Simulation of Gas Condensate Reservoir Performance
Authors: Coats Keith H. 
Published: December 2007

Summary
This paper presents a generalized equation of state (EOS) that represents several widely used cubic EOS'S. The generalized form is obtained by manipulation of Martin's EOS and is applied in this study. A component pseudoization procedure that preserves densities and viscosities of the pseudocomponents and the original mixture as functions of pressure and temperature is described. This procedure is applied with material balance requirements in generation of two-component, black-oil properties for gas condensates. Agreement between resulting black-oil and fully compositional simulations of gas condensate reservoir depletion is demonstrated for a very rich, near-critical condensate. Also, agreement between EOS compositional results and laboratory expansion data is shown. The fully compositional simulation necessary for below-dewpoint cycling is performed for the near-critical condensate with a wide range of component pseudoizations. Results show the well-known necessity of splitting the C7+ fraction and indicate a minimal set of about six total components necessary for acceptable accuracy.

Introduction
Gas condensate reservoirs are simulated frequently with fully compositional models. This paper presents a pseudoization procedure that reduces the multicomponent pseudoization procedure that reduces the multicomponent condensate fluid to a pseudo two-component mixture of surface gas and oil. This allows the use of a simpler, less expensive, modified black-oil model that accounts for both gas dissolved in oil and oil vapor in the gas. A major question in the use of the black-oil model is whether the two-component description can represent adequately the compositional phenomena active during the depletion or the cycling of gas condensate reservoirs. This question is especially pertinent to near-critical or very rich gas condensates. This paper, therefore, includes a comparison of black-oil and compositional simulations for depletion and below-dewpoint cycling of a naturally occurring, rich condensate only 15 deg. F [8.3 deg. C] above its critical temperature. Like a number of unreported cases for leaner condensates, the two models give very similar results for depletion. In addition, the two models give identical results for cycling above dewpoint provided that certain conditions are satisfied. However, the black-oil model is not applicable to cycling below dewpoint, so results of the compositional model are compared for different multi-component descriptions to estimate the minimal number and identity of components necessary for acceptable accuracy. The compositional calculations reported here use variants of the Redlich-Kwong and Peng-Robinson EOS's. This paper discusses a general cubic EOS form based on work by Martin that encompasses all these EOS's. A general-component pseudoization procedure is presented, followed by its application to gas condensates. presented, followed by its application to gas condensates. The black-oil PVT properties obtained and the agreement between laboratory test data and EOS calculated results are given for the rich condensate. Black-oil and compositional simulation results are then compared for depletion and below-dewpoint cycling of the condensate. Finally, the compositional-model cycling results are compared for different degrees of pseudoization (lumping) of components.

A General Form for Cubic EOS's
Use of an EOS in compositional simulation of reservoir performance and laboratory tests requires two basic performance and laboratory tests requires two basic equations that give the compressibility factor z and the fugacity of each component for a homogeneous mixture (phase). The two equations,
(1a)(1b)
give these quantities as functions of pressure, temperature, and phase composition × = {xi}. A number of EOS's have been developed and are in wide use. These are the Redlich and Kwong (RK), modifications by Zudkevitch and Joffee and Joffee et al. (ZJRK) and by Soave (SRK), and the Peng and Robinson (PR) EOS. Martin shows that all cubic EOS's can be represented by a single general form. Use of Martin's work and basic thermodynamic relationships yields generalized forms for Eqs. 1a and 1b as follows:
(2a)
JPT
P. 1870