(475g) Surfactant- or Nanoparticle-Stabilized CO2-in-Oil and Natural Gas-in-Oil Emulsions and Foams for Completely Waterless Hydraulicfracturing

One of the most promising, completely waterless hydraulic fracturing fluids that reached commercialization during this decade was gelled liquid propane and gelled LPG. The thickening was accomplished via the introduction of carefully designed LPG-soluble phosphate esters tailored to remain somewhat soluble in the LPG upon the dispersion of an aqueous solution with trivalent metal ions that cross-linked the phosphate esters. In an attempt to generate completely waterless hydraulic fracturing fluids based on fluids other than propane, we began by assessing these phosphate-ester-based formulations in liquid ethane and liquid CO₂. In this presentation, we will demonstrate that cross-linked phosphate esters are indeed incredibly effective in propane, however when the aqueous crosslinking solution was added to single phase ethane-phosphate ester or CO₂-phosphate esters solutions, the cross-linked complex immediately precipitated and no thickening resulted.

As a result, we turned our attention to combinations of CO₂ and mineral oil (oils were selected that would not become completely miscible with CO₂ at high pressure). CO₂-energized mineral oil (this term refers to a CO₂-oil mixture at high pressure in which a substantial amount of CO₂ dissolves in the mineral oil) has been commercially applied for water-sensitive formations during this decade, but these mixtures contain relatively low proportions of CO₂ (e.g. 30wt%). In an attempt to increase the proportion of CO₂, we successfully designed non-fluorous, mineral oil-soluble, silicone oil-alkyl copolymer surfactants capable of stabilizing high apparent viscosity CO₂-in-oil (C/O) emulsions up to ~90 vol% liquid CO₂.

In this study, we demonstrate that one can also stabilize these (C/O) emulsions with non-fluorous, surface-functionalized, mineral oil-dispersible nanoparticles (NP). NP-stabilized (C/O) emulsions with apparent viscosities up to 14 cP can be generated with surface-modified nanoparticles despite a low driving force for the nanoparticles to adsorb to the interface, given the low interfacial tension. We systemically designed the nanoparticles to be soluble in mineral oil and interfacially active at the CO₂-oil interface through the nature of the surface modification. We initial grafted a C₁₆ alkane, a lipophilic ligand, to the surface to make the nanoparticles soluble in mineral oil and then grafted a secondary non-fluorous CO₂-philic ligand to make the nanoparticle interfacially active. The nanoparticles were readily soluble in mineral oil and lowered the CO₂-oil interfacial tension from 4.7 mN/m to as low as 3.1 mN/m. Furthermore, the most interfacially active nanoparticles were able to stabilize a C/O emulsion at shear rate as low as 370 s^-1 and the emulsion droplet size decreased to 10 mm with increasing shear rate. The droplet size and apparent viscosity are compared for various nanoparticle surface modifications. The emulsion apparent viscosity generally increased with an increase in the mineral oil phase viscosity. The emulsions were stable for at least 72 hrs, a long enough time frame for application as a fracturing fluid to suspend proppant. The emulsions formed with nanoparticles had comparable viscosities and stabilities relative to those formed with our previously designed silicone oil-alkyl copolymer surfactants. The mechanism of emulsion stability is compared for the polymer and nanoparticle stabilized emulsions.

Finally, we will present out initial findings for another type of completely waterless hydraulic fracturing fluid; natural gas-based fluids. Since methane is a far weaker solvent than ethane or CO₂, phosphate esters cannot be used to thicken high pressure natural gas. Therefore, we turned our attention to natural gas-in-mineral oil foams. The benefits of this waterless fluid include the ability to acquire the natural gas from the same field’s production and field storage as LNG, the ability for the flowback natural gas to simply blend with the natural gas production, and the ability of the flowback mineral oil to simply blend with the produced hydrocarbon liquid products (condensate and/or crude oil). This minimizes trucking requirements and water-related processing and eliminates the need to vent gases. Further (unlike the cross-linked phosphate ester components), the novel non-fluorous, mineral oil-soluble surfactants that we have designed or selected for stabilizing these natural gas-in-mineral oil (NG/O) foams will not cause downstream processing problems in refineries. Using methane as a model component for a lean natural gas, we have shown that it is possible to generate NG/O foams with high apparent viscosity, although their stability (to date) is not as great as that exhibited by the C/O emulsions.