Determination of Dew Point Pressure With and Without a Visual Cell

Abstract

The dew point pressure of a gas condensate fluid is used for planning the production profile of gas condensate reservoirs and for determining their economic success. Usually the best method of the determination of the dew point pressure loci is by visual observation when a constant mass expansion (CME) test is performed on a sample in a high-pressure cell fitted with a glass window. In this test the cell pressure is reduced in steps and the pressure at which the first sign of a liquid droplet is observed and is recorded as the dew point pressure. The gas volume of the saturated fluid at dew point pressure is used as a reference volume and the total hydrocarbon volume measured for each pressure depletion step is reported relative to this volume. For lean gas condensate systems the amount of heavy components that can condense during pressure reduction is less than 4 mol% (<50 bbls/MMscf). Such experimental determination of dew point pressure for these gases require many small pressure reduction steps which make the observation method tedious, time consuming, and expensive. In this study the authors demonstrate that a graphical method of determining dew point pressure, which does not require a windowed cell for visual observation is possible. The authors have extended the Y-function, which is often used to smooth out CME data for black oils to gas condensate systems. They started from the initial measured pressure and volume and by plotting log of the extended Y-function, which they call the Y_RRR-function, with the corresponding pressure so that two straight lines were obtained: one above and one below the dew point. The point at which these two lines intersect is the dew point pressure. The Y_RRR-function method was applied to determine the dew point pressure for both a lean and a rich gas condensate system. The difference between the dew point pressures determined by the authors’ proposed Y_RRR-function method and the observation method was less than ±2.0% for both gas systems. This extension of the Y-function to determine dew point pressure was not found elsewhere in the open literature.

Keywords:

• constant mass expansion
• dew-point pressure
• gas condensate
• relative volume
• visual observation
• Y-Function