(436b) Low Molecular Weight CO2 Thickener Design for Enhanced Oil Recovery

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. To overcome this problem, there is a need to develop CO2-soluble additives that will increase the effective viscosity of CO₂ without the use of a co-solvent. The only known polymeric direct thickener poly(fluoroacrylate-co-styrene) (polyFAST) has been shown to significantly increase the viscosity at dilute concentrations (<1wt%) measured by falling cylinder viscometry and Berea sandstone core mobility experiments. However, high molecular weight polymers like polyFAST require significant amounts of expensive fluorinated moieties in order to impart solubility. In addition to their cost, fluorinated compounds have undesirable environmental impacts. Therefore, we are exploring non-fluorous, self-associating, low molecular weight thickeners as a potentially safer and more economically viable route to thickening CO₂.

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 CO₂ thickener is then detailed along with phase behavior and viscosity of dilute solutions composed of thickener candidates in either pure CO₂.or CO₂-rich environments. Each thickener candidate is assembled with both CO₂-philic segments (e.g. an oligomer or low molecular weight polymer of dimethyl siloxane or propylene glycol, sugar acetate) to impart favorable solvent-solute interactions and CO₂-phobic functional groups (e.g. aromatics, hydroxyl aluminum, tin fluoride, carboxylic acid, benzoic acid, hydroxyl groups, amides) that induce associative intermolecular interactions (e.g. aromatic dimerization, hydrogen bonding, Lewis acid-base interaction) which can lead to the formation of viscosity-enhancing supramolecular structures in solution.