Abstract

This paper examines the performance of methodologies and computational tools commonly used in the prediction of phase equilibria of mixtures in typical temperature and pressure range, at hyperbaric conditions. VLE and PVT data of binary and synthetic multicomponent mixtures up to 2000 bar are used. Correlated and prediction results are presented with the translated Peng-Robinson (t-mPR) EoS and EoS/GE models.

Correlation of high pressure VLE data is possible with one kij value for systems where Tc, Pc and experimental values are available. Predicted kij values for high asymmetric systems with hydrocarbons of Nc>20 are totally unacceptable at high pressure region. There is a great uncertainty on the critical properties and the acentric factor of hydrocarbons with Nc>20 and the relative importance of these parameters is very significant at high pressure conditions. Volume translation is essential for PVT predictions. Temperature independent correlations give very satisfactory results. LCVM, although it provides the best results of the EoS/GE models studied, suffers at high pressures.

Introduction

In the last few years as higher depths are explored by drilling, wider ranges of temperature (up to 250 C) and pressure (up to 1200 bar) are met in practice. Reservoir fluids at such conditions are commonly referred to as hyperbaric fluids. They consist of methane (greater than 40% in mole fraction) and high amounts of heavy hydrocarbons mainly n-alkanes. They exhibit gas condensate behavior at high temperatures but they can become oils at moderate ones because of the presence of heavier compounds. Some of these methane-rich fluids are near critical as well. Crystallization of heavy hydrocarbons have also been observed for temperatures 10-30 C lower than the pure component melting temperature. These characteristics cause a significant difficulty in studying their thermodynamic behavior, both from an experimental or from a theoretical point of view. Such mixtures have not been extensively studied in the literature and experimental data at such conditions is very limited.

The objective of this work is the examination of the applicability of the models used for low pressure reservoir fluids, at hyperbaric conditions and the identification of the problems encountered in the VLE and PVT calculations.

Methodology

The modified and translated Peng-Robinson EoS (t-mPR) as well as three predictive EoS/GE models (MHV2, PSRK and LCVM) are used to investigate the limits of our expectations in the quality of the predictions. The applied methodology consists of the followings:

VLE predictions. For systems including high molecular weight compounds such as nC24 and nC36, different methods of evaluating the critical properties and the acentric factor were tested as shown in Table 1.

kij values between C1/HC pairs derived from generalized correlations or from high pressure VLE data have been utilized. PVT predictions. The effect of three different approaches in the evaluation of the volume translation (t) has been examined. Table 2 presents the values used. One-parameter and multiparameter tuning has been applied to investigate possible improvement of the obtained results.

Vapor-Liquid Equilibria

Table 3 presents the results for the systems under study. Correlations with kij values appear satisfactory at high pressure. LCVM provides good results even at very high pressures of N2/C14 system but MHV2 and PSRK EoS/GE models fail, especially at high asymmetric systems.

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