Virtually all of the published correlations for solution gasoil ratios for pressures at and below bubble-point pressures suffer from two major deficiencies. First, calculated values of solution gas-oil ratios do not match the concave up - point of inflection - concave down shapes as pressure declines below the bubble-point that are evident in experimental data, especially at high initial gas-oil ratios. Second, bubble-point pressure must be estimated using the same correlations; there is no convenient way to impose a known bubble-point pressure - such as one derived from reservoir pressure data. We propose a correlation where the equations are formulated to solve both of these problems.
Published correlations for oil formation volume factors for pressures at and below bubble-point pressures suffer the same two deficiencies plus a third problem: the material balance connection between oil formation volume factors, solution gas-oil ratios, and reservoir oil densities is not honored. We propose an oil formation volume factor calculation which is based on the solution gas-oil ratio correlation described above so that the first two deficiencies are alleviated. Further, the proposed correlation connects these fluids properties in material balance format.
In addition to solving the deficiencies discussed above, the proposed correlations fit our data base of 2097 laboratory measured values much more closely than other published correlations.
Also, we propose a modification of an existing bubble-point pressure correlation which can be used in the event that a field-data derived bubble-point pressure is not available. This correlation fits our data base of 728 laboratories measured bubble-point pressures more closely than other published correlations.
All three of the proposed correlations require the usual field data; solution gas-oil ratio at the bubble-point, reservoir temperature, separator gas specific gravity, and stock tank oil gravity.
Many correlations for estimating crude oil PVT properties have been published in the past 50 years. Most of these correlations yield reasonably accurate results when applied at the bubble-point pressure. However, for pressures below the bubble-point, the computed PVT properties may have considerable error.
The results of several correlations (Standing,1 Vasquez and Beggs,2 Petrosky and Farshad,3 and Kartoatmodjo and Schmidt4) were compared with laboratory data. To illustrate the problem, two sets of laboratory data were selected: one with relatively high initial solution gas-oil ratio and one with low initial solution gas-oil ratio. Figures 1 and 2 show the typical concave up - point of inflection - concave down shapes of both solution gas-oil ratios and oil formation factors as pressure declines below the bubble point for a reservoir oil with high initial gas-in-solution. Figures 3 and 4 show data which illustrate that the concave up portion of the curves are more nearly linear for oils with lower initial gas-in-solution.
Note that the correlations result in curves which do not have the correct shapes. This is important because many applications of these fluid properties require differences as pressure declines, i.e., the slopes of the lines are critical. The incorrect shapes of these correlations persisted throughout the entire data set.
