An Efficient Phase Behavior Package For Use In Compositional Reservoir Simulation Studies
Authors: Besserer George J. , Serra John W. , Best Dennis D. 
Published: December 2007

Abstract
A compositional reservoir simulation study has been done at costs approaching those of black oiltype studies. The cost reductions are possible through the special handling of the vapour-liquid equilibrium constants which are reduced to analytical functions of pressure and total cell fluid composition. This paper describes the techniques used and their application to an eleven component system which includes hydrogen sulphide, carbon dioxide, nitrogen, and eight hydrocarbons. The compositional simulation work was done to evaluate the effects of injecting nitrogen into a sour gas condensate reservoir incentral Alberta, Canada.
Addition of nitrogen to a gas condensate causes the dewpoint pressure to increase dramatically as well as causing the convergence pressure of the system to increase to values as high as 30,000 psi. This drastic compositional effect is beyond the capability of the standard K-value chart equilibrium data packages used in most compositional simulators. Simulators with a built-in equation of state can be used for this type of system, however, they are very expensive to run. The technique used in this study allowed the simulator to run several times faster than simulators with the standard package.
An equation of state was used to match laboratory vapour-liquid equilibrium data on the virgin gascondensate and gas condensate-nitrogen mixtures. A database was generated with the equation of state and parameters for the analytical functions were parameters for the analytical functions were determined from the data base. The analytical functions were then used in the compositional simulator. This technique keeps the complex equation of state calculations out of the simulator and yet allows accurate and efficient calculation of the vapour-liquid equilibrium constants over the whole composition and pressure range. This procedure makes compositional pressure range. This procedure makes compositional modelling of complex systems affordable.

Introduction
Analysis of the performance of petroleum reservoirs using mathematical simulation models cannot always be satisfactorily accomplished with black oiltype simulation tools. Such tools may overly simplify the representation of the fluid properties and the phase behaviour relationships. when mass transfer effects between two or more phases become dominant, it is usually necessary to use some type of compositional model. Traditionally compositional reservoir simulation models are considerably more expensive to run than a black oil simulation model. Although the increased accuracy and more rigorous treatment of the phase behaviour aspects is often justified, it is desirable to minimize the computer run times and associated costs.
Compositional simulation models represent the hydrocarbon phases as multi-component mixtures. INTERCOMP has previously published a paper describing a compositional simulation model which computes the changing composition of the liquid and vapor phases in a reservoir using the principles of mass conservation and phase equilibrium. The basic simulation model combines Darcy's law for fluid flow with the continuity equation to derive differential equations that conserve the mass of water and of each component required to characterize the fluid property behaviour of the hydrocarbon vapor and liquid phases.
The compositional model assumes that mass transfer in the reservoir is rapid relative to the movement of fluids everywhere in the reservoir. The mole fraction of each of the components in the vapor phase is related to the mole fraction in the liquid phasephase constants (K factors). phase constants (K factors). using equilibrium
The detailed description of the equations and techniques used to solve the equations in an iterative fashion have been published. The current work describes a technique which was applied in a specific case to speed up the model and result insignificant cost savings.