An Efficient Tuning Strategy to Calibrate Cubic EOS for Compositional Simulation
Authors: Aguilar Zurita Rafael A. , McCain William D. 
Published: December 2007

A new strategy for preparing an equation of state (EOS) for use in compositional simulation is presented. The strategy consists of three major steps: extending the plus fraction and matching the saturation pressure, grouping, and matching of volumetric data.

The plus fraction is extended into 45 single carbon number groups (SCN) using a gamma probability function. Then critical pressure, critical temperature, and acentric factor are assigned to each SCN using the best available correlations. The laboratory measured saturation pressure of the fluid is matched by adjusting the molecular weight of the heaviest SCN.

The extended composition is grouped into two multiple carbon number groups (MCN) with the intermediates grouped into two pseudocomponents and methane as a pure component. The nonhydrocarbon components are included as required. The critical properties for the grouped components are assigned with a methodology that preserves the coefficients, a and b, of the equation of state that previously matched the saturation pressure. The slight change in the match of the measured saturation pressure caused by the grouping is overcome by adjusting the critical properties of the heaviest MCN.

Finally, volumetric data are matched using the volume translation parameters as regression variables.

Introduction

Equations of state (EOS) are widely used in compositional simulation and surface facility design. Volatile oils and gas condensates require special treatment due to their near-critical nature. Classical material balance cannot be applied to these near-critical fluids because their molar compositions change as the reservoir is depleted. The gas coming out of solution releases signficant amounts of condensate at the surface facilities. Thus, compositional simulation is necessary to determine the best economical and technical exploitation.

However, the performance of cubic equations of state is questionable in their predictive mode when they are applied to petroleum mixtures. Some of the parameters of an equation of state must be adjusted in order for the calculations to reproduce experimental data of a reservoir fluid. This is a time consuming and sometimes frustrating process.

Currently, engineers dealing with such fluids do not have a clear methodology to follow to make these modifications to an equation of state. Instead, several approaches are proposed in the literature for tuning the EOS parameters. The process is more of an art than a science.

Objective

The objective of this study is to introduce a new strategy that is logical, systematic, easily applicable, suitable to any naturally occurring petroleum fluid, and that requires the modification of only a minimum number of EOS parameters.

Approach

The proposed strategy is based on the results of laboratory tests of ten naturally occurring near-critical petroleum fluids.

This strategy is applicable to any cubic EOS. However, the Peng-Robinson EOS1 (PREOS) has been used in this study because of its popularity among researchers and engineers. Moreover, the PREOS is claimed to be more suitable for volumetric predictions. In this study, volume shift parameters2,3 are used to improve the volumetric capability of the PREOS.

Binary interaction coefficients (BIC) between hydrocarbon components are assumed to be zero while BIC between non-hydrocarbons and hydrocarbon components are set different than zero.

The following steps are necessary to adjust an equation of state so that calculations will match laboratory data:Extend the measured plus fraction to SCN45Assign critical properties and acentric factors to the components of the extended compositionMatch the saturation pressure using the extended composition