Summary

Certain fluid properties are required for studies related to management of gas/condensate reservoirs or prediction of condensate reserves. Often these studies must begin before laboratory data become available, or possibly when laboratory data are not available. Correlations to estimate values of these properties have been developed that are based solely on commonly available field data.

These properties are the dewpoint pressure of the reservoir fluid, changes in the surface yield of condensate as reservoir pressure declines, and changes in the specific gravity of the reservoir gas as reservoir pressure declines. No correlations based solely on field data have been published for any of these properties.

The field data required are initial producing gas/condensate ratio from the first-stage separator, initial stock-tank liquid gravity in °API, specific gravity of the initial reservoir gas, reservoir temperature, and selected values of reservoir pressure.

The dewpoint pressure correlation is based on data of 615 samples of gas condensates with worldwide origins. The other two correlations are based on 851 lines of constant-volume-depletion data from 190 gas-condensate samples, also with worldwide origins.

Introduction

Correlation equations for gas condensates based on readily available field data have been developed. The correlations can be used to predict dewpoint pressures, decreases in surface condensate yields after reservoir pressure has decreased below dewpoint pressure, and decreases in reservoir-gas specific gravity at reservoir pressures below dewpoint pressure.

A value of dewpoint pressure is essential data for any reservoir study. A reasonably accurate estimate of dewpoint pressure for a specific reservoir fluid is necessary in situations in which laboratory data are not available or before laboratory data are obtained. Laboratory measurements of dewpoint pressure and other gas properties of 615 gas condensates with worldwide origins were used to develop a dewpoint-pressure correlation based on initial producing gas/condensate ratio, initial stock-tank oil gravity, and specific gravity of the original reservoir gas. This is the first proposed dewpoint-pressure correlation that does not require some laboratory-measured quantity.

Estimation of decreases in producing yields after the reservoir pressure drops below the dewpoint pressure is necessary for accurate prediction of condensate reserves. The reduction in surface yields can be as much as 75% during the primary production of a gas condensate. This reduction must be taken into account in the prediction of ultimate recoveries of condensate. A surface-yield correlation has been developed that is a function of a selected reservoir pressure, initial stock-tank oil gravity, specific gravity of the original reservoir gas, and reservoir temperature. The data set included laboratory studies of 190 gas-condensate samples. This is the first proposal offered in petroleum literature of a correlation to estimate the decreases in surface yield.

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