Modeling of Asphaltene Precipitation Due to Changes in Composition Using the Perturbed Chain Statistical Associating Fluid Theory Equation of State

Abstract

Asphaltene precipitation and deposition adversely affect production assurance and are key risk factors in assessing difficult environments such as deepwater. Deposition in the near well bore regions and production tubulars implies high intervention and remediation costs. Prediction of asphaltene precipitation onset represents a challenge for the flow assurance area. The applicability of the PC-SAFT equation of state model to predict asphaltene onset of the precipitation in live oils is demonstrated by studying representative examples from field experiences with asphaltene problems during production. These examples not only validate the proposed model but also confirm the theory that asphaltene phase behavior can be explained based only on molecular size and van der Waals interactions. The PC-SAFT equation of state estimates also properties such as densities and bubble points of live oil systems using a minimum number of real components and “realistic” pseudocomponents. The amount and composition of the asphaltene precipitated phase is also determined as part of the equilibrium calculations. This paper presents experimental observations and simulation results at reservoir conditions using the PC-SAFT equation of state on the effect of compositional changes in live oils caused by two common processes in the oil industry: oil-based-mud (OBM) contamination and reinjection of associated gas. In the first case, the downhole oil samples can be contaminated with OBM, causing laboratory measurements of the bubble point and asphaltene precipitation to be different from the reservoir fluid. In the second case, the reinjection of gas into the field increases the gas−oil-ratio (GOR) of the oil. The addition of gas into the oil can cause asphaltene precipitation and deposition due to the increase of light components that reduces asphaltene solubility. Asphaltenes in this example are treated as both monodisperse and polydisperse pseudocomponents. From the obtained results, it can be concluded that the live oil modeled using the PC-SAFT equation of state properly predicts the asphaltene phase behavior under these compositional changes. The PC-SAFT equation of state model is a commercially available, proven tool.