Author(s): JJ Lee, S Cummings, A Dhuwe
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The only known CO2 thickener (a compound that dissolves in CO2 and increases it viscosity significantly when present in dilute concentration) is a fluoroacrylate-styrene random copolymer that is probably too expensive for commercial application. High pressure CO2 has also been thickened via the dissolution of high molecular weight polydimethylsiloxane (PDMS, silicone oil) or polyvinyl acetate (PVAc), but this strategy requires several wt% of the polymer and the addition of large concentrations of an organic solvent to the CO2 (e.g. 20% toluene + 80% CO2), which is also impractical for commercial use. Because the utilization of high molecular weight polymers no longer appears to be a viable strategy for affordably thickening CO2 at EOR conditions, we are assessing the use of novel small molecules that self-assemble into viscosity-enhancing supramolecular structures in dense CO2. Small molecules can actually increase fluid viscosity just as effectively as high molecular weight polymers when compared at similar concentrations. For example, tributyltin fluoride and hydroxyaluminum di(2-ethyl hexanoate) are remarkable thickeners of light hydrocarbons even when present at concentrations well below 1wt%. In this presentation, three types of novel CO2 thickening candidates are designed, synthesized and assessed for solubility in CO2 and viscosity-increasing capabilities. Each small molecule possesses a “CO2-philic” segment that promotes dissolution in CO2; the CO2-philic segments are low-cost oligomeric versions of CO2-soluble polymers. Three different types of slightly “CO2-phobic” functional groups known to promote intermolecular associations in hydrocarbon and/or aqueous systems are also included in the thickener structure. The foremost challenge in the molecular design is selecting the appropriate type and number of associating groups needed to enhance viscosity, while not rendering the compound insoluble in CO2. A variety of prospective CO2 thickeners have been synthesized and the solubility of these candidates in CO2, and their ability to thicken CO2, will be presented.
This article was published in Society of Petroleum Engineers
and referenced in Journal of Petroleum & Environmental Biotechnology