Experimental and simulation insights on the high viscosity of heavy oil I: Low-concentration compounds

Asphaltene and resin can form microscale colloids; in the literature, their content is the key to heavy oils' viscosities. However, our recent experimental results indicate some heavy oil samples with the same content of asphaltene and resin have a 2× discrepancy in terms of their viscosities, which cannot be explained by previous work. In this paper, we hypothesize that this discrepancy corresponds to the total content of low-concentration compounds. We present the molecular configuration and interactions of heavy oil using a combination of atomic force microscopy (AFM), gas chromatography–hydrogen flame-ionization detection tests, and molecular dynamics simulations. With the improvement of experimental accuracy, we found that 7089 of 7376 detected compounds in a heavy oil sample had a low concentration of <0.05% (mol/mol). These compounds could form nanoscale aggregations (radius <102 nm) as a continuous network with a substantial surface area of ∼150 nm2 / 92 nm3. AFM and scanning electron microscopy–energy-dispersive X-ray spectroscopy results indicate that the surface area of heavy oil was 103 greater than that of light oil. Simulation results indicate that the calculated molecular diffusion of each detected compound differed. Thus, the intermolecular interactions on these surface areas are the main contributors to internal friction during flow. Our experimental results reveal the substantial impact of low-concentration compounds on the measured viscosity. These findings are the first to present the importance of low-concentration compounds on the viscosity of heavy oil, and will facilitate the development of viscosity reducers for lowering the low-concentration compounds' surface areas.