Dynamic Light Scattering Monitoring of Asphaltene Aggregation in Crude Oils and Hydrocarbon Solutions

Abstract

An overview of dynamic light scattering (DLS) studies of asphaltene aggregation in hydrocarbons and crude oils is presented. A special optical scheme, to make light scattering measurements in “nontransparent” light-absorbing colloids possible, has been designed and tested. The modified DLS is an effective technique for real-time monitoring of petroleum colloids. Combined with accurate viscosity measurements, DLS is a powerful tool for investigating the colloid nature of crude oils, heavy petroleum fractions, and asphaltene solutions. Various regimes of asphaltene aggregation have been investigated by DLS in crude oils and hydrocarbon mixtures. Crossover between reaction-limited aggregation and diffusion-limited aggregation has been observed in hydrocarbon solutions of asphaltenes. Asphaltene colloidal structures, originally persisting in some crude oils, have been detected. Using n-heptane as precipitant, we have studied stability of crude oils with respect to asphaltene aggregation. We have found that the oils with different aromatics/saturates ratios exhibit different aggregation kinetics. Formation of stable asphaltene aggregates, originating from ultrasonic agitation and surfactant addition, has been also studied. A DLS microrheology in a crude oil has been demonstrated. Crude oils and heavy petroleum fractions are complex multicomponent fluids exhibiting properties of either molecular solutions or colloids depending on the origin and on the properties under consideration. Non-Newtonian rheology of petroleum systems (especially those with high-content asphaltenes and paraffins) indicates the presence of a supra-molecular mesoscopic structure. However, the physical nature of this structure is not yet well understood. It is commonly accepted