1. oilfield waterSampling
Introduction
• One of the first items of interest in oilfield waterhandling is to
sample it and determine its composition.
• This is our primary means of detecting present and potential
problems.
• However, the oilfield watersample must be representative of the
oilfield waterof interest or our analysis will lead to false
conclusions.
• The importance of good sampling practices cannot be
overemphasized. An extremely accurate chemical analysis of a
oilfield water sample, followed by a brilliant assessment of the
problems indicated by the analysis, is worthless if the sample does
not represent the oilfield waterin your system.
2. Produced oilfield waterSurface Sample
Sample Containers
• Clean (preferably new) plastic bottles with tightly
fitting plastic caps are recommended for routine
oilfield watersamples.
• Several different types of plastic and different cap
designs are available.
• Once you have selected a particular bottle and cap
combination, fill it with water, cap it, and squeeze it
to make sure that the cap will not pop off or leak.
3. • Label the bottle (not the cap) so that the
sample can be identified.
• Cardboard mailing tubes or cartons should be
obtained if the samples are to be shipped to a
laboratory or carried a considerable distance.
4. • If the sample is to be analyzed for oil content or
for the concentration of any other organic
constituent, a glass bottle should be used.
• Oil or other organic materials will adhere to the
walls of a plastic container or be absorbed by it.
• Extra precautions are necessary in packing the
sample for shipment.
• Freezing can be a problem. Use a plastic cap with
a plastic liner.
5. • Never use a metal container or a metal cap.
The oilfield waterwill corrode them and
become contaminated with corrosion
products.
Sample Volume
• A minimum sample volume of one pint (500 or
litre ) is recommended for routine analysis.
6. Sampling Procedure
If a sampling valve is available, connect a piece of plastic
tubing to the end of the valve. A small nipple or hose
connector is handy for this purpose.
• Open the valve and let the oilfield waterrun for at least
one minute. Watch to see if the color of the oilfield wateris
changing. If it is, wait until the color becomes constant.
• Remember that there is probably some debris in the valve
or in the bottom of the line and this will have to be flushed
out before you can get a good sample.
7. Once you have a representative oilfield waterflow, the
following sampling procedures are recommended:
• Sample for Routine Analysis - Rinse the bottle out
three times, then place the end of the hose in the
bottom of the bottle and let the bottle overflow for
an estimated 10 volumes.
8. Then slowly pull out the hose and quickly cap it to minimize
a- oxygen contamination and
b- the escape of dissolved gases.
Sample for Oil-in-oilfield waterAnalysis –
-Fill a clean glass bottle to the neck directly from the sample point and
cap quickly.
-Do not rinse or overflow the bottle with the oilfield waterto be sampled
as in the case of the sampling procedure given for routine analyses.
- Oil will tend to adhere to the bottle wall and separate from the water. If
the bottle is filled more than once, oil from each filling is likely to stay
in the bottle, giving erroneously high results.
9. Produced oilfield waterSampling Locations
• 1. Take wellhead samples at the wellhead, not at the
heater-treater or at the tank.
• 2. If a tank is to be sampled, sample it at several
levels, preferably top, middle and bottom.
• 3. Take samples from the system when it is
operating normally. It is often most convenient to
take samples when the system is shut down.
Needless to say, this type of sample is of
questionable value. Make sure the flow rate is
"normal" and that nothing unusual is going on
upstream of the sampling point.
10. • 4. If you are sampling a surface oilfield
wateras a possible source for a waterflood
remember that the oilfield watercomposition
(especially the turbidity, oxygen concentration and
microbial population) may change considerably
with the time of the year. It is advisable to sample
at several points over a period of several months.
•
14. Subsurface Sample
• The composition of subsurface oilfield
watercommonly changes with depth, and also
laterally in the same aquifer. Changes may be
brought about by the intrusion of other waters,
and by discharge from and recharge to the aquifer.
It is thus difficult to obtain a representative sample
of a given subsurface body of oilfield waterbecause
any one sample is a very small part of the total
mass, which may vary widely in composition.
15. Subsurface Sample
• Considerations in selecting sampling sites are as follows:
(1) Which sites will better fit into an overall plan to
evaluate the chemistry of the waters on a broad basis?
• (2) Which sites will yield the better information for
correlation with data obtained from other sites?
• (3) Which sites are more representative of the total
chemistry of brines from a given area?
• The value of the sample is directly proportional to the facts
known about its source; therefore, sites should be selected
for which the greater source knowledge is available.
17. Subsurface Sample
Few of the samples collected by drill-stem test are truly
representative formation-oilfield watersamples.
During drilling, the pressure in the well bore is intentionally
maintained higher than that in the formations.
Filtrate from the drilling mud sweeps into the permeable strata,
and this filtrate is the first liquid to enter the test tool.
• The most truly representative formation-oilfield watersample
usually is obtained after the oil well has produced for a period
of time and all extraneous fluids adjacent to the wellbore
have been flushed out.
18. Samples taken immediately after the well is
completed may be contaminated with drilling
muds, with drilling fluids, and/or with well
completion fluids, such as filtrate from
cement, tracing fluids, and acids, which
contain many different chemicals.
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