Geoscience Reference
In-Depth Information
Values of solution gas-oil ratio at the bubblepoint pressure should be
obtained from production test data or early production data measured
prior to the reservoir pressure declining below the bubblepoint pressure
of the reservoir oil. Field-reported values of gas production are usually
sales gas metered from the first-stage separator. Estimates of the volume
of gas vented from the stock tank must be added to the volume of
separator gas in order to obtain values of solution gas-oil ratio at the
bubblepoint pressure, which should represent the total gas-in-solution
initially in the reservoir. A method of determining the quantity of
stock-tank gas will be discussed.
Sometimes the bubblepoint pressure of the reservoir oil can be
estimated from field-measured average reservoir pressures.
1
If the
necessary pressure measurements are available, bubblepoint pressures
estimated in this way will improve the accuracy of the results of almost
all of the fluid property correlations.
Many correlation equations require the specific gravity of the
separator gas as an independent variable. Some correlations and
equations give better results if the weighted average of the total surface
gas is used. A method of estimating the specific gravity of the stock-tank
gas for use in calculating the weighted-average specific gravity of the
total surface gas will be discussed.
Solution gas-oil ratios at bubblepoint pressures from field data
Solution gas-oil ratio at the bubblepoint pressure must be estimated
by adding the volume of gas from the separator and the volume of gas
from the stock tank.
(3.1)
The producing gas-oil ratio from the separator,
R
SP
, is obtained
from early separator gas and stock-tank oil production data (i.e., while
reservoir pressure is still above the bubblepoint pressure of the oil). The
producing gas-oil ratio from the stock-tank,
R
ST
, is estimated using a
correlation. The best available correlation is given as equation (3.2).
2
(3.2a)
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