A Novel Graphical Method for Determining Dewpoint Pressures of Gas Condensates
Authors: Potsch K.T. , Braeuer L. 
Published: December 2007

This paper describes a novel graphical method for determining the dew point pressure as a backup for the visual readings of the total volume (gas and liquid) during a constant composition expansion and the Z factor. The latter is taken from the overall composition of the gas condensate and should be provided with high accuracy.

Theoretically, this method will work even if no visual data are available. The method was tested on several gas condensate systems. The results exhibit a relative accuracy of less than 3% or maximum 5 bar.

Introduction

The dew point pressure is one of the most important quantities for characterizing a gas condensate. The experimental determination in a window PVT cell is often cumbersome especially when the condensate is a lean one and the liquid drop out is therefore very low. The visibility of the first drops of liquid is masked by the numerous reflections of the menisci and not enhanced at all by the grey backside of the cell. A better option is to observe the visual onset of liquid precipitation in a through-window cell.

Traditionally the determination of the liquid drop out is done by reading the distance between the meniscus of the mercury and the hydrocarbon liquid with a kathetometer (Fig. 1). A calibration curve then gives the actual volume of the liquid. The liquid saturation curve - i.e. the volume of the liquid divided by the total volume (gas and liquid) of the hydrocarbons in the cell - is extrapolated to S1=0 in order to find the dew point (Fig.2.)

Theory

The dewpoint pressure is sought experimentally in a constant composition expansion (CCE) with the temperature held constant. The content of the cell is not changed so that the number of moles stays constant as well

(1)

In this formula m denotes the number of moles, p is the pressure, Vt the volume of the PVT cell, Z is the compressibility factor, R the universal gas constant and T the absolute temperature. As long as the system pressure is above the dew point pressure there exists only one phase and one can use the single phase compressibility factor for the gas which is calculated from the overall composition of the system.

Below the dewpoint pressure naturally two phases form and one has to split the expression in Eq. 1 into gas and liquid moles

(2)

Making use of the definition of the liquid saturation one arrives finally at

(3)

where Z1 stands for the single phase compressibility factor and S for the saturation. The subscripts characterize the phases - gas and liquid. Except for Z1, which has to be calculated, the factor in front of the square bracket on the right hand side consists of measurable quantities. In the single phase region the expression in the square bracket will always yield the numerical value of one because the liquid saturation is non existent and therefore Z1 = Zg.

The key idea of this work is to detect the onset of the liquid formation by the following steps:–Calculate the number of moles in the cell at each pressure point (Eq. 1), starting well above the dew point with at least four points above the dew point using the single phase compressibility factor Z1.–Plot the moles versus pressure. This procedure should yield a straight horizontal line in the single phase region.–Continue with the same method also in the two phase region.

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