Phase Behaviour Computations For Reservoir Fluids: Effect Of Pseudo-Components On Phase Diagrams And Simulation Results

Abstract

In phase behaviour calculations and compositional simulations with an equation of state, it is advantageous to represent the oil and gas systems by a small number of pseudo-components. This paper presents a scheme for determining these pseudocomponents based on the K-values of all constituents that are present in the reservoir oil and gas systems. The scheme is simple and applicable to both the light and heavy components. Sensitivity studies are carried out to investigate the effect of different pseudo-component representations on the shape of various phase diagrams, and on the compositional simulation results. Introduction Phase behaviour calculations and compositional simulations with an equation of state require the use of pseudocomponents to represent the oil and gas mixtures. Because of the large number of components which form the heavy fractions (e.g. C6 + fractions), it is necessary to group them into pseudo-components. Furthermore, to minimize simulation costs, it is also advantageous to lump the light fractions. This paper presents a procedure for characterizing the heavy fractions of oil and gas and a systematic scheme for lumping the components into pseudo-components based on the K-values at a specified operating condition which is typical of the process under study. Phase diagrams are then generated to investigate the effects of different lumping schemes on the shape of the phase boundaries and quality lines. The construction of these phase diagrams provides a rapid method for examining the sensitivities of the computed phase behaviour to different lumping schemes over a wide range of pressure, temperature and composition. Compositional simulation with different lumping schemes is also carried out to investigate the effect of different pseudo-component representations on the simulation results. Extended Analysis of Heavy Fractions Extended analyses of the heavy fractions (e.g. C6 + fraction) are required for accurate predictions of oil and gas phase behaviour from an equation of state (EOS)(1). True-boiling-point (TBP) analyses yield directly the boiling point, specific gravity and the molecular weight of each carbon-number (CN) group(2) from which molar distribution is found directly. Analyses from gas-chromatograph (GC) measurements, on the other hand, provides only the weight fraction of each CN group. The molar distribution is then obtained by using either the molecular weight of the normal alkane corresponding to each CN group, or the data of Katz and Firoozabadi(3, 4). The results obtained from ac measurements are less accurate than those obtained from TBP experiments. If only a partial extended analysis is available, it can be extended to higher CN group by using the method of Whitson(1), Pedersen et al.(5) or the one described in Appendix A. The critical properties and eccentricities of the CN groups are estimated from their specific gravities and boiling points by using Kesler-Lee correlations(6). If the specific gravity and boiling point of any CN group are not available, they can be estimated from the data of Katz and Firoozabadi(3, 4) or from the correlations reported by Whitson(7). Lumping into Pseudo-Components Because of the large number of components in the heavy fraction, it is necessary to lump them into pseudo-components before an EOS can be used efficiently.

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