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Summary

Asphaltene precipitation occurs when the pressure of a crude oil system falls below the asphaltene onset pressure (AOP) which may lead to subsequent deposition and plugging of the production tubing. The AOP curve is usually obtained experimentally and serves as an input to Equation of State (EoS) models. In this work, we highlight the importance of defining asphaltene precipitation by both the AOP curve and the amount of asphaltene precipitated. This is because experiments performed on wellhead samples are most likely missing the heaviest asphaltene fractions that have already deposited in the wellbore. Although the absence of this fraction can significantly shift the AOP curve, it will not have a notable effect on the amount precipitated since it only accounts for a small fraction of the total precipitated asphaltenes. Additionally, using the AOP curve along with the amount of asphaltene precipitated, we have shown that modeling asphaltenes as a monodisperse fraction gives a lower onset pressure and magnifies the amount of asphaltene precipitated as compared to the actual polydisperse case. Providing a more accurate prediction gives a better understanding of the system. This allows the use of general strategies, such as simple pressure and temperature control to prevent asphaltene deposition problem.

Introduction

Asphaltenes are a polydisperse mixture of the heaviest fraction of crude oil and are defined as a solubility class. They are soluble in aromatic solvents and insoluble in paraffinic solvents (Vargas et al. 2009). This polydisperse mixture can be fractionated using different n-alkanes which represent different precipitating strengths. n-Pentane insoluble asphaltenes are referred to as n-C5 asphaltenes and n-octane insoluble asphaltenes are referred to as n-C8 asphaltenes. The asphaltene fractions differ significantly in structure, solubility, and properties (Wang et al. 2006; Khaleel et al. 2015).

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