A New Depletion-Performance Correlation for Gas-Condensate Reservoir Fluids
Authors: Eaton B.A. , Jacoby R.H. 
Published: December 2007

This paper presents the depletion-performance correlations developed using data from PVT studies and well-completion tests of 27 rich gas-condensate and volatile-oil reservoir fluids. The PVT behavior for each fluid was determined experimentally in the laboratory, and the depletion performance of each fluid was then calculated. Well-completion test data consisting of reservoir pressure and temperature, initial total gas-oil ratio, and stock-tank oil gravity were used with the depletion-performance data to develop general performance correlations.

Three correlations are presented. The first equation shows the empirical relation of gas in place per barrel of hydrocarbon pore space with reservoir pressure and temperature, and initial total gas-oil ratio. The second equation is the correlation of stock-tank oil in place per barrel of hydrocarbon pore space. The third equation relates stock-tank oil production (to 500-psia depletion pressure) with pressure, temperature and oil gravity. All three correlations are also presented in convenient graphical form. Gas production to 500-psia depletion pressure is approximately 92.6 per cent of the gas in place for each fluid, so no correlation was developed for gas production.

Introduction

The rate of discovery of shallow, low-pressure and low-temperature oil reservoirs in the United States has been declining in recent years. For this reason, the search for oil gas gone to greater depths where high pressures and temperatures are encountered. A high-pressure and high-temperature reservoir is the environment in which many rich gas condensates and highly volatile oils are found. Because of this trend, the performance prediction of such reservoirs has become an important task of the reservoir engineer. The need and usefulness of a performance correlation based on field data that are available from the usual well-completion tests have previously been established.6 These earlier attempts have shown that the phase behavior of rich gas-condensate systems and volatile-oil and temperature.1 However, very little information on gas-condensate depletion performance5 was available at the time of these earlier attempts.

The available of more recent data has made possible the development of an improved correlation. The data, as listed in Tables 1 and 2, are for 25 gas-condensate and two volatile-oil systems that were studied. The problem was to correlate the data for in-place fluids and the production data with the initial total gas-oil ratio, initial reservoir pressure and temperature, and stock-tank oil gravity.

These correlations were obtained by multiple regression analysis, whereby the data were fitted to an equation by the method of least squared error.4

Basis of the Correlations

The data used in the development of these correlations were obtained from depletion-performance calculations6 and well-test or production measurements. All of the independent variables used in the correlations are available from the usual well-test completion data. The reservoir volume considered is 1 bbl of hydrocarbon pore space (HCPS). The gas-oil ratio used is the total standard cubic feet of gas (at 14.7 psia, 60F) coming off the entire separator train per barrel of stock-tank oil. This minimizes the effect of separator conditions which might vary from system to system.