Prediction of extreme asphaltene onset pressures with PC- SAFT for petroleum reservoir fluids

Abstract

In this work, four versions of the perturbed chain- association fluid theory equation of state (PC- SAFT EoS) were used to predictive upper- asphaltene onset pressures (UAOPs), minimum upper critical solution temperatures (MUCSTs), and hyper- asphaltene onset pressures (HAOPs) for real petroleum reservoir fluids. As a result, the PC- SAFT models using the Liang and Kontogeorgis universal constants (LKUCs) predicted lower UAOPs and MUCSTs than the PC- SAFT versions utilizing the original Gross and Sadowski set (GSUCs). That is especially valid for the PC- SAFT EoS that combines the LKUCs with a temperature- and density-dependent effective diameter. Moreover, all the PC- SAFT versions predict ascending branches of UAOPs. Nevertheless, none of these branches tends to infinity. In addition, LKUCs try to suppress or eliminate the predictions of HAOP curves.

Introduction

Perturbed chain- statistical association fluid theory equation of state (PC- SAFT EoS) [1] is based on the first-order thermodynamic perturbation theory (TPT) for chain molecules developed by Wertheim [2], [3] and Chapman et al. [4]. This model includes attractive (dispersion) interactions by using the second-order perturbation theory of Barker and Henderson [5]. Not long ago, Abutaqiya et al. [6] reported that the original PC- SAFT [1] predicts a minimum upper critical solution temperature (MUCST) and a hyper- asphaltene onset pressure (HAOP) curve for petroleum mixtures prone to precipitate asphaltenes. The HAOPs result because the UAOP curves return from the respective MUCST up to more elevated temperatures (see Fig. 2 in [6]). This last finding appears to be independent of the characterization method chosen to describe the petroleum fluid [6]. In this work, UAOP and HAOP curves are predicted for six real petroleum reservoir fluids by employing four distinct PC- SAFT versions [7]. In short, these four PC- SAFT models result by combining two distinct sets of universal constants for the second-order dispersion with two different expressions of effective diameter [7]. The first model considered is the original PC- SAFT EoS [1], hereafter called as GS- PC- SAFT. This one employs the Gross and Sadowski universal constants (GSUCs) and the original PC- SAFT temperature-dependent effective diameter, $d\left(T\right),$ displayed by Eq. (3) in [1]. The second one is the result of combining Liang- Kontogeorgis universal constants (LKUCs) [8] with the original PC- SAFT effective diameter, named hereafter as LK- PC- SAFT [7]. The third PC- SAFT version combines the GSUCs with a recently published temperature-and density-dependent effective diameter given by Eq. (4) in [9], resulting in the GS- PC- SAFT-$d\left(T,\rho \right)$ model. Wherein $d\left(T,\rho \right)$means an effective diameter dependent upon temperature and density. This diameter is theoretically supported, and its contribution occurs principally at low temperatures, high pressures, and for dense (condensed) phases like asphaltenes [9]. Ultimately, the latter PC- SAFT model is given by combining the LKUCs with $d\left(T,\rho \right),$ resulting in the LK- PC- SAFT-$d\left(T,\rho \right)$ version. In fact, these four PC- SAFT EoS models are amply discussed in [7]. Thus, the results published in this manuscript complement those presented in [7] and [9].

Liang and Kontogeorgis [8] showed that under actual application conditions, the original PC-SAFT [1] predicts more than three volume (density) roots or two stable liquid volume roots. In addition, discontinuous volume roots were obtained as the pressure increases, which produces unrealistic predictions of phase behavior [7], [8]. Those authors reformulated the universal constants used in the dispersion part of PC- SAFT to eliminate that drawback. The resulting universal constants are here called LKUCs. In addition, those constants usually produce lower densities at reduced temperatures or higher pressures than GSUCs [1]. This last fact increases the pressure and temperature range wherein the fluid is isotropic, relegating the metastable conditions to very low temperatures or very high pressures. In fact, in conditions practically out of the most relevant engineering applications. It is important to highlight that the TPT of Barker and Henderson, the basis for PC- SAFT, was really formulated for fluid phases under isotropic conditions [5].