Effects of inhibitor concentration and thermodynamic conditions on n-octylphenol-asphaltene molecular behaviours

The asphaltene stability in crude oil can be disturbed due to changes to thermodynamic conditions during production, raising flow assurance concerns. The addition of chemical inhibitors, such as surfactants, to a crude oil can improve the asphaltene stability. The impact of chemical inhibitors on the asphaltene aggregation intensity and aggregate characteristics, which is needed for design of an efficient inhibitor for a target crude oil, is poorly understood. In this paper, a molecular dynamics simulation tool is employed to simulate the molecular behaviour of the asphaltene and n-octylphenol (OP) at various concentrations of OP (0-15 wt%) and thermodynamic conditions (T = 300-360 K and P = 1-60 bar). To meet the objectives, the average aggregation number of different asphaltenes and asphaltene mixtures is measured besides the aggregate characteristics, including aggregate gyration radius, density, and shape. The results show that the archipelago asphaltenes (A1) do not self-aggregate severely, and the gyration radius distribution of the aggregates is similar after the OP addition. Nevertheless, the average aggregation number and the aggregate density analysis show that the aggregation intensity reduces slightly when the OP concentration is increased. Because the continental asphaltene (A2) can form hydrogen bonds, the addition of OP increases the aggregate dispersity significantly, which is identified with the highest peaks at 28 and 35 A after the addition of 7 and 15 wt% OP, respectively. It is found that OP increases the variations in the aggregate shape and type for A2 aggregates. It is concluded that a high concentration of OP (above 7 wt%) is needed to avoid the asphaltene aggregation. The pressure increase does not change the aggregate shape both with or without the OP. However, adding OP increases the aggregate gyration radius and decreases the aggregate density at 30 bar. Increasing temperature results in a severe aggregation in the system without the inhibitor as the aggregate gyration radius increases, and aggregates become more compact and spherical. However, by adding OP, both the average aggregation number and aggregate gyration radius decrease (especially at 360 K). According to the sensitivity analysis results, integrating GPUs with CPUs can speed up the simulation approximately three times, compared to the system processing with only CPUs. Outcomes from this research work can help for a better understanding of the molecular behaviour of asphaltene and inhibitors at various thermodynamic situations, leading to the improvement of inhibitor design. This paper highlights the competence of molecular dynamics for exploring the optimal inhibitor concentration and the thermodynamic conditions in which the inhibitor has the highest impact on asphaltene aggregation. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Chemical inhibitors, such as surfactants, can improve the asphaltene stability in crude oil by disrupting the asphaltene aggregation intensity and aggregate characteristics. Molecular dynamics simulations show that increasing the concentration of chemical inhibitors can reduce the aggregation intensity of asphaltenes. The research also reveals that the addition of certain inhibitors can significantly affect the dispersity and shape of aggregates, especially for specific types of asphaltenes.