Asphaltenes and Waxes Do Not Interact Synergistically and Coprecipitate in Solid Organic Deposits

Abstract

Waxes and asphaltenes are the major components in organic deposits from petroleum fluids. A key unresolved issue is whether there are significant intermolecular interactions between wax and asphaltene molecules during precipitation that would lead to synergy and coprecipitation, i.e., the phase change of one class inducing the precipitation of the other class. To address this, we studied six organic deposits in great chemical and physical detail: three that we categorized as predominantly wax deposits, two that were asphaltenic deposits, and one that seemed to be an organic acid deposit. Chemical analyses included elemental analysis, Fourier transform infrared (FTIR) spectroscopy, high-temperature simulated distillation, melting range measurement, and solubility experiments, performed on both parent crude oils and their corresponding deposits. The wax deposits contain in excess of 44 wt % waxes, and the complement of the deposits was occluded oil, water, and inorganic solids. Less than 6.2% (w/w) of these deposits were asphaltenes. Waxy-deposit waxes have higher melting points, longer straight-chain methylene, higher carbon number distributions, and higher molecular weights than their parent crude waxes. Asphaltenic-deposit waxes have properties that are almost identical to their parent crude oil waxes. Asphaltenic deposits contained >27 wt % asphaltenes. The properties of asphaltenes isolated from asphaltenic deposits were compared with the properties of asphaltenes isolated from their parent crude oils. The metal content and apparent VPO molecular weight of asphaltenic-deposit asphaltenes are significantly higher than those of corresponding crude-oil asphaltenes. The solubility of asphaltenic-deposit asphaltenes is much lower than the solubility of asphaltenes from their parent crude oils. The results indicate that the asphaltenes in the wax deposits are likely a component of the occluded oil in the deposit. A key conclusion of our work is that there is no evidence for any intermolecular interaction between waxes and asphaltenes to suggest synergy in precipitation. In fact, the evidence very much points to only one of these components undergoing a phase change and precipitating in an organic deposit and the other component simply being a portion of the crude oil occluded in the deposit.