Small Molecule Associative CO₂ Thickeners for Improved Mobility Control

Mobility control is one of the largest problems in carbon dioxide (CO2) miscible enhanced oil recovery. This can be traced back to the very low viscosity of high pressure carbon dioxide, 10-100 times lower than the original oil in place, which gives it an unfavorably high mobility ratio that results in viscous fingering, early CO2 breakthrough, decreased sweep efficiency, and high CO2 injected:oil recovered utilization ratios. CO2’s viscosity can also cause conformance control issues in stratified formations because it promotes CO2 flow into higher permeability, watered-out zones leaving a much smaller fraction of CO2 available to flow the lower permeability, oil-bearing zones of interest. An economical, direct CO2 thickener that is effective at dilute concentrations would be disruptive technology because it would not only mitigate all of the problems associated with an unfavorable mobility ratio, but it would also eliminate the need for the water-alternating-gas process for the reduction of CO2 relative permeability. These effects would be especially pronounced in horizontal, relatively homogeneous porous media. However, CO2 has never been thickened using an affordable or small molecule. To circumvent these obstacles, we have designed novel small molecules that self-assemble into viscosity enhancing supramolecular structures. Generally, our designs utilize both CO2-philes to enhance dissolution and steric effects that promote linear supramolecular structures. CO2-phobic groups are also included to promote self-assembly. This work primarily focuses on molecular designs based on highly CO2-philic silicones and CO2-phobic hydrogen bonding groups such as aromatic amides and ureas. Initial phase behavior studies on un-functionalized oligomeric silicones of varying molecular weight in CO2 served as solubility limits in that the inclusion of a CO2-phobic associating group(s) will result in a decrease in solubility (i.e. an increase in cloud point pressure). Along with exploration of various functional groups, we also demonstrate the effect of molecular geometry on final solution properties. Specifically, terminally functionalized branched silicones required a much higher mass concentration than silicone tailed core-associative molecules to achieve similar solution viscosities. The most promising thickening results are obtained with trisureas functionalized with three relatively short, branched CO2-philic silicone-based functionalities, most notably benzene tris((tri(trimethylsiloxy)silyl)propyl) urea. Replacement of one or two of these branched (tri(trimethylsiloxy)silyl) functional groups with linear oligomers of dimethyl siloxane renders compounds more CO2-soluble but less effective at thickening. In most cases, however, an organic co-solvent is required to attain solubility levels great enough for viscosity enhancement to occur