(19a) Interactions of Synthetic Additives with Petroleum Aggregates Probed by Small-Angle Neutron Scattering

Gawrys, K. L., Nalco Company, Energy Services Division
Lindemuth, P. M., Nalco Company, Energy Services Division

The mechanisms, interactions, and association behaviors of synthetic additives necessary for crude oil production, refining, and flow assurance are poorly understood. For example, refinery desalter operations involve the intimate mixing of wash water and crude oil to remove water-soluble contaminants and oil-insoluble particulates. The desalting process involves the formation of water-in-crude oil emulsions that are treated with synthetic demulsifying agents to promote water droplet coalescence. Furthermore, synthetic dispersants are used to mitigate asphaltene deposition in various refinery process units and flow assurance applications. The mechanisms of interaction between these chemicals and petroleum asphaltenes are not well characterized. Small-angle neutron scattering (SANS) is a useful tool for studying the aggregation behavior in complex petroleum fluids, whereas other characterization methods employing visible light are often precluded due to the high refractive index and visual opacity of petroleum mixtures. SANS experiments were performed on mixtures of raw crude oil with various amounts of perdeuterated heptane (0, 0.25, 0.5, and 0.75 w/w) to induce asphaltene flocculation. SANS scattering curves for untreated crude oils at each solvent condition were compared to those in which the crude was treated with 1000 ppm of dispersant and/or demulsifier. Two heavy crude oil blends (i.e., Crude A and Crude B) were initially studied. Guinier analyses indicated that neither of the synthetic additives affected the aggregation state of asphaltenes in Crude A. Dispersant addition to Crude B appeared to decrease the nano-particle volume fraction (i.e., with respect to the untreated crude) at some solvent strengths, but it did not modify the aggregate size. This observation seems to contradict the widely accepted belief that dispersants work by disrupting micro-particle flocs, thus increasing the concentration of nano-particles. The addition of demulsifier disrupted micro-particle flocs, as manifested by a decrease in low Q scattering intensity. It also modified the size of nano-particle aggregates with respect to the untreated crude. Observations concerning the role of chemical additives on asphaltene aggregation behavior were directly related to the chemical composition of the crude blends, SARA fractionation, and abilities to form stable water-in-oil emulsions.