Molecular-scale mechanistic investigation of asphaltene precipitation and deposition control using chemical inhibitors

Ghamartale, Ali (2022) Molecular-scale mechanistic investigation of asphaltene precipitation and deposition control using chemical inhibitors. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Asphaltene stability can be perturbed during oil production and transportation, leading to asphaltene precipitation and deposition. Chemical inhibitors are usually added to the crude oil to postpone asphaltene deposition. The interaction and potential bonds between asphaltene and inhibitor molecules describe the mechanisms and efficiency of inhibitors during the prevention of precipitation and deposition. As the asphaltene type varies from oil to oil, screening, designing, and developing inhibitors for a target oil are necessary. The screening process of selecting an effective inhibitor will be much more efficient if actual inhibition mechanisms are known. Although chemical inhibitors have been used in the industry for a long time, understanding of interaction mechanisms between asphaltenes and chemical inhibitors is vital in developing an efficient inhibitor. Disclosing the interaction mechanisms using an experimental strategy needs high technology tools, and it is demanding and costly. This research aims to develop a simulation workflow to understand the interaction between asphaltene molecules, inhibitors, and surfaces, which will help to figure out the main prevention/aggregation mechanisms during precipitation and deposition. In this study, molecular dynamics (MD), an advanced computational chemistry method, is employed to analyze the inhibitory effect of n-octylphenol (OP) and two 1-Butyl-3-methylimidazolium ionic liquids for three different asphaltene structures. The employed asphaltene structures include one archipelago and two continentals. Based on the knowledge gaps, we first study the impact of asphaltene structure, inhibitor concentration, pressure, and temperature on the efficiency of asphaltene aggregation inhibitors. Then, the impact of inhibitors on asphaltene binding arrangement during the aggregation process is investigated. Finally, we explore the inhibitory effect of chemicals on asphaltene deposition in the calcite pore. The asphaltene aggregation, aggregate characterization, and deposit characterization are studied in oil bulk and confined oil systems (pore structure) to meet the objectives. This thesis begins with an extensive literature review, and the first sets of simulations focus on the impact of asphaltene structures during the precipitation process. In this section, systems with singular and binary asphaltene types in n-heptane are simulated in the absence and presence of OP, as a surfactant inhibitor. The results show that the OP can delay the aggregation of the continental asphaltene with the potential of forming hydrogen bonds. It is also concluded that the average aggregation number needs to be coupled with the gyration radius analysis to evaluate the hierarchical paradigm of asphaltene aggregation. After that, a sensitivity analysis is conducted to investigate important aspects such as the inhibitor concentration, thermodynamic condition, and computational hardware. This part aims to optimize the screening and designing of inhibitors from both technical and economic viewpoints. It is concluded that a minimum inhibitor concentration is needed so that the inhibitor appears to be impactful. It also shows that in the pressure range of 1-60 bar, OP has the most inhibitory effect at 30 bar since the asphaltene-asphaltene aggregation energies are extremely high at 1 bar such that the asphaltene-inhibitor energy can not cope with this situation. In comparison, the asphaltene-asphaltene interaction energy is extremely reduced at 60 bar such that the aggregates are already unstable even in the absence of OP. OP is most impactful at 360 K in the temperature range of 300-360 K. The significant impact of the inhibitor on aggregate shape in this study motivates us to study the detailed arrangement of asphaltene aggregation, which is directly related to the strength and stability of the aggregation. Therefore, we concentrate on more mechanistic details by investigating the impact of two types of inhibitor, including surfactant and ionic liquid, on the asphaltene binding arrangement. The outcome reveals the OP mainly forms bonding through hydrogen bonds, and the quadrupole-quadrupole interaction between the OP benzene ring and asphaltene core is weaker than the quadrupole-quadrupole between asphaltenes. In contrast, the ionic liquid reduces the asphaltene stacking association as the cation part of ionic liquids approaches the aromatic core of the asphaltene and beats the quadrupole-quadrupole interaction between asphaltenes. In the next step, the asphaltene deposition is studied when asphaltene-heptane is placed in a calcite pore without/with chemical inhibitors. According to the results, the OP adsorbs on the calcite surface and reduces LJ and Coulomb energies between asphaltene and calcite by 400 and 1000 kJ/mol, respectively, which reduces the asphaltene deposition on the surface. The selected ionic liquid has a short alkyl tail on its cation, which cannot provide a hindrance layer near the calcite surface. The combination of two inhibitors minimizes the precipitation and deposition of asphaltene when OP to IL ratio is 3:1. At this ratio, the aggregation number reduces from 20 to less than 10, and the deposition rate reduces from 1 to 0.8 compared to the case with no inhibitor. This thesis is a pioneering study to demystify the asphaltene-inhibitor behaviors during asphaltene precipitation and deposition, which can provide a useful workflow to screen, select, and design the effective inhibitor for the target crude oil (and asphaltene) besides saving time and money for the industry with effective pre-selection instead of conducting trial and error lab tests.

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