The effect of resins on the aggregation behavior of asphaltenes
Date
2020
Authors
Derakhshani Molayousefi, Mortaza, author
McCullagh, Martin, advisor
Szamel, Grzegorz, committee member
Van Orden, Alan, committee member
Ettema, Robert, committee member
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Abstract
Millions of barrels of crude oil are extracted on a daily basis. Crude oil has four main components separated by the SARA fractionation method.1 Asphaltenes are the heaviest component of the cured oil. They are known to be responsible for clogging oil wellbores and pipelines, which bedevils the oil industry financially. Additionally, the cleaning chemicals and the clogging waste has a huge negative impact on our environment. The majority of the research on understanding the clogging problem is focused on the asphaltenes as a fraction of crude oil without much consideration for the effects of specific chemical structure. Moreover, the role of other components of the crude oil such as resins is not clear. Here, we have performed structure specific studies of asphaltenes by performing all-atom molecular dynamics (MD) simulations to quantify the aggregation behavior of asphaltenes in the absence and presence of resins. In this research, we have studied the aggregation tendency of asphaltenes in connection with their molecular properties. Systems with 20 counts of model asphaltene molecules were studied for nanoaggregation behavior of eight model asphaltenes in their neat state. We have quantified the aggregation tendency of asphaltene molecules in n-heptane with isodesmic free energy of aggregation, ∆Giso, as well as a quantity called aggregation propensity (AP). Using ∆Giso and AP value, we have classified model asphaltene molecules to three main category of non-aggregating, mildly-aggregating, and readily-aggregating asphaltenes. Each category of asphaltene have different aggregation behavior. They differ in their molecular features that ultimately is related to their aggregation propensity. Subsequently, we have studied the aggregation tendency of asphaltene in the presence of resin with total of 48 systems comprising 8 model asphaltene molecules in the presence of 6 model resins. We wanted to determine the role of resins in the aggregation behavior of asphaltenes by observing the effect of presence of resin on the ∆Giso and AP values. Additional to ∆Giso, we have defined a normalized quantity called aggregation propensity ratio (APR) to compare the effect of resin on the aggregation of asphaltenes. Resins studied in this work had no promoting effect on the aggregation tendency of asphaltenes. In general, both ∆Giso and APR metrics suggest that aggregation of asphaltene in presence of resin is either not affected or is prevented to different degrees. We have studied the aggregation behavior of asphaltenes in nanoaggregation, clustering and flocculation stages proposed by Yen-Mullins model. Resins have from minimal disruptive to highly disruptive effect on the nanoaggregation of asphaltenes. We investigated the further aggregation of stable nanoaggregates into clustering and flocculation with 500 counts of mildly-aggregating and readily-aggregating asphaltene molecules. We found that both clustering and flocculation stages occur for the readily-aggregating asphaltenes and do not occur for the mildly-aggregating asphaltenes. Readily-aggregating asphaltene molecules with large negative ∆Giso and large AP values lead to clustering and flocculation whereas the mildly-aggregating asphaltenes stay in the form of nanoaggregates. Our results show that in order for asphaltenes to flocculate, there is a threshold for existence of adequate favorable molecular features. Asphaltenes containing large enough aromatic cores and/or heteroatom reach clustering and flocculation stages. Furthermore, we found that in the presence of a highly disruptive resin, clustering and flocculation does not occur. For the readily-aggregating asphaltenes the aggregation stops in the nanoaggregation stage and for the mildly-aggregating asphaltenes the size of the nanoaggregates decreases. Our results explain what kind of resins are capable of potentially solving the deposition problem with providing insight on the molecular features of both asphaltene and resin molecules. Such molecular insights paves the road to explore more natural based solutions in preventing the clogging problem in the oil industry by informed characterization of each oil reservoir and its capability to form aggregate or prevent aggregates within itself and in another reservoir.
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Subject
asphaltene
molecular dynamics
resin
inhibition
aggregation
precipitation