Estimation of asphaltene precipitation and equilibrium properties of hydrocarbon fluid phases using the PC-SAFT equation of state

Abstract

Most of the components of heavy oil contain coarse grained molecules and it is difficult to accurately estimate the density and phase behavior of such fluids by cubic equations of state (EOS). Meanwhile, equations of Statistical Associated Fluid Theory (SAFT) have shown much success due to their high ability to calculate fluid's density. Among them is the PC-SAFT theory which despite being introduced about a decade ago, has not been vastly used in the industry and commercial software packages. In this survey, an alternative to describe fluids containing coarse grained molecules at high pressures is presented, using the PC-SAFT theory. Then, some properties of oil such as fluid's density, bubble- and dew point, phase envelop and asphaltene precipitation, are estimated. Precipitation of asphaltene in different stages of crude oil production causes formation damage, choking in pipe lines and wells and the production system. Therefore, estimating the asphaltene behavior in a crude oil looks vital. One of the major purposes of the present work is to investigate asphaltene phase behavior which is solely accomplished by modeling as a function of temperature, pressure and composition. Despite the previous SAFT modeling, the method of calculating the amount of asphaltene precipitation is based on the solid model. Qualitatively, this model is compared with experimental data which are obtained from adjoining a solvent (nC5 to nC9) with heavy oil, to observe its prosperity in predicting asphaltene precipitation. In this comparison, the average error percentage yielded an interval of 5 to 12% for the asphaltene precipitation weight percentage. Except the association term, the repulsive and attractive molecular interaction terms are considered for the present approach. Therefore, the model input includes the oil pseudo-components, PC-SAFT parameters and interaction coefficients between components. To model asphaltene precipitation using the PC-SAFT equation of state, the system is considered as vapor–liquid–quasi-liquid so that its equilibrium helps modeling, considering asphaltene as a quasi-liquid pure phase. The main privilege of estimating asphaltene precipitation by the method presented in this study over the solid model is that it does not need to adjust the volume shift parameter, whereas it is essential when using cubic EOSs.