Is polymer of 1-decene an effective CO₂ thickener?

This study addresses a point of contention in the literature regarding the extent to which the viscosity of supercritical CO₂ can be enhanced by non-associating, low-molecular-weight polymeric direct thickeners. Significant viscosity enhancement could allow more efficient water displacement during aquifer sequestration of CO₂. Among such direct thickeners, a commercial polymer of 1-decene (P1D) with reported average number of repeat units in the range six to seven (relative molar mass ∼9⋅102) has been studied by several research groups. However, the reported results show large discrepancies in outcome as to CO₂ thickening effect. The reported results vary from virtually no effect to 15-fold increase in viscosity. This discussion has recently expanded to include custom-synthesised poly-1-decenes with higher numbers of repeat units, some of which have shown remarkable thickening effects. However, these custom polymers are not readily available for independent verification, and the application potential of the more readily available commercial P1D remains unresolved. To address this uncertainty, and because the performance of the commercial P1D may inform the potential of the custom polymers, we present experimental results on the solubility, cloud point pressure, and viscosity of mixtures of commercial P1D and CO₂. For mixtures with polymer concentration of 1 wt% our experiments yielded relatively high cloud point pressures (>25 MPa) and low thickening effect (<5 % viscosity increase). To be economically viable for field applications, the polymer would need to exhibit significant effect at such low concentration, which seems not to be the case for the commercial P1D. Only slightly improved thickening effect (∼15 % viscosity increase) was observed by increasing the polymer concentration to 3 wt%. Anyway, the increased polymer concentration caused the cloud point pressure to increase to such extent as to make the polymer less suitable for field applications. At 3 wt% polymer, the cloud point pressure at 358 K was more than 50 MPa and apparently increasing with decreasing temperature. To be viable for aquifer sequestration of CO2, the cloud point pressure would need to be lower than the aquifer pressure. Given that recent reports claiming significant CO2 thickening with custom P1D also report substantial effects with the commercial P1D, our findings raise concerns about the validity of those claims, particularly for the custom polymers.