Abstract

A limited number of multiple carbon number groups are used in compositional simulation to represent the heptanes plus fraction of the petroleum mixture.  The selection of the single carbon number groups that are grouped into the multiple carbon number groups has a severe effect on the calculation of the position of the critical point of the mixture. This in turn strongly affects the maximum constant volume line of the phase diagram at reservoir temperature. If the maximum liquid volume at reservoir temperature is nearly correct then the match of other pressure-volume-temperature data of a laboratory study is relatively simple. A correlation has been developed to determine the correct grouping of the multiple carbon number groups. Also it was found that only two multiple carbon number groups are required to represent the heptanes plus fraction of a petroleum mixture. 

Introduction

The selection of the number and compositions of the pseudo-components that represent the heptanes plus fraction of a petroleum mixture causes the calculation of seriously different values of the critical temperature and critical pressure for that mixture. Of course, there should be a single critical point for a single mixture, so all but one of these calculations of critical temperature and critical pressure must be in error.

More important than the position of the critical point is the shape of the phase diagram. The position of the critical point in pressure and temperature controls the shape of the phase diagram since the bubble point pressure and the dew point pressure lines converge at the critical point. Also the quality lines (also called constant liquid volume lines or isovolume lines) all converge at the critical point. Thus the calculated value of the critical point must be nearly correct so that the isovolume lines of the phase diagram at reservoir temperature are approximately correct. This aids the “tuning” process for the equation of state because the process is primarily an adjustment of the parameters of the equation of state to cause a fit of the phase diagram at reservoir temperature.

This paper illustrates the affect on the phase diagram of altering the compositions of the pseudo-components (also called multiple carbon number groups, MCNs) representing the heptanes plus part of the composition.

A correlation was developed to permit selection of the compositions of the MCNs for gas condensates and volatile oils so that the starting point of the “tuning” process calculates approximately the correct phase diagram at reservoir temperature. This correlation is based on the composition of the heptanes plus component of the petroleum mixture.

This research found that only two MCNs are required to represent the heptanes plus component, with the intermediates represented by two pseudo-components, and methane as a pure component. The non-hydrocarbon components are included as pure components as needed. Suggestions are made for increasing the number of MCNs to three if deemed necessary.

Twenty-two laboratory PVT reports were used to develop the correlation to select the correct MCN’s. The compositions of these fluids cover the full range of gas condensates and volatile oils. The results agree with previous work that was based on ten PVT reports so a total of thirty-two equations of state were “tuned” in developing this procedure.