Dimensionless PVT Behavior of Gulf Coast Reservoir Oils

Abstract

The empirical correlations presented here show that for "typical" Gulf Coast reservoir oil, cumulative gas evolution and oil shrinkage are about proportional to pressure drop below the bubble point. The correlations proportional to pressure drop below the bubble point. The correlations should be useful for the Gulf Coast, and the correlation technique, with modifications, might also serve in other geologic provinces. Introduction Often, for an oil reservoir where a PVT analysis is not available, various aspects of depletion-drive behavior must be estimated. over the years various correlations have been published; and reliable empirical relations between bubble-point pressure, solution GOR, formation volume factor, etc., are available. Thus, for a given oil and reservoir conditions these correlations may be used to estimate the properties of the reservoir oil at initial (bubble point) conditions and at any desired pressure below the bubble point. An alternate approach to the use of empirical correlations is the use of a PVT analysis for an oil with properties and reservoir conditions similar to those properties and reservoir conditions similar to those of the reservoir being investigated. With either approach there usually is the question of whether the data so derived are representative of the oil and reservoir being investigated. The purpose of this paper is to present a method of comparing the PVT behavior of different reservoir oils by reducing the PVT data from a large number of oils to sets of dimensionless plots. The relations so developed, which are somewhat easier to use than previously published correlations, should provide previously published correlations, should provide additional insight into the probable PVT behavior of many Gulf Coast oils. These relations are not intended as a substitute for a PVT analysis, should the latter be available. They are designed to be used with other, previously published correlations and reservoir information for determining bubble-point pressure, initial (bubble point) formation volume factor, pressure, initial (bubble point) formation volume factor, reservoir temperature, and formation volume factor at atmospheric pressure and reservoir temperature. Procedure Procedure Sets of dimensionless ratios were calculated for each PVT analysis. These ratios are "dimensionless PVT analysis. These ratios are "dimensionless pressure," "dimensionless cumulative gas evolution," and pressure," "dimensionless cumulative gas evolution," and "dimensionless shrinkage." Dimensionless pressure is defined as pressure divided by bubble-point pressure. Dimensionless cumulative gas evolution is defined as cumulative gas liberated to a pressure divided by total gas liberated by a pressure drop from the bubble point to atmospheric pressure. Dimensionless point to atmospheric pressure. Dimensionless shrinkage is defined as shrinkage to a pressure below the bubble point divided by total shrinkage from the bubble point to atmospheric pressure. (See Nomenclature.) Some of the PVT analyses did not report a value for the formation volume factor at atmospheric pressure and reservoir temperature (Boa); therefore, pressure and reservoir temperature (Boa); therefore, for these analyses the correlation published by Katz was used to estimate this parameter, An example of the computer output showing the original data and the corresponding dimensionless ratios for one of the PVT analyses is shown in Table 1. PVT analyses is shown in Table 1. Dimensionless ratios were calculated from the PVT data from 80 oils in 31 Gulf Coast fields. The range of oil properties and reservoir temperatures is listed in Table 2. The frequency distribution of each variable is shown by the histograms in Figs. 1 through 5. In these histograms each asterisk denotes one oil sample or PVT analysis. JPT P. 538