Understanding the role of iron (III) tosylate on heavy oil viscosity reduction

Heavy oil is an abundant energy resource, but its recovery remains challenging primarily due to its high viscosity. Thermally enhanced oil recovery in the presence of metal-ligand compounds (MLCs) has been studied as a promising method for in situ viscosity reduction and oil quality upgrading. In spite of its importance, the interactions between MLCs and crude oil components at the molecular-level are poorly understood, and their mechanistic details for viscosity reduction are unclear. Here we studied viscosity changes of Peace River oil in the range of 80–295 °C in the presence and absence of iron (III) tosylate (para-toluenesulfonate) MLC and analyzed reaction products via viscometry, TGA-MS, XPS, GC-MS, SARA analysis, and elemental analysis. Whereas thermal treatment lowered viscosity at 190 °C and above, thermal treatment with the iron tosylate MLC decreased viscosity only at temperatures above 230 °C. The MLC effect was most substantial at 280 °C, at which viscosity decreased by 58% (compared to 39% in absence of the MLC at the same temperature). The MLC likely lowered oil viscosity by catalytically reacting with the asphaltene to decrease its total content in oil, and by releasing a ligand to form 4-methylbenzenethiol (MBT) that interfered with asphaltene intermolecular interactions. At temperatures below 230 °C, the MLC unexpectedly raised oil viscosity, likely due to bridging interactions with asphaltene. This understanding of MLC-induced deviscosification provides a selection rationale for appropriate metals and ligands for enhancing heavy oil recovery.