Page 188 - PVT Property Correlations

162 PVT Property Correlations

G p
R p 5 ð7:4Þ
N p
Fig. 7.8 shows that the field producing GOR is a constant value (around
580 scf/STB) for some time until cumulative oil production has reached 0.5
MMSTB. GOR starts to increase and reaches 1000 scf/STB. The GOR data
generally show fluctuations. In this example, considerable variation in GOR
data occurred in 2005 and after, due to changes in wells completion and
poor maintenance of gas meters.
Consideration of the entire field GOR rather than individual wells GOR
is important for selecting an appropriate value for use in PVT correlations.
In addition, taking a 3 or 4-month moving average for the data reduces the
scattering in GOR data and obtains a more representative average initial
GOR. The correct PVT correlations input is the total GOR (and not the sepa-
rator GOR). In the majority of oil field operations, the gas produced from
the stock-tank (and sometimes the low-pressure separator) is vented or sent
to the flare. Therefore, the value of the stock-tank GOR is not usually avail-
able. Correlations are used to estimate the stock-tank GOR. These correla-
tions are dependent on primary separator conditions in addition to the usual
correlations input parameters.

Estimating Stock-Tank Gas Oil Ratio
Investigating the summary tables for oil correlations shows that four correla-
tions exist for calculating stock-tank GOR. Further information on their
applicability ranges is given in Appendix C, Oil Correlations Range of
Applicability. These correlations require primary separator conditions in addi-
tion to separator GOR and stock-tank oil gravity to calculate an estimate for
stock-tank GOR. Once the stock-tank GOR is calculated, it is added to the
separator GOR to estimate the total GOR required for oil PVT correlations.

Estimating Stock-Tank Oil Gravity
Stock-tank oil gravity is usually available from the oil sales data.
Theoretically, stock-tank oil gravity is constant as long as the oil is above
the bubble point. Stock-tank oil gravity slightly decreases in black oils, while
it slightly increases for volatile oils when the reservoir pressure falls below
the bubble point. The change in API gravity value occurs due to changes in
oil composition when gas is released from the oil in the reservoir.
Occasionally, these changes in API are not recorded properly. The API value
that is required as input in oil correlations is the initial value (when the oil is
still undersaturated).