Ultrasonic and geochemical characterisation of asphaltene aggregation in water-in-oil emulsions

Abstract

Asphaltenes constitute the heaviest, most polar and aromatic fraction of petroleum crucial to the formation of highly-stable water-in-crude oil emulsions. The latter occur during petroleum production as well as spills and cause difficulties to efficient remediation practice. This work investigates the aggregation of asphaltene ‘monomers into nanoaggregates, the latter analogous to surfactant micelles. It is generally accepted in the literature that in nanoaggregate form, asphaltenes create elastic layers around water droplets enhancing the stability of emulsions. A better understanding of the nanoaggregation process would enable a more efficient spill remediation practice. Ultrasonic charaterisation is deployed to infer asphaltene nanoaggregation in toluene. Ultrasonic velocity measurement is a high-resolution non-invasive tool in colloidal analysis shown to successfully identify surfactant micelle formation and has been applied to asphaltene nanoaggregation in toluene. The high sensitivity of acoustic velocity to molecular rearrangements and ease in implementation renders it an attractive method to study asphaltene phase properties. Currently, the onset of aggregate formation is thought to correspond to an intersection of two concentrationultrasonic velocity regressions that suggest a critical nanoaggregate concentration. In this work, measurements indicate a variation in the proximity of nanoaggregation which could be investigated further. This uncertainty is attributed to physicochemical heterogeneity of the asphaltene fraction driven by variation in molecular size and a critical nanoaggregation region is proposed. Asphaltenes were obtained from four petroleum samples and treated with ruthenium ion catalysed oxidation to obtain information about their molecular structure. Statistical analysis was performed to investigate the coupling between asphaltene structures and velocity measurements and their impact on aggregation. A geochemical characterisation of the parent oils was also performed. Finally, Bayesian modelling of the ultrasonic measurements was performed to estimate the statistical likelihoods of a single aggregation concentration versus the aggregation region hypotheses.