Retrograde GOR Behavior Of Condensate Reservoirs From Well Test Data
Authors: Joiner David S. , Long Gerald B. 
Published: December 2007

This paper describes an empirical technique that can quickly give the approximate retrograde GOR behavior of volumetric condensate reservoirs. The prediction is of a preliminary nature and is not prediction is of a preliminary nature and is not intended to replace the more rigorous, conventional prediction methods. Data needed for the technique prediction methods. Data needed for the technique are initial well stream gas-oil ratio, dew point pressure, reservoir temperature, well stream gas pressure, reservoir temperature, well stream gas gravity, and gravity of the separator liquid. The only item not normally known from well test information is the dew point pressure. It may be known approximately, or can usually be assumed equal to the initial reservoir pressure without too much error. The procedure involves solution of three empirical equations and the final result is a plot of gas-oil ratio versus reservoir pressure. This plot can be combined with a volumetric gas material plot can be combined with a volumetric gas material balance (P/Z versus cumulative plot) to obtain gas-oil ratio performance versus cumulative gas production for a volumetric condensate reservoir. production for a volumetric condensate reservoir. This performance can be related to time by using recognized techniques to time-rate the gas production. production

Introduction

In working with condensate reservoirs, accurate predictions of gas oil ratio vs. pressure are predictions of gas oil ratio vs. pressure are possible using compositional material balance or possible using compositional material balance or compositional model methods. However, these methods require compositional analyses of the reservoir gas, and months, or sometimes over a year may be required to gather samples, obtain special laboratory tests, and perform the ensuing studies. Preliminary studies, for planning and budgeting purposes, are usually required in a much shorter time frame. Long lead time equipment may even be ordered based on the preliminary studies. Compositional studies may preliminary studies. Compositional studies may never be made for many of the smaller condensate reserves being developed today and economics will be run based only on preliminary predictions of performance. For these reasons, every effort should performance. For these reasons, every effort should be made to improve the accuracy of early predictions. predictions. Although conventional gas material balance techniques (P/Z vs. cumulative gas production curves) provide a sufficiently accurate and fast prediction of gas production, there seems to be no prediction of gas production, there seems to be no easy way to predict liquid production. It was desirable to find or develop a quick and accurate empirical method to time-rate condensate liquid production using only the field data normally production using only the field data normally available from a condensate well test. This need was apparent in at least one area of operation which was finding and developing small condensate reservoirs. Economics were required before drilling each well, and initial performance of nearby wells indicated that there would be retrograde behavior. Time did not permit, and sizes of the reservoirs did not justify detailed compositional model studies.

Such a quick predictive method would also be useful in estimating the base case for preliminary economic analysis of cycling projects considered for large condensate reservoirs.

CASE HISTORIES FOR MATCHING

It was hoped that a correlation to accomplish these objectives could be found in the literature. The only correlation found was that of Eaton and Jacoby, which gave a prediction of ultimate liquid recovery, but did not provide for its time-rating. Since time-rated liquid production was essential for economic calculations, it was decided to attempt to develop such correlations.

Twelve condensate reservoirs were selected, where both initial well test data and a compositional analysis of the initial well stream fluid were available. This basic data is compiled in Table 1. For each of these cases, depletion performance predictions were made using a performance predictions were made using a compositional material balance program. These predictions were accepted as actual performances, predictions were accepted as actual performances, and the characteristics of these plots were studied preparatory to finding an empirical predictive preparatory to finding an empirical predictive method which would match them.