(644f) Liquid CO2-in-Mineral Oil Emulsions Stabilized By Siloxane-Long Chain Alkyl Surfactants and Application As a Waterless Hydraulic Fracturing Fluid

CO₂ has long been considered an attractive fracturing fluid for water-sensitive formations, but the very low viscosity of CO₂ (e.g. 0.1 mPa s at 25^oC and 2500 psig) inhibits the formation of large hydraulic fractures and makes it difficult to transport high concentrations of large proppant particles into fractures. The CO₂-rich waterless emulsions described in this study may be a practical way of dramatically increasing the apparent viscosity of CO₂ for fracturing applications while offering an easy way to add both the surfactant and the proppant to the mineral oil at ambient pressure (rather than blending surfactant and/or proppant with high pressure CO₂ at the surface).

A water-free, high pressure CO₂-in-oil emulsion (sometimes referred to as foam) of liquid CO₂ droplets within continuous films of mineral oil can be stabilized by a novel, completely hydrophobic, oil-soluble surfactant. The surfactant comprised of an oleophilic (CO₂-phobic) alkyl segment and a CO₂-philic oleophobic polydimethylsiloxane segment adsorbs at the CO₂-oil interface despite the very low interfacial tension without surfactant present. The surfactant increases the oil viscosity by 40% at a concentration of 2 wt % aiding emulsion stabilization. The emulsions were formed by either mixing with an impeller (CO₂ quality or volume fraction up to 60 vol%) or by simultaneously co-injecting the liquids through a packed bed of 22 Darcy unconsolidated sand (CO₂ quality up to 90 vol% with CO₂ droplet sizes in the 20 – 150 micron range). These 50-90% quality CO₂-in-mineral oil emulsions exhibited an apparent viscosity of about 3 - 18 cP at 25^oC and 2500 psia with a falling ball and capillary viscometer. Ultimately, these emulsions could be used to greatly reduce water requirements and minimize wastewater produced in hydraulic fracturing.