Unlocking the mechanism of asphaltene dispersion in petroleum fluids

Asphaltene precipitation and subsequent deposition in production tubing and topside facilities present a significant challenge to the petroleum industry because they reduce well productivity and limit fluid flow. In order to mitigate this problem, chemical dispersants are usually used to keep asphaltene aggregates in colloidal suspension. Most of the existing studies on asphaltene dispersion is of experimental nature and utilizes proprietary commercial dispersants of unknown chemical structure, some of which are deployed in oilfields with limited success. Studies with dispersants of known structure revealed that alkylbenzene-derived amphiphiles could be relatively effective in delaying the onset of asphaltene precipitation and reducing the size of aggregates. However the mechanism by which amphiphiles stabilize asphaltenes is still a subject of speculation. The main objective of this thesis research is to provide novel insights into the mechanism of asphaltene dispersion and to assess the effect of important structural parameters. For this purpose, several ionic and nonionic dispersants with various structures were selected. The dispersant performance was evaluated by impedance analysis (IA) where the onset of asphaltene precipitation in heptane-toluene solutions containing dispersants was measured and the size of nanoaggregates was calculated from their electrical conductivity. Precipitation onsets using IA technique were validated by refractive index measurements. The interactions between dispersants and asphaltenes were systematically studied by molecular dynamic (MD) simulations using high performance computing. For each dispersant, the interaction energies between the dispersant and asphaltene were computed in heptane. The mechanism of asphaltene-dispersant interactions was also captured by umbrella sampling and the dispersant role in reducing asphaltene aggregate sizes was determined through long-term simulations in heptane. Our results revealed two important competitive factors that define the efficiency of dispersants: 1) dispersant self-association, and 2) asphaltene-dispersant interaction. The MD simulation results were in agreement with IA experiments and with direct HRTEM imaging of asphaltene flocculates in heptol.