Role of Characterization in the Accuracy of PC-SAFT Equation of State Modeling of Asphaltenes Phase Behavior

Abstract

The ability to capture asphaltene precipitation conditions is key to minimize costs associated with production caused by asphaltene depositing and blocking wellbores and pipelines. In this work, modeling of asphaltene precipitation of crude oils is performed using the perturbed chain form of the statistical associating fluid theory (PC-SAFT) equation of state (EoS). The characterization of choice is crucial to the accuracy obtained from any equation of state. Therefore, an alternative characterization is proposed where the molecular weight of asphaltenes is assumed to be temperature-dependent. This allows more flexibility in matching experimental data. Moreover, it compensates for deficiencies of near-infrared (NIR) spectroscopy and high-pressure microscopy (HPM) techniques used to measure asphaltene onset pressure (AOP). Results from six crude oils ranging from Middle East to Gulf of Mexico with varying gas injections are shown and compared. The proposed characterization shows some advantage over the original characterization especially in the low-temperature region. Most importantly, it illustrates that better accuracy can be obtained without the need to consider association as proposed by some. In general, both characterizations give asphaltene precipitation tendency predictions within acceptable range of error. However, the modified characterization can be most beneficial when high accuracy is needed such as in calculating the amount of asphaltene anticipated to deposit.

Characterization of crude oils and asphaltenes using the PC-SAFT EoS: A systematic review

“…AlHammadi et al [104] have reported that the experimental onsets that are measured by LST(NIR) and HPM methods generally follow a specific pattern, featuring an abrupt increase in the U-AOP values at low temperatures similar to the modeling trend exhibited by the PC-SAFT. They reported that some crude oils demonstrate atypical behavior with the U-AOP curve increasing linearly with a decrease in temperature, as demonstrated in the case with a crude oil from Kuwait (Fig.…”

“…3 MD/PD -Monodisperse/polydisperse modeling (asphaltene fraction represented by one/several pseudo-component(s)). 4 Liquid phases: Asphaltene-rich and wax-rich phases and bulk crude oil phase searchers [35,[54][55][56][57][60][61][62]104,105]. This method discriminates the stock tank oil (STO) components into four distinct groups including saturates, aromatics, resins, and asphaltenes according to SARA analysis results.…”

“…Laboratory measurement techniques used for characterization of petroleum fluids suffer from some limitations. For example, the LST + HPM results are sensitive to depressurizing rate and equilibration time; also, the smallest size of the aggregates that can be detected is limited by the wavelength of the emitted light source [61,103,104,125,151]. These limitations lead to uncertainties in experimental measurements.…”

“…The unusual behavior of the model was explained by the nature of experimental procedures that are used for the U-AOP detection. The slow aggregation of asphaltenes at low temperatures combined with lack of sufficient aging time during the experiments are hypothesized to cause the onset of asphaltene precipitation to be detected at lower pressures and the true onset to be bypassed by experiments [104]. Despite of this explanation, the researchers [104] suggested modifications to the SARA-based method that helps match low temperature U-AOP calculations with experimental measurements.…”

Thermodynamic Assessment of the Partitioning of Acetone between Supercritical CO2 and Polystyrene Using the Polar PC-SAFT Equation of State

“…The SAFT equation of state was proposed by Chapman et al , based on Wertheim’s theory. − Its ability to accurately predict phase splitting in systems with large variations of size and shape made it one of the most widely used equations of state. − Tremendous success has been shown ,− in modeling phase behavior of systems of large size differences such as polymers and asphaltene using the SAFT equation of state and its variations (e.g., PC-SAFT). These include systems of complex interaction mixtures of associating and nonassociating components. − Examples are the work of Aghaie et al and Kondori et al where they used the PC-SAFT equation of state to model CO 2 solubility in ionic liquids and to evaluate gas hydrates .…”