Page 116 - Fundamentals of Reservoir Engineering

PVT ANALYSIS FOR OIL 55

gas oil ratio if the pressure in the vicinity of the well is at or above bubble point
pressure. If not, the surface gas oil ratio will be too low or too high, depending upon
whether the free gas saturation in the reservoir is below or above the critical saturation
at which gas will start to flow. In this respect it should be emphasized that PVT samples
should be taken as early as possible in the producing life of the field to facilitate the
collection of samples in which the oil and gas are combined in the correct ratio.

2.4 DETERMINATION OF THE BASIC PVT PARAMETERS IN THE LABORATORY AND
CONVERSION FOR FIELD OPERATING CONDITIONS

Quite apart from the determination of the three primary PVT parameters B o, R s and B g,
the full laboratory analysis usually consists of the measurement or calculation of fluid
densities, viscosities, composition, etc. These additional measurements will be briefly
discussed in section 2.6. For the moment, the essential experiments required to
determine the three basic parameters will be detailed, together with the way in which
the results of a PVT analysis must be modified to match the field operating conditions.

The analysis consists of three parts:

− flash expansion of the fluid sample to determine the bubble point pressure;

− differential expansion of the fluid sample to determine the basic parameters B o,
R s and B g;

− flash expansion of fluid samples through various separator combinations to
enable the modification of laboratory derived PVT data to match field separator
conditions.

The apparatus used to perform the above experiments is the PV cell, as shown in
fig. 2.8. After recombining the oil and gas in the correct proportions, the fluid is charged
to the PV cell which is maintained at constant temperature, the measured reservoir
temperature, throughout the experiments. The cell pressure is controlled by a positive
displacement mercury pump and recorded on an accurate pressure gauge. The
plunger movement is calibrated in terms of volume of mercury injected or withdrawn
from the PV cell so that volume changes in the cell can be measured directly.

The flash and differential expansion experiments are presented schematically in
figs. 2.9(a) and 2.9(b). In the flash experiment the pressure in the PV cell is initially
raised to a value far in excess of the bubble point. The pressure is subsequently
reduced in stages, and on each occasion the total volume v t of the cell contents is
recorded. As soon as the bubble point pressure is reached, gas is liberated from the oil
and the overall compressibility of the system increases significantly. Thereafter, small
changes in pressure will result in large changes in the total fluid volume contained in
the PV cell. In this manner, the flash expansion experiment can be used to "feel" the
bubble point. Since the cell used is usually opaque the separate volumes of oil and
gas, below bubble point pressure, cannot be measured in the experiment and
therefore, only total fluid volumes are recorded. In the laboratory analysis the basic unit
of volume, against which all others are compared, is the volume of saturated oil at the