An Empirical Correlation for Oil FVF Prediction

Abstract

Abstract A new empirical correlation for predicting oil FVF at bubble point pressure is developed from 420 experimentally obtained oil FVF on 119 oil-gas mixtures collected from four different unpublished sources. When applied to all data, the newly developed correlation clearly out-performs the existing oil FVF correlations, based on low values of average per cent difference, absolute average per cent difference, and standard deviation. The present and the existing correlations are evaluated within pre-determined ranges of oil API gravity. Five main ranges of this parameter are selected for best correlations. Introduction The oil formation volume factor, Bo, is a factor used to estimate the change in volume of oil as it undergoes a change in pressure and temperature. The change in volume is a combination of thermal expansion, compressibility effects, and mass transfer, which is represented by gas evolving from or going into solution. Therefore oil FVF is the volume of oil at P and T occupied by one stock tank barrel (STB) of oil and the gas in solution. Several empirical correlations have been published that can be used to estimate Bo at and below bubble point pressure (Pb). The most widely used correlations are shown in Table 1. The scope of the proposed research consisted of developing a new correlation for estimating Bo at and below Pb. Data Acquisition and Preparation The analysis of 119 bottom-hole fluid samples were made available for this study. The data bank used (consisting of a total of 425 PVT tests), is obtained from four different unpublished sources. In these sources, the flash separation data were used in conjunction with the values of the flash and differential process to calculate the combination formation volume factor. The calculations were performed assuming that the relationship between the flash and differential liberation processes is constant at any reservoir pressure (the procedure of calculations is described, in detail, in Ref. 2, p. 18). Each of the PVT tests is examined for unreasonable values. If the observed FVF for a PVT test seemed unreasonable when compared to the FVFs estimated by all the existing FVF correlations (Le. the per cent errors of all the correlations are greater than 10%), the PVT test is eliminated from the data set. As a result, five data points are removed. The revised data bank now contains 420 data points (Table 2). A brief description of the revised data bank is given in Table 3. Development of the Correlation Using non-linear multiple regression analysis, the following relation is developed from 420 experimentally obtained oil FVF on 119 different oil-gas mixtures: Equation (Available In Full Paper) The same units apply for Rs, Gag, Gao, and T as in the equations shown in Table 1. The developed correlation meets approximately the requirement for the formation volume factor of 1.0 at 60 °F and atmospheric pressure (R=O). Figure 1 is a plot of the observed Bo values vs the estimated Bo values for the data used.