(406f) Novel Surfactants for Stabilizing CO2-in-Oil Emulsions and Natural Gas-in-Oil Foams for Dry Hydraulic Fracturing

Liquid CO₂ is an attractive fluid for many hydraulic fracturing applications because it contains no water (i.e., it is â??dryâ? does not damage water-sensitive sandstone or shale formations) and it is easily removed from the wellbore after the fracturing process is complete. Despite these attributes, very few â??dryâ? fracks with liquid CO₂ are conducted. The foremost technical reason that pure CO₂ remains an ineffective dry fracking fluid is its very low viscosity at typical fracturing pressures. Based on models of rock mechanics, a fracture formed with pure CO₂ is significantly smaller than fractures formed with â??slickwaterâ?. Further, the low viscosity of CO₂ hinders its ability to effectively transport high concentrations of large proppant particles into the fractures. There are several companies that combine liquid CO₂ with an aqueous surfactant solution to generate CO₂-in-water emulsions that have a high apparent viscosity. However, the presence of water in these CO₂-in-water foams renders them â??wetâ?. We propose a â??dry foamâ? composed of bubbles of liquid CO₂ within films of an oil phase. Several research teams have previously explored CO₂-in-oil emulsions that were stabilized with conventional surfactants composed of hydrophilic and hydrophobic segments, often with the addition of water or ethylene glycol to the mixture. However the dissolution of these surfactants in the oil often results in gelation of the oil that makes it difficult to pump, and upon mixing of the oil + surfactant mixture with CO₂ the resultant emulsions are often unstable. Therefore we have designed and synthesized novel non-fluorous surfactants for stabilizing CO₂-in-oil emulsions. In accordance with Bancroftâ??s rule, these surfactants are more oil-soluble than CO₂-soluble. Further, each surfactant has two hydrophobic segments; one that is CO₂-philic and oil-phobic, while the other segment is oil-philic and CO₂-phobic. Finally, the surfactants have also been designed to not gel the oil. High pressure CO₂-in-mineral oil emulsion stability measurements and high pressure CO₂-mineral oil IFT results will be presented that demonstrate the remarkable efficacy of these novel surfactants relative to conventional surfactants.

Preliminary results for novel surfactants designed to stabilize dry natural gas-in-oil foams will also be presented. Such foams may be attractive for dry fracturing because the foam would have a much higher apparent viscosity than pure methane, natural gas could be obtained from nearby producing wells, and the flowback hydrocarbons (natural gas and oil) would mix with the produced gas and oil.