Asphaltene inhibitor (AI) refers to chemical additives that help prevent asphaltene deposition when dosed at very low concentrations into the oil (up to a few hundred parts per million by volume), unlike asphaltene solvents that typically require much higher concentrations to work (percent level).
Although generally referred to as asphaltene inhibitors by the industry, this class of additives can actually further divided as asphaltene inhibitors and asphaltene dispersants, depending on their mechanisms of preventing asphaltene deposition. It has been reported that asphaltene inhibitor performance from lab testing does not necessarily reflect the performance in the field.
Mechanisms of inhibition
In general asphaltene inhibitors are polymeric chemistries with charged groups that are aimed to function as artificial resins to keep the asphaltenes in solution. They provide real inhibition since they prevent the aggregation of asphaltene molecules and therefore shift the asphaltene onset pressure. If an asphaltene inhibitor contains long alkyl long chains, it can help disperse any formed asphaltene aggregates as well. Therefore, some asphaltene inhibitors can function as asphaltene dispersants.
A thermodynamic model has been proposed for asphaltene inhibition by treating asphaltenes as micelles. The absorption interaction between an asphaltene and an amphiphile molecule (natural resin or synthetic additive) is considered the most important parameter for the stabilization of asphaltene micelles in crude oil.
It is well-known that the performance of asphaltene inhibitors are oil-specific. A product that works well on one type of oil may not work well on other fluids. This is partially contributed to the presence of organic acids in the oil. Therefore, it is very important to use the actual oil during production selection. If not chosen properly, some asphaltene inhibitors can actually make the asphaltene problem worse by precipitating more asphaltenes compared to an untreated system.
Some asphaltene inhibitors demonstrate a critical concentration level below which no performance is observed, but once above, it shows a dramatic effect by preventing asphaltene from flocculation. However, some asphaltene inhibitors do now show a critical concentration, but instead show an accumulative effect in reducing the amount of asphaltene precipitation. In general, asphaltene inhibitor is not 100% proof in preventing asphaltene deposition.
Since asphaltene inhibitors prevent precipitation, they are best applied upstream of the location where asphaltene onset pressure occurs, typically downhole. It is usually injected on an continuous basis through a downhole capillary string. Asphaltene squeeze into the formation has been proposed, but has not been proven effective most due to short squeeze life.
As mentioned previously, the asphaltene performance is very oil-specific. Therefore, it is very important to conduct inhibitor screening on the actual crude sample. Dead oil can be used as a preliminary screening method, but live oil testing is recommended as it more closely mimics the field conditions.
Unlike asphaltene inhibitors, asphaltene dispersants do not prevent asphaltene precipitation but reduce the size of asphaltene particles and keep them in suspension to reduce asphaltene deposition.
Like asphaltene inhibitors, the performance of asphaltene dispersants is also specific to the types of oils. They typically do not show a critical concentration effect and the effectiveness is proportional to the concentration.
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