Page 415 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Properties of Hydrocarbon Mixtures 181

of drawdown in a saturated reservoir cannot be eliminated; therefore, it is
necessary to reduce the pressure drawdown by reducing the flowrate to the
lowest possible stable rate while sampling.
There are two basic methods of sample collection: subsurface (bottomhole)
and surface (separator). The fluid sampling method to be used dictates the
remainder of the conditioning process. If the bottomhole samples are to be
collected, the period of reduced flowrate will generally last from one to four
days, depending on the formation and fluid characteristics and the drainage
area affected. After this reduced flowrate period, the well would be shut in and
allowed to reach static pressure. The shut-in period would last about one day
or up to a week or more depending on formation characteristics. For the case
of the saturated reservoir, the shut-in period has the resultant effect of forcing
gas into solution in the oil, thus raising the saturation pressure. In some cases,
the desired value of P,, is obtained; however, in most cases this value is only
approached and the final difference is a function of well productivity, production
rate and fluid properties. At the conclusion of the shut-in period, the well would
be properly conditioned and ready for bottomhole sampling. Subsurface sam-
pling is generally not recommended for gas-condensate reservoirs; the same is
true for oil reservoirs producing substantial quantities of water. If separator gas
and liquid samples are to be collected, the gas and liquid rates must be
monitored continually during the period of stable flow at reduced flowrates. A
minimum test of 24 hr is recommended, but more time may be needed if the
pressure drawdown at the formation has been high. Surface sampling, called
sepamtm sampling has wider applications than subsurface sampling, and is the
only recommended way of sampling a gas-condensate reservoir, but often can
be used with good success for oil reservoin as well. There are three requirements
to successful separator sampling:
1. stable production at a low flowrate
2. accurate measurement of gas and liquid flowrates
3. collection of representative samples of first-stage gas and first-stage liquid
The above procedure is described in detail in API Standard 811-08800 [9].
The reservoir process is stimulated in the laboratory by flash differential
vaporization (Figures 6-7 and 6-8). Based on both figures, it is possible to
prepare the reservoir f hid data for engineering calculations.
In the laboratory, the differential liberation consists of a series-usually 10
to 15-of flash liberations. An infinite series of flash liberations is the equivalent
of a true differential liberation. At each pressure level, gas is evolved and
measured. The volume of oil remaining is also measured at each depletion
pressure. This process is continued to atmospheric pressure. The oil remaining
at atmospheric pressure is measured and converted to a volume at 60°F (15.6"C).
This final volume is referred to as the residual oil. The volume of oil at each
of the higher pressures is divided by the volume of residual oil at 60°F (15.6OC).
Example 1 [ll]

Surface separator samples were collected from a well on completion of a
2-hr test on June 8, 1984. The gas/liquid ratio measured on this test was 4,565 ft3
of separator gas per barrel of separator liquid and was used as the basis for
this recombination. The resultant reservoir fluid exhibited a dew point of 4,420
psia at T, = 285°F. The reservoir fluid exists as a gas (an undersaturated gas)
at P,, = 12,990 psia.