Estimation of Solution GOR of Black Oils

Abstract

Summary. An empirical equation was developed to estimate stock-tank GOR from field data for black oils. Solution GOR at or above bubblepoint pressure can be obtained as the sum of the estimated stock-tank GOR and the measured separator GOR. The equation was derived with data from two-stage separations of 301 black-oil samples. Introduction Volumetric and material-balance calculations of oil in place require estimates of oil FVF and bubblepoint pressure. Several correlations, the most applicable of which was proposed by Standing, I are available for determining bubblepoint pressure and oil FVF when an estimate of solution GOR is available. Frequently solution GOR, Rsb, is estimated for pressures at or above the bubblepoint by setting its value equal to the initial producing GOR. However, the initial producing GOR often does not include gas vented from the stock tank. This omission can cause the estimate of Rsb to be low by 20 % or more. 2 The inaccurate estimate of Rsb then introduces error into estimates of bubblepoint pressure and FVF, and hence into oil-in-place calculations. This paper presents a method of using readily available field data to obtain an accurate estimate of Rsb. Total producing GOR, R, is equal to the sum of stocktank GOR, Rst, and separator GOR, Rsp.