Page 70 - Introduction to Petroleum Engineering

54 PROPERTIES OF RESERVOIR FLUIDS
a system is the ratio of the change in volume ΔV to the initial volume V. The fractional
volume change ΔV/V may be estimated from
∆V =− cp (3.10)
∆
V
where c is the average compressibility of the system and Δp is the pressure change.
The minus sign is applied so that an increase in pressure (Δp > 0) will result in a
decrease in the volume of the system. Similarly, a decrease in pressure (Δp < 0) will
result in an increase in the volume of the system.
Formation Volume Factor. The volume of oil swells when gas is dissolved in the
oil. The FVF for oil, B , expresses this swelling as a ratio of the swollen volume to
o
the volume of the oil phase at a reference condition, usually the stock tank pressure
and temperature. This ratio is expressed as reservoir volume divided by stock tank
volume. In this sense “reservoir” refers to pressure, temperature, and composition
that exist in a reservoir. An example of a unit for oil FVF in oil field units is RB/STB
where “RB” refers to reservoir barrels and STB refers to stock tank barrels or it could
be rm /sm for reservoir meters cubed per stock tank meters cubed in metric units.
3
3
For example, an FVF of 1.5 RB/STB means that for every barrel of oil produced to
the stock tank, 1.5 barrels were taken from the reservoir. The 0.5 barrel volume
difference represents the volume of oil phase lost as volatile species escaped from the
liquid phase during the reduction in pressure from the reservoir up through the well
to the separator and stock tank.
Usually, most of the change in volume from stock tank to reservoir results from
the volume of gas dissolved in the oil. But pressure and temperature also play a role.
The increase in pressure from stock tank to reservoir compresses the oil a small
amount, while the increase in temperature from stock tank to reservoir thermally
expands the oil.
FVF for oil usually ranges from 1 to 2 RB/STB. FVF for water is usually about 1
RB/STB because gas is much less soluble in water than in oil. Gas FVF varies over a
wider range than oil FVF because gas volume is more sensitive to changes in pressure.

3.4 GAS PROPERTIES

Formation Volume Factor. For this text, we use the ideal gas law to estimate the
FVF B for gas:
g
T ° ( R)
g (
B RB MCF) = 503 (3.11)
.
/
(
p psia)
where the units of each variable B , T, p are given in parentheses and MCF denotes
g
1000 ft . The coefficient on the right‐hand side of the equation includes conversion
3
factors. For this correlation, the temperature in degrees Rankine is required.
To improve the estimate of B , the real gas law should be used in place of the ideal
g
gas law.