevent nanoparticle aggregation, but not enough to stop all flocculation - may induce particle bridging in the brine phase and improve overall foam stability. This work examines the effect of ligand concentration at the nanoparticle surface on Pickering foam stability and viscosity. We generated silica particles (50-80 nm) with the Stöber method and grafted them with polyethylene glycol (PEG), varying the surface concentration of PEG from 0 to 2 μmol/m2 at finely controlled intervals. We characterized particles before and after grafting with dynamic light scattering, thermogravimetric analysis, and transmission electron microscopy to determine the particle diameter, zeta potential, and ligand concentration. As we increased PEG concentration on the particle surface, we observed slightly larger particle diameters in deionized water and measured less hydrophilic zeta potentials. Particles with moderately concentrated PEG coatings mildly flocculated in brine and exhibited larger particle diameters, whereas particles with low or no PEG coatings aggregated completely. At elevated temperature and pressure, we will generate CO2-in-brine Pickering foams in a 180 μm beadpack (22 d permeability) and measure the pressure drop to determine viscosity. Foam stability is inferred from visual observations in a microscopy cell over time. We hypothesize that bridging between moderately coated nanoparticles increases foam stability and viscosity, yielding a specific optimal PEG concentration for foam generation.

Effect of nanoparticle surface modification on the stability of CO2-in-brine Pickering foams

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.