Asphaltene Nanoaggregates Measured in a Live Crude Oil by Centrifugation

Abstract

Asphaltene nanoaggregates have recently been observed in live crude oil by observation of gravitationally induced asphaltene gradients in four different reservoir sands with oil columns up to 1000 m vertical. When the liquid phase is invariant, these gradients can be fit using Archimedes buoyancy in the Boltzmann distribution; the only adjustable parameter in data fitting is the size of the asphaltene nanoaggregate; ∼2 nm is obtained in four reservoir sands and is similar to laboratory results for asphaltene nanoaggregates in toluene. Here, a live crude oil (with dissolved gases) has been spun at modest g forces for long times designed to create a large, equilibrium asphaltene gradient for the presumed 2 nm aggregates. Elevated temperatures (∼91°C) were employed during centrifugation to mimic reservoir conditions for asphaltene aggregation and prevention of a possible wax phase. Elevated pressures were employed on the hot, live crude oil to maintain dissolved gas concentrations. A total of 13 alliquots of crude oil were removed after centrifugation, and the asphaltene concentrations were determined by optical spectroscopy. Indeed, a large asphaltene gradient was observed, and a 2.6 nm diameter nanoaggregate was obtained using Archimedes buoyancy in the Boltzmann distribution. In addition, a solubility model accounting for the gas/oil ratio (GOR) gradient was used to analyze the asphaltene gradient, giving an asphaltene particle size of 2.0 nm, thus, the same as field observations. In addition, the gradient in bulk resins was shown to be quite small, showing the stark contrast of asphaltene versus bulk resin aggregation. The heaviest resins (or lightest asphaltenes) do show some gradient. These observations allow for the determination of the maximum and minimum asphaltene aggregation number; the range is roughly 3-8. Some modest resin association with asphaltenes, one resin molecule in every asphaltene nanoaggregate, is consistent with our data. These results are discussed within the increasingly successful modified Yen model of asphaltenes.

A study of asphaltene-resin interactions

“…15,16 These fundamentals must hold true for resins as well. Nonetheless, the limit concentration at which either asphaltenes or resins start forming aggregates, or clusters, was not investigated in our experimental studies.…”

The aggregation and diffusion of asphaltenes studied by GPU-accelerated dissipative particle dynamics

“…They also found the nanoaggregate weights determined by vapor pressure osmometry were about 5 times larger than the asphaltene molecular weights. The live crude oil centrifugation study supported that asphaltenes were presented in at least some crude oils as 2 nm nanoaggregates and the aggregation numbers of these nanoaggregates were ranged from 3 to 8 [10]. Based on the Yen model proposed in 1967, Mullins et al have reviewed the collected data and proposed a modified Yen model or the ''Yen-Mullins model'' [11].…”

The influence of asphaltene-resin molecular interactions on the colloidal stability of crude oil

“…Based on highresolution transmission electron microscopy (HRTEM) and fluorescence depolarization techniques, the size of asphaltene stacks is estimated to be about 1 nm ðwidthÞ Â 1 nm ðheightÞ, mainly comprised of two or three individual asphaltene molecules [3][4][5][6][7]. However, new findings of laboratory centrifugation of a live crude oil indicate that the size of asphaltenes in crude oil is $2 nm, and their aggregation number (in nano-aggregates) is between 3 and 8 [8].…”