Rethinking the modeling approach for asphaltene precipitation using the PC-SAFT Equation of State

Abstract The Perturbed Chain version of the Statistical Associating Fluid Theory Equation of State (PC-SAFT EoS) has been successfully applied to the modeling of the phase behavior of petroleum systems at reservoir conditions and the prediction of asphaltene precipitation. Although the predictive capabilities of this method and its wide range of applications are much better than other modeling techniques based on conventional cubic equations of state, there are certain features that are still missing, such as the polydisperse nature of asphaltenes. Furthermore, the simulation parameters are usually tuned to Asphaltene Onset Pressure (AOP) data –which are usually obtained by depressurizing live oil samples− or titration experiments of dead oil samples at ambient conditions. These techniques are subject to significant uncertainties caused by slow kinetics of asphaltene aggregation and the limited sensitivity of the instruments used. The focus of this work is on the experimental study and modeling of asphaltene precipitation from n-alkane - diluted crude oil mixtures considering the polydispersity of asphaltenes. To investigate the effect of asphaltene polydispersity, a crude oil sample is studied by sequential precipitation with different normal alkanes. A recently developed experimental technique called the “Indirect Method” is used for the detection and quantification of asphaltene precipitation. Based on our previous research using the indirect method, it was found that both the onset and the amount of asphaltene precipitation change with the aging time for a wide range of volume fractions of the added n-alkane. However, the data showed that the amount of precipitated asphaltene at 90 volume percent of a given precipitant is nearly time invariant. This experimental data point is not controlled by the kinetics of asphaltene aggregation and, for this reason, is used in this study to adjust the simulation parameters. The experimental data are modeled using the Perturbed Chain form of the Statistical Associating Fluid Theory. An enhanced characterization method is used to minimize the number of adjustable parameters. In this work, a distribution function is proposed to represent the asphaltene molecular weight distribution for different asphaltene sub-fractions. It has been found that the inclusion of four asphaltene sub-fractions to represent the polydispersity of asphaltenes is necessary to match the amount of asphaltene precipitated. The modeling results show a reasonable agreement for the precipitated amount at 90 volume percent of n-pentane, n-hexane, n-heptane and n-octane. The PC-SAFT EoS predicts an onset point that requires less addition of n-alkane than the corresponding experimental point after one day of aging time. This result suggests that a thermodynamic limit exists for the onset of asphaltene precipitation and that it corresponds to experiments that require very long aging times, as it has been previously reported by Maqbool et al. [1]. By fitting the simulation parameters to the amount of precipitated asphaltenes (and not the detected onset points) at 90 volume percent of added asphaltene precipitant, we can successfully remove the variability of the data with respect to the aging time. This tuning procedure also enables a more accurate prediction of the amount of precipitated asphaltenes at reservoir conditions, which is a more important result for the modeling of asphaltene deposition than the Asphaltene Onset Point itself. The integration of this novel characterization procedure as part of the Asphaltene Deposition Tool (ADEPT) [2–4] developed at Rice University is a topic of ongoing research.