This paper discusses time-resolved small-angle neutron scattering results that were used to investigate asphaltene structure and stability with and without a precipitant added in both crude oil and model oil. A novel approach was used to isolate the scattering from asphaltenes that are insoluble and in the process of aggregating from those that are soluble. It was found that both soluble and insoluble asphaltenes form fractal clusters in crude oil and the fractal dimension of the insoluble asphaltene clusters is higher than that of the soluble clusters. Adding heptane also increases the size of soluble asphaltene clusters without modifying the fractal dimension. Understanding the process of insoluble asphaltenes forming fractals with higher fractal dimensions will potentially reveal the microscopic asphaltene destabilization mechanism (i.e., how a precipitant modifies asphaltene-asphaltene interactions). It was concluded that because of the polydisperse nature of asphaltenes, no well-defined asphaltene phase stability envelope exists and small amounts of asphaltenes precipitated even at dilute precipitant concentrations. Asphaltenes that are stable in a crude oil-precipitant mixture are dispersed on the nanometer length scale. An asphaltene precipitation mechanism is proposed that is consistent with the experimental findings. Additionally, it was found that the heptane-insoluble asphaltene fraction is the dominant source of small-angle scattering in crude oil and the previously unobtainable asphaltene solubility at low heptane concentrations was measured.

Asphaltenes are the heaviest and most polar fraction of crude oil. They can precipitate at any point of oil production or transportation as a result of changes in the temperature, pressure, or composition. Their precipitation and deposition are having a serious economic impact on petroleum production. It was previously shown that that the precipitation of asphaltenes is a kinetic phenomenon. In this work, it is demonstrated that the kinetics of precipitation is universal among many different crude oils. However, the aggregation rates strongly depend upon the properties of the crude oil or the solvent used to stabilize asphaltenes. A new method of analysis is developed to estimate the aggregation rates by accounting for the properties governing the aggregation kinetics using Smoluchowski's aggregation model. It is shown that the viscosity and solubility parameter of the solution play an important role in controlling the aggregation behavior of asphaltenes in different crude oils and different model oils. However, with certain key assumptions in the kinetic analysis, all disparities in the aggregation rates of asphaltenes for different types of asphaltenes and crude oils disappear and the rates collapse onto a single curve. These findings will lead to a better understanding of the properties that govern the destabilization and growth processes, which can, in turn, give rise to new predictive models to foresee precipitation kinetics under different operational conditions.

This study discusses experimental and modeling results of asphaltene aggregation and deposition using various nalkanes as precipitants to destabilize asphaltenes from a crude oil. The amount of asphaltenes precipitated as a function of precipitant carbon number and concentration was obtained after monitoring the slow kinetic aggregation process. A geometric population balance was used to estimate the asphaltene−asphaltene collision efficiency during bulk aggregation. The results revealed that, for a fixed volume fraction of precipitant, the collision efficiency decreases with increasing precipitant carbon number, resulting in slower aggregation. The tendency for asphaltenes to deposit was measured using capillary flow experiments under similar conditions. Similar asphaltene deposition behavior was obtained when the results were normalized by the asphaltene solubility and other experimental factors. A modified aggregation model was applied to the results and revealed that the difference between the asphaltene and solution solubility parameters is a dominant predictor of asphaltene aggregation. The time required to form an initial deposit inside the capillary apparatus was also found to correlate with the difference between asphaltene and solution solubility parameters. However, the deposition rate of asphaltenes in the capillary apparatus did not correlate with the collision efficiency or solubility parameter difference, contrary to initial expectations. The results suggest that mass transport barriers in the apparatus provided sufficient resistance to deposition as to limit observable correlation between the deposition rate and collision efficiency.

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Destabilized asphaltenes can easily adhere to the oil well production equipment, the transportation pipelines, and also the refining and storage facilities, resulting in significant remediation costs. Therefore, it is of great importance to the energy industry to understand the factors that govern the kinetics of asphaltene aggregation and precipitation. In this study, the effect of asphaltene concentration on their aggregation and precipitation tendencies after their destabilization with heptane is investigated for three different types of asphaltenes. It is intuitively expected that any increase in asphaltene concentration will accelerate the precipitation kinetics after heptane addition. For asphaltene concentrations below 1 wt % in toluene, this expected trend is indeed experimentally confirmed. However, for asphaltene concentrations above 1 wt %, an increase in concentration leads to slower aggregation instead. We believe that this counterintuitive decline in the aggregation rate is due to the stabilizing effect of stable or soluble asphaltenes. This effect has been overlooked in the existing aggregation models, and our research provides a better understanding of the factors controlling aggregation process. Accounting for the solubilizing effect of stable asphaltenes can provide successful predictions for the aggregation rate of asphaltenes at different asphaltene concentrations using Smoluchowski’s model.