Abstract

This paper presents and discuses a complete phase behavior package for determining PVT properties of mixtures of hydrocarbon and non-hydrocarbon species commonly encountered in the Petroleum Industry. All calculations are performed using a generalized Equation Of State, which upon selection of appropriate parameters, can be used to represent any of five Equations of State commonly employed in the Petroleum Industry; i.e., Redlich - Kwong, Zudkevitch - Joffe - Redlich - Kwong, Soave - Redlich - Kwong, Peng - Robinson and Schmidt - Wenzel.

A robust algorithm, which utilizes a combination of the modified successive substitution and Minimum Variable Newton-Raphson methods, is used to calculate flash composition and saturation points. The algorithm is optimized to facilitate convergence near the critical point. The modified successive substitution method is used to determine the number of phases present at a given pressure and temperature. A new simple method is used to ascertain whether the system temperature is greater or less than the critical temperature. The method does not require estimation of the critical temperature for the specified mixture. Straight forward application of a method proposed by Baker and Luka allows the user to efficiently compute all saturation points (i.e., a bubble point and a dew point, or, an upper dew point and a lower dew point) at a given value or temperature. This latter unique feature enables the user to define the two-phase envelope without having to perform a multitude of time consuming flash calculations. Finally, results from our phase package are compared to those presented previously in the literature in order to verify the accuracy and efficiency or the computational procedure.

Introduction

Calculation of fluid properties and phase equilibria is important as a general petroleum engineering tool. Numerical computation of phase equilibria using Equations of State is very important because laboratory analysis is expensive and time consuming.

In this work, we discuss a robust phase package that can be used to perform phase equilibria calculations based on any of five Equations of State commonly used in the oil industry; that is, the Redlich - Kwong1 (RK), Zudkevitch-Joffe - Redlich - Kwong2 (ZJRK), Soave - Redlich - Kwong3 (SRK), Peng - Robinson4 (PR) and Schmidt - Wenzel5 (SW) Equations of State.

The package is designed to enable the user to generate a wide variety of fluid properties, such as densities, compressibility factors, and viscosities. Moreover, the package offers the option of using one, or a combination of two numerical techniques to solve the single-stage separation equations that describe the thermodynamic equilibrium of fluids, depending on whether the mixtures considered are single phase, single phase saturated, or two phase (liquid and vapor).

The first numerical technique is a special version of the commonly known successive substitution method6. Its two major advantages are its ability to detect the single phase region without having to compute the saturation pressure, and its simplicity. Unfortunately, this method converges very slowly (or not at all) near the critical region. When the successive substitution method is converging slowly, our computer program automatically switches to a more robust technique called the Minimum Variable Newton Raphson (MVNR) method.7

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