(597d) Development of Associative Small Molecule CO2 Thickeners

The low viscosity of CO₂ at typical enhanced oil recovery (EOR) conditions is responsible for a poor mobility ratio that causes viscous fingering and poor sweep efficiency, leading to reduced efficiency and yield. A CO₂ thickener could be a game changer in that it would mitigate the unfavorable mobility ratio and reduce or eliminate the need for water alternating gas methods (WAG). The ideal CO₂ thickener would be an affordable, safe, water-insoluble additive that could dissolve in CO₂ at typical wellhead and reservoir conditions during CO₂ EOR and elevate the viscosity of CO₂ to a comparable value of the oil. In a distinct petroleum recovery application, a CO₂ thickener would allow CO₂ to more effectively transport sand into fractures during hydraulic fracturing of tight sands or shales, thereby allowing liquid CO₂ (rather than water) to be used for “fracking” in water-sensitive formations.  A brief history of prior attempts to thicken CO₂ with either high molecular weight polymers or small associating compounds is presented. Our strategy for designing a novel small molecule, associating, CO₂ thickener is then detailed along with phase behavior and viscosity of dilute solutions composed of thickener candidates in CO₂. Each thickener candidate is assembled with a CO₂-philic segment (e.g. an oligomer or low molecular weight polymer of dimethyl siloxane or propylene glycol) and one or more somewhat CO₂-phobic functional groups (e.g. aromatics, hydroxyl aluminum, tin fluoride, carboxylic acid, benzoic acid, hydroxyl groups, amides) that induces associative intermolecular interactions (e.g. aromatic dimerization, hydrogen bonding, organometallic coupling) that can lead to the formation of viscosity-enhancing supramolecular structures in solution. In addition to the small molecule thickening candidates, several new polymeric thickeners based on high molecular weight silicone oils and a new type of environmentally benign polyfluoroacrylate will also be presented.