ABSTRACT

The paper presents correlations of bubble-point pressures, formation volumes of bubble-point liquids, and formation volumes of gas plus liquid phases as empirical functions of gas-oil ratio, gas gravity, oil gravity, pressure, and temperature. Although the correlations are on California crude Oils and gases, comparisons are made for the various crudes reported by Katz. In order to facilitate the use of the data, the results of the correlations are presented in the form of calculating charts.

The solution of reservoir-performance problems requires that the physical properties of the reservoir fluids be known. These properties may be determined m the laboratory either from bottom-hole samples or from proper recombination of surface trap samples If the results of laboratory tests are not available, however, the physical properties must be estimated from field measurements The purpose of this paper is to give the results of several correlations between the variables normally measured in the field and the physical properties necessary for the solution of reservoir performance problems. Other correlations of this type have been reported by Gosline and Dodson,1a and by Katz3 Sage and Olds4 have recently reported an excellent correlation of formation volumes of condensate systems. The accuracy of the following P-V-T correlations is restricted by two factors. 1 The varied and complex multi-component hydrocarbon systems which are dealt with are defined by only three simple parameters gas gravity, oil gravity, and gas-oil ratio, and, 2. These parameters themselves depend upon the process by which the oil and gas are separated The method used by Sage and Reamer2 in the RIO Bravo Field studies of specifying hexanes and heavier material as "oil" and all pentanes and lighter material as "gas" would overcome these difficulties. However, this approach does not lend Itself to field usage. A further aid to preparing correlations would be to make use of a standard procedure in separating the oil and gas when gas-oil ratios are determined. However, as the P-V-T data are prepared for individual field conditions, it is normally not possible to use such a method. The gas-oil ratios, gas gravities, oil gravities, and formation volumes presented in this paper are laboratory values. They are the result of a 2-stage flash separation at 100 deg F-the first stage normally being within the pressure range of 250 psi to 450 psi and second stage at atmospheric pressure. This procedure is considered to approximate the average California field practices.

Bubble-Point Pressure Correlations

One of the important functions of P-V-T data is to indicate whether the reservoir oil is under saturated or saturated, or whether free gas is being produced from the sand. This required a knowledge of the gas solubility-bubble-point-pressure relationship of the oil and gases associated in the reservoir. In considering the manner in which the several variables affect the bubble-point pressure of a mixture of an oil and a gas, it seems reasonable to postulate a correlation of the form (mathematical equation available in full paper)

In developing the specific equation relating the bubble-point pressure to the variables on the right side of equation (1), the general relationship between the variables was used to suggest graphical methods of determining specific relationships.

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