The Impact of Asphaltene Chemistry and Solvation on Emulsion and Interfacial Film Formation

Abstract

Petroleum asphaltenes (n-heptane insolubles and toluene solubles) and fractions more and less soluble in mixtures of heptane and toluene (heptol) were analyzed chemically and by small angle neutron scattering (SANS). Asphaltene chemistry and their propensity to aggregate in solution were correlated to the stability of water-in-oil emulsions and the strength of adsorbed films at oil-water interfaces. Solubility profiles of the more soluble and less soluble fractions in heptol indicated strong cooperative asphaltene interactions. The less soluble asphaltene fractions had lower H/C ratios, higher N, V, Ni, and Fe contents than the more soluble or unfractionated asphaltenes. Neutron scattering studies at 25 and 80°C indicated asphaltenes near their solubility limit formed the largest aggregates. Highly aromatic solvents and the presence of petroleum resins disrupted intermolecular pi and hydrogen bonding and reduced the degree of aggregation. Less soluble fractions formed aggregates considerably larger than the unfractionated asphaltenes (as high as 520 Å), while soluble asphaltenes formed the smallest aggregates (as low as 22 Å). Enhanced aromatic pi-pi bonding, dispersion forces and hydrogen bond interactions within the less soluble fraction likely caused large aggregate formation and low solubility. Emulsion stability was gauged by the volume percentage of water resolved after centrifugation at high speed (15,000 rpm). Strong emulsion formers were characterized by intermediate to large aggregates with lower aromaticity and higher nitrogen polarity, Ni and V. Asphaltenes aggregating due primarily to H-bonding and by pi-pi interactions were presumed to form a cohesive interfacial oil-water film and stable emulsions. At low aromaticities (< 30 % toluene), asphaltenes were poorly solvated without resins and did not form stable emulsions. Increasing the resin-asphaltene ratio to 5 or 10:1 of the less soluble asphaltene fractions significantly enhanced emulsion stability (< 20 vol % water resolved). Higher R/A (> 20) led to emulsions with partial stability (40-80 % water resolved). A biconical bob interfacial shear rheometer was used to study the properties of asphaltene films at oil-water interfaces. The degree of film consolidation was determined from ratios of elasticity (G')/film mass and yield stress/G'. Asphaltenes with higher concentrations of heavy metals (Ni: 330-360 ppm, V: 950-1000 ppm), lower aromaticity (H/C: 1.24-1.29), and higher polarity (N: 1.87-1.99) formed films of high elasticity, yield stress and consolidation.