Correlation of PVT Properties for UAE Crudes (includes associated papers 26135 and 26316 )

Abstract

Summary. Available PVT data from UAE reservoirs was used to obtain correlation equations for bubblepoint pressure and oil FVF as functions of oil specific gravity, gas gravity, solution GOR, and reservoir temperature. Error bounds of the obtained correlations are calculated and compared with other available correlations. The correlations of this study result in lower errors and should work better for UAE crudes. Introduction PVT data are essential in reservoir engineering calculations. It is important to obtain reservoir fluid samples to determine PvT properties. In case no fluid samples are taken, correlations can be used to estimate PVT data. This is particularly true during the early development phase where fluid properties are only available from surface flow tests. Correlations of bubblepoint pressure, formation pressure, and FVF, were published as early as 1947 by Standing, who used California reservoir samples. The bubblepoint pressure, Pb. and the oil FVF, Bob, were correlated with gas solubility, Rs gas and oil gravity, and reservoir temperature, T. The correlation was based on 22 different crude oil/natural gas mixtures and a total of 105 samples. Other correlations were made from data available in other regions. Glaso used 45 samples to obtain a correlation for North Sea oils. Sutton and Farshad reviewed empirically derived PVT properties for Gulf of Mexico crude oils, and Al-Marhoun presented PVT correlations for Middle East crudes using data from 69 bottomhole samples. All correlations cited above were concerned with crudes from different locations and presumably of different characteristics. Each study claimed that the resulting correlation could be applied to crude conditions anywhere. The particular composition of each crude and the presence of certain components that characterized them and affected their properties are well-known. The effect of crude paraffinicity, and the presence of CO2, N2, and H2S on PVT properties have been studied. Lassater reported that the predicted bubblepoint pressure decreases as the CO2 content in the crude oil increases, while Glaso reported the opposite; i.e., saturation pressures increase with increasing CO2 content. Jacobson and Glaso both noted an increase in the Saturation pressure with increasing N2 content. Saturation pressure was found to decrease in the presence of H2S. The paraffinicity and the presence of CO2, N2 and H2S are not the only factors expected to affect crude PVT properties. Other factors may include the total sulfur content and the asphaltene content. Thus, a universal correlation would be difficult to obtain and instead, correlations for local regions, where crude properties are expected to be uniform, are a reasonable alternative. In the current study, measurements on 51 bottomhole samples from UAE reservoirs and regression analysis are used to obtain bubblepoint pressure and oil FVF correlations. Bounds of errors and comparisons with previous correlations are presented. Regression Analysis Bubblepoint Pressure. Standing presented the following general relation for the saturation pressure of an oil sample and solution GOR, gas and oil gravities, and formation temperature. (1) A particular model that expresses the relationship among variables can be written as (2) Taking the logarithm of both sides, (3) The unknowns in Eq. 3 are a0 through a4. Applying the known experimental data of Pb Rs, and Tin Eq. 3 allows us to solve for the five unknowns. The system can be solved for the a unknowns with . Details of the solution method are given elsewhere. Bubblepoint FVF. The bubblepoint oil FVF is related to solution GOR, gas and oil gravities, and formation temperature with the following general correlation: (4) For the current correlation, Eq. 5 was written in the form presented by Al-Marhoun. (5) The constant b is determined by trial and error, with the initial value assumed to equal the reciprocal of the average temperature, or (l/ST)/n. Then, Eq. 5 becomes - (6) which is solved in the unknowns through . A function M is defined as (7) Then, the bubblepoint FVF is derived from (8) PVT Correlations Table 1 gives the experimental data used to develop the following correlations. Bubblepoint pressure correlation: (9) Bubblepoint oil FVF correlation: (10) (11) Correlation Errors The deviation of correlated bubblepoint pressure and FVF from experimentally determined values was calculated by several statistical equations given below. Percent relative error: (12) SPEFE P. 41⁁