(677e) CO2-in-Mineral Oil Emulsions, CH4-in-Mineral Oil Foams and N2-in-Mineral Oil Foams Stabilized By Novel Oil-Soluble Surfactants As Waterless Hydraulic Fracturing Fluids

Objectives/ Scope: Viscous, completely water-free, high pressure liquid CO₂-in-oil (C/O) emulsions, methane-in-oil (M/O) foams and N₂-in-oil (N/O) foams have been stabilized by novel, interfacially active, oil-soluble, polymeric surfactants. For example, for (C/O) emulsions, the surfactant is composed of an oleophilic long-chain alkyl pendant groups on a CO₂-philic, oil-phobic, polydimethylsiloxane (PDMS) backbone. For (N/O) and (M/O) foams, neither N₂ nor CH₄ has appreciable solvent strength therefore the surfactant design is based upon oilphilic/oilphobic considerations. These emulsions and foams can serve as hydraulic fracturing fluids to eliminate the need for water and reduce formation damage caused by water.

Methods, Procedures: We designed and synthesized series of polymeric CO₂-philic PDMS- oil-phobic alkyl surfactants for (C/O) emulsions; polymeric oil-philic/oil-phobic surfactants were designed for the N/O and M/O foams. The emulsions/foams were formed by either mixing with an impeller (quality or volume fraction up to 60 vol%) or by simultaneously co-injecting the fluids through a packed bed of 22 Darcy unconsolidated sand (quality up to 90 vol%).

Results, Observations, Conclusions: The novel, completely hydrophobic surfactants adsorb at the (C/O), (N/O) or (M/O) interface despite the very low interfacial tension (driving force) without surfactant present. For C/O emulsions, at a concentration of 2 wt % in oil, the surfactant stabilizes the emulsions and no gelation of oil occurs. The 50-90% quality C/O emulsions, with CO₂ droplet sizes in the 5 – 150 micron range, exhibited an apparent viscosity of about 3 - 18 cP at 25^oC and 2500 psia as determined with falling ball and capillary viscometers. The emulsions were stable for over a day as indicated by photographs and determination of the bubble size with microscopy. In contrast, N/O foams had an apparent viscosity ranging from 15-65 cP. Oil-soluble surfactants with optimum oilphilic/oil-phobic balance, based on the decrease in the interfacial tension, were the only surfactants that stabilized the N/O and M/O foams.

Novelty/Additive Information: We have addressed a major limitation in the generation and stabilization non-aqueous CO₂ and N₂ energized emulsions and foams for water-sensitive formations, by providing general criteria for surfactant design. The surfactant design criteria are based on a fundamental understanding of the relationship between foam apparent viscosity in terms of interfacial properties and phase behavior, as well as foam texture. We also compare C/O emulsions and N/O and M/O foams to understand the role of gas solubility in oil on apparent viscosity. The waterless, viscous emulsions and foams are hydraulic fracturing fluids that offer an efficient way to add both the novel surfactant and the proppant to the mineral oil at ambient pressure (rather than blending surfactant and/or proppant with high pressure CO₂ or N₂) for improved oil or gas recovery in water-sensitive formations.