Carbon dioxide-in-oil emulsions stabilized with silicone-alkyl surfactants for waterless hydraulic fracturing

The design of surfactants for CO₂/oil emulsions has been elusive given the low CO₂-oil interfacial tension, and consequently, low driving force for surfactant adsorption. Our hypothesis is that waterless, high pressure CO₂/oil emulsions can be stabilized by hydrophobic comb polymer surfactants that adsorb at the interface and sterically stabilize the CO₂ droplets. The emulsions were formed by mixing with an impeller or by co-injecting CO₂ and oil through a beadpack (CO₂ volume fractions (ϕ) of 0.50-0.90). Emulsions were generated with comb polymer surfactants with a polydimethylsiloxane (PDMS) backbone and pendant linear alkyl chains. The C₃₀ alkyl chains are CO₂-insoluble but oil soluble (oleophilic), whereas PDMS with more than 50 repeat units is CO₂-philic but only partially oleophilic. The adsorbed surfactants sterically stabilized CO₂ droplets against Ostwald ripening and coalescence. The optimum surfactant adsorption was obtained with a PDMS degree of polymerization of ∼88 and seven C₃₀ side chains. The emulsion apparent viscosity reached 18 cP at a ϕ of 0.70, several orders of magnitude higher than the viscosity of pure CO₂, with CO₂ droplets in the 10-150 µm range. These environmentally benign waterless emulsions are of interest for hydraulic fracturing, especially in water-sensitive formations.