Page 189 - PVT Property Correlations

Black Oils Chapter | 7 163

Estimating Separator Gas Gravity
Separator gas gravity is usually measured by taking a sample of the separator
gas (sales gas) and measuring its composition. The separator gas is usually
sold, and therefore it is necessary to calculate its heat content. The heat con-
tent calculation requires that the gas composition be known. The standard
procedure for calculating gas specific gravity from composition (see
Chapter 3: Dry Gases) is used.

Estimating Temperature
If oil PVT properties are required for a reservoir oil sample, the PVT proper-
ties will be calculated at the reservoir temperature. The reservoir temperature
is usually obtained from logging tools and is usually available in the log
header. If reservoir temperature data is unavailable, the following equation
can be used to estimate an approximate reservoir temperature (assuming nor-

mal temperature gradient). The temperature unit in this equation is F and
the depth is recorded in feet.

Depth
T 5 T surface 1 15 ð7:5Þ
1000
The temperature gradient varies from one location to another, and the
local temperature gradient is usually available.

Knowledge of Bubble-Point Pressure
If bubble-point pressure is not known for the reservoir fluid, we first calcu-
late the bubble-point pressure from a correlation, then use it in the following
calculations for other PVT properties. In some cases, bubble-point pressure
is known or can be estimated from field data. Fig. 7.10 is a plot of static res-
ervoir pressure (measured from several wells in the field) versus cumulative
oil production from the reservoir. Also shown in the plot is the cumulative
GOR versus cumulative oil production. A weighted moving averaging
method was used to estimate the reservoir pressure curve within the mea-
sured points. At cumulative oil production of approximately 0.5 MMSTB, it
is observed that the static pressure curve flattens and changes in slope to a
gentler slope. This point indicates that the reservoir has reached the bubble-
point pressure. The use of this value of bubble point (2700 psia) as input to
the oil PVT correlations to compute other PVT properties will result in more
accurate PVT properties than will the use of an estimated bubble point from
a correlation. For this particular example, a bottom-hole fluid sample was
available. The sample indicated that the bubble point pressure is 2760 psia
which is close to the value estimated from the static pressure versus cumula-
tive oil production plot.