Correlation of Black Oil Properties at Pressures Below Bubblepoint Pressure—A New Approach

Virtually all of the published correlations for solution gas-oil ratios for pressures at and below bubblepoint 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 bubblepoint that are evident in experimental data, especially at high initial solution gas-oil ratios. Second, bubblepoint pressure must be estimated using the same correlations; there is no convenient way to impose a known bubblepoint 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 bubblepoint pressures suffer the same two deficiencies plus a third problem: the material balance connection of 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 fluid properties in material balance format. In addition to solving the deficiencies discussed above, the proposed correlations fit the data set of 2097 laboratory measured values much more closely than other published correlations. Also, we propose a modification of an existing bubblepoint pressure correlation which can be used in the event that a fielddata derived bubblepoint pressure is not available. This correlation fits the data set of 728 laboratory measured bubblepoint pressures more closely than other published correlations. All three of the proposed correlations require the usual field data; solution gas-oil ratio at the bubblepoint, reservoir temperature, separator gas specific gravity, and stock tank oil gravity. FIGURE 1: Comparison of solution gas-oil ratios from several correlations with laboratory data for a reservoir oil with high initial solution gas-oil ratio. FIGURE 2: Comparison of oil formation volume factors from several correlations with laboratory data for a reservoir oil with high initial solution gas-oil ratio. portion of the curves are more nearly linear for oils with lower initial gas-in-solution. Note that all 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 produced by these correlations persist throughout the entire data set. This paper introduces new correlations to model the performance of the solution gas-oil-ratio, Rs, and the oil formation volume factor, Bo, for black oils for pressures at and below the bubblepoint. The solution gas-oil ratio correlation is based on a “reduced” variable approach which permits use of bubblepoint pressure from any source. Oil formation volume factors are calculated with a material balance equation using solution gas-oil ratios and oil densities. The paper also proposes a bubblepoint pressure correlation developed using reservoir temperature, gas specific gravity, oil gravity, and the solution gas-oil ratio (at the bubblepoint pressure) which fits our data set more closely than the other correlations.