Page 141 - Hydrocarbon Exploration and Production Second Edition

128 Reservoir Fluids

reservoir conditions of temperature and pressure is calculated from the mapping
techniques discussed in Section 6.4.
As the reservoir pressure drops from the initial reservoir pressure towards the
bubble point pressure (P b ), the oil expands slightly according to its compressibility.
However, once the pressure of the oil drops below the bubble point, gas is liberated
from the oil, and the remaining oil occupies a smaller volume. The gas dissolved in
the oil is called the solution gas, and the ratio of the volume of gas dissolved per
3
3
volume of oil is called the solution GOR (R s , measured in scf/stb of sm /stm ).
Above the bubble point, R s is constant and is known as the initial solution GOR
(R si ), but as the pressure falls below the bubble point and solution gas is liberated,
R s decreases. The volume of gas liberated is (R si R s ) scf/stb.
As solution gas is liberated, the oil shrinks. A particularly important relationship
exists between the volume of oil at a given pressure and temperature and the volume
of the oil at stock tank conditions. This is the oil formation volume factor (B o , measured
3 3
in rb/stb or rm /stm ).
The oil formation volume factor at initial reservoir conditions (B oi , rb/stb) is
used to convert the volumes of oil calculated from the mapping and volumetrics
exercises to stock tank conditions. The value of B oi depends upon the fluid type and
the initial reservoir conditions, but may vary from 1.1 rb/stb for a black oil with a
low GOR to 2.0 rb/stb for a volatile oil. Whenever volumes of oil are described,
the volume quoted should be in stock tank barrels, or stock tank cubic metres, since
these are the conditions at which the oil is sold. Quoting hydrocarbon volumes at
reservoir conditions is of little commercial interest.
Figure 6.23 shows the change in oil volume as pressure decreases from the initial
pressure, the amount of gas remaining dissolved in the oil and the volume of
liberated gas.
If the reservoir pressure remains above the bubble point, then any gas liberated
from the oil must be released in the tubing and the separators, and will therefore
appear at the surface. In this case, the producing GOR (R p ) will be equal to R s , that is
every stock tank barrel of oil produced liberates R s scf of gas at surface.
If, however, the reservoir pressure drops below the bubble point, then gas will be
liberated in the reservoir. This liberated gas may flow either towards the producing
wells under the hydrodynamic force imposed by the lower pressure at the well, or it
may migrate upwards, under the influence of the buoyancy force, towards the crest
of the reservoir to form a secondary gas cap. Consequently, the producing GOR (R p )
will differ from R s . This is further discussed in Chapter 9.
In a saturated oil reservoir containing an initial gas cap, the producing GOR
(R p ) may be significantly higher than the solution GOR (R s ) of the oil, as free gas in
the gas cap is produced through the wells via a coning or cusping mechanism. Free
gas is the gas existing in the gas cap as a separate phase, distinct from solution gas
which is dissolved in the oil phase.

6.2.6. Fluid sampling and PVT analysis

The collection of representative reservoir fluid samples is important in order to
establish the PVT properties – phase envelope, bubble point, R s and B o – and the