1. NAME: UMA JOSHUA AJIKE
MATRICULATION NUMBER: 12CN014027
DEPARTMENT: PETROLEUM ENGINEERING
TITLE: METHODS OF FLUID SAMPLING
DATE: 01-04-2015

2. METHODS OF FLUID SAMPLING
Reservoir fluids should be sampled as early as possibleduring the producing
life of a reservoir. When the reservoir pressurefalls below the initial saturation
pressure, thehydrocarbon phaseforms two phases of gas and liquid. The mole
ratio of the two phaseflowing into the well is not generally equal to that formed
in the reservoir. Hencethe collection of a representative sample becomes a highly
demanding, and in some cases an impossibletask.
As long as the reservoir pressurehas never been below its saturation pressure,
and a single phasesample flows into the sampling bottle, the chance of collecting
a representative is high. Producing fluids, however ae at two phase conditions.
Hence the sampling procedureshould aim at collecting both phases at such
conditions where the recombination procedureprovides the original fluid.
Sampling of reservoir fluids can be accomplished by three methods
- Subsurface(bottomhole) sampling
- Wellhead sampling
- Surface(separator) sampling
The samples of reservoir fluids arecollected at these particular locations.
Whichever technique is used the samebasic problemexists, and that is, to ensure
that the proportion of gas to oil in the compositesample is the sameas that
existing in the reservoir.
The TYPES of fluid samples are
-Bottomhole Samples (singleor two phase)
-Separator Samples (Two phase)
Thus, sampling a reservoir under initial conditions, each stock tank barrelof oil
in the sample should be combined with Rsi standard cubic feet of gas.
The following data should be recorded when using any sampling technique
- A volume of oil in the separator compared with a volume of oil in the stock tank.
This information permits the field calculation of a shrinkage factor for separator

3. oil. The final shrinkagefactor for separator oil is determined in the laboratory by
flashing to stock-tank conditions.
-The temperature and pressureof the separator.
-The temperature and pressureof the stock tank.
- The specific gravity of the stock-tank oil.
- The amount of separator gas produced per stock-tank barrel(GOR,
gas-oilratio).
- The gravity of the separator gas obtained in field or laboratory to correct meter
measurements.
- The flowing bottom-hole pressureand temperature.
- The shut-in bottom-hole pressureand temperature.
SUBSURFACESAMPLING
This is the direct method of fluid sampling. A special sampling bomb is run in
the hole, on wireline, to the reservoir depth and the sample collected fromthe
subsurfacewellstreamat the prevailing bottomhole pressure. Either electrically
or mechanically operated valves can be closed to trap a volumeof the borehole
fluids in the sampling chamber. This method will obviously yield a representative
combined fluid sample providing that the oil is undersaturated with gas to such a
degree that the bottom hole flowing pressureat which the sampleis collected, is
above the bubble point pressure. In this casea single phasefluid, oil plus its
dissolved gas, is flowing in the wellboreand therefore, a sample of the fluid is
bound to havethe oil and gas combined in the correct proportion. Many
reservoirs, however, areinitially at bubble point pressureand under these
circumstances, irrespectiveof how low the producing rate is maintained during
sampling, the bottom hole flowing pressurewillbe less than the bubble point.
In sampling a gas saturated reservoir, two situations can arise depending on
the time at which the sample is collected. If the sampleis taken very early in the
producing life it is possiblethat the fluid flowing into the wellbore is deficient in
gas. This is becausethe initially liberated gas mustbuild up a certain minimum gas
saturation in the reservoir pores beforeit will start flowing under an imposed
pressuredifferential.

4. Figure 1: Subsurface Sampling method
The effect on the producing gas oil ratio, immediately below bubble point
pressure, is shown in as the small dip in the value of R for a shortperiod after the
pressurehas dropped below bubble point. As a resultof this mechanism there will
be a period during which the liberated gas remains in the reservoir and the gas oil
ratio measured froma subsurfacesamplewillbe too low. Conversely, oncethe
liberated gas saturation exceeds the critical value the producing well will
effectively steal gas frommore remote parts of the reservoir and the sample is
likely to havea disproportionately high gas oil ratio.
One of the main drawbacks in the method is that only a small sample of the
wellbore fluids is obtained, the typical sampler having a volumeof only a few
litres. Therefore, one of the only ways of checking whether the gas oil ratio is
correctis to take several downholesamples and compare their saturation
pressures atambient temperature on the well site.

5. WELLHEAD SAMPLING
Wellhead sampling is only possible for fluids that are single phaseunder
wellhead conditions. Therefore samples are obtained directly from the wellhead
when it is known that the flowing conditions at wellhead are within the single
phaseregion. For this purpose, the wellhead flowing pressuremustbe sufficiently
above the saturation pressureof the fluid at wellhead temperature. Hence, some
information on the phaseenvelope of the fluid must be available in advance.
Under Suitable conditions, wellhead sampling is the most reliable and efficient
sampling method.
SURFACE SAMPLING
In collecting fluid samples at the surface, separatevolumes of oil and gas are
taken at separator conditions and recombined to give a composite fluid sample.
The surfaceequipment is shown schematically in the figure below.
The well is produced at a steady rate for a period of severalhours and the gas
oil ratio is measured in SCF of separator gas per stock tank barrelof oil. If this
ratio is steady during the period of measurementthen one can feel confident that
recombining the oil and gas in the sameratio will yield a representative
composite sampleof the reservoir fluid. In fact, a slight adjustment mustbe made
to determine the actual ratio in which the samples should be recombined. This is
because, as shown in the diagrambelow, the oil sample is collected at separator
pressureand temperature whereas the gas oil ratio is measured relative to the
stock tank barrel, thus the required recombination ratio is

6. Figure 2: Surface Sampling Method
One of the attractive features of surfacerecombination sampling is that
statistically it gives a reliable value of the producing gas oil ratio measured over a
period of hours; furthermore, itenables the collection of large fluid samples. As it
was for subsurfacesampling, thesurfacerecombination method will only provide
the correctgas oil ratio if the pressurein the vicinity of the well is at or above
bubble point pressure. If not, the surfacegas 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 PVTsamples should be taken as early as possiblein the
producing life of the field to facilitate the collection of samples in which the oil
and gas are combined in the correctratio.
This method is often used when
- A large volume of both oil and gas are required for analysis (as in the case of gas
condensatefluids).
- The fluid at the bottom of the well is not representativeof the reservoir fluid
(i.e., gas condensatereservoirs and oil reservoirs producing largequantities of
water).

7. REFERENCES
1) Danesh A., “PVT and Phase Behavior of PetroleumReservoir Fluids”.
Department of Petroleum Engineering, Heriot Watt University. Edinburgh. 1998,
pg. 36.
2) Dake L. P., “Fundamentals of Reservoir Engineering”. Developments in
Petroleum Science 8. Shell InternationalePetroleum Maatschappij B. V. The
Netherlands. 1978, pg. 52-54
3) “IntroductionTo Well Testing”. Schlumberger Wireline & Testing. Bath,
England. 1998. Section 4-6.