Hydrocarbon Processing - November 2021 - 68

Water Management
costs, revenues and taxes among multiple
players collaborating on field development
and the production of oil and gas.
When a well test is carried out, waterin-oil
or water cut is measured for the
oil stream exiting the separator oil outlet.
Together with the rate of the water
stream, the total water production rate
from a specific well can be calculated.
With the advancement of multi-phase
flow measurement technologies, well
testing is increasingly carried out using
a multi-phase flow measurement device,
which can measure the amount of water,
oil and gas simultaneously without the
use of a test separator.
When oil and/or gas are exported, a
pipeline is commonly used. Due to the
density difference between water and oil,
and between water and gas, water-in-oil
or water-in-gas can drop out and accumulate
within certain parts of the pipeline,
which can then lead to corrosion problems
(FIG. 2). The presence of water-ingas
during transportation can also potentially
lead to the formation of gas hydrate
within the pipeline under certain temperature
and pressure conditions. Gas hydrate
is essentially ice that contains natural
gas that, when present, can potentially
lead to a pipeline blockage, resulting in
operational and safety issues. Knowledge
of the level of water-in-oil or water moisture
in gas can help determine the likelihood
of corrosion taking place and/or gas
hydrate formation inside the pipeline. It
can also help determine the level of antihydrate
formation chemicals that may be
required to prevent gas hydrate formation
or the scheduling of the pipeline inspection
and maintenance regime.
The terms " custody transfer " and " fiscal
metering " are often used interchangeably.
They refer to the transactions involving
transporting oil and gas commodities
from one operator (or owner) to another
and includes the transferring of oil and gas
between tanks and tankers, tankers and
ships, and other transactions. Custody
transfer in oil and gas measurement is defined
as a metering point (location) where
oil or gas is being measured for sale from
one party to another. For fiscal metering
or custody transfer, water-in-oil must be
measured accurately and then discounted.
Crude oil BS&W and refineries.
Crude oil received in a refinery usually
contains water, salts, clay and sand, which
are generally included in the water-in-oil
or BS&W measurement. Salts present include
those of magnesium, sodium and
alkaline earth metals and are generally
dissolved in the water component of the
crude. These components are unwanted
by refineries, as they can cause problems
to operations that include:
* Salt water can lead to corrosion of
piping and process equipment.
* Sand, salt and clay particles can
cause serious erosion or damage
to pumps and pipelines. They can
also lead to deposits and foul heat
exchangers and cause blockages.
* Salts can poison the catalyst used
in crude oil refining processes.
To avoid their potential impact on operations,
BS&W components must be removed
before the crude oil undergoes refining.
Fresh water is often added to wash
the crude oil, which then goes to a desalter
where the wash water is separated,
collected and treated before discharging.
In Europe, according to a June 2020
" 2016 survey of effluConcawe
report,1
ent quality and water use at European
refineries, " of the 72 refineries in Europe
that responded to the survey, 2.46% of the
total fresh water used (352 MMm3
, exFIG.
2. Accurate measurement of oil-in-water
reduces the likelihood of pipeline corrosion
formation.
68 NOVEMBER 2021 | HydrocarbonProcessing.com
cluding the once-through cooling water)
was for desalting. This is compared to the
total treated effluent water of 330 MMm3
reported in 2016.
From a refinery standpoint, knowing
the level of BS&W in the incoming crude
oil is important, as it will determine the
amount of freshwater required for the desalting
process, and the subsequent effluent
water treatment process.
Measurement methods and technologies.
Water-in-oil can be measured
using online technologies, as well as laboratory
standard methods. Online technologies
are increasingly used for fiscal,
custody transfer and process operations,
in particular, after the publication of the
American Petroleum Institute (API)
MPMS TR 2570 / EI HM56 technical report,
" Continuous online measurements
of water content in petroleum (Crude oil
and condensate) " in 2010.2
These online
instruments offer many benefits in terms
of cost savings, real-time continuous water-in-oil
information and increased productivity.
Laboratory standard methods
are also important and have been traditionally
used to measure water-in-oil concentration
and to calibrate and validate
the performance of online water-in-oil
measurement devices.
Laboratory standard methods. For
fiscal, custody transfer and allocation,
laboratory-based water-in-oil concentration
measurement standard methods
have played, and will continue to play, an
important role. They are well-established
and have been practiced by the oil and
gas industry for decades. Many standard
methods are available, including centrifuge-based,
distillation-based and Karl
Fischer titration-based.
All Karl Fischer methods are generally
better and more accurate than distillation
and centrifuge-based water-in-oil analysis
methods. In general, the coulometric
Karl Fischer method is better and more
accurate than the volumetric Karl Fischer
method. However, Karl Fischer titrationbased
methods are affected by the presence
of mercaptan and sulfide (S- or
H2
S). Also, Karl Fischer titration methods
require the use of expensive, hazardous
chemical reagents and delicate glass pieces,
and the reagents must be replenished
continuously. In addition, routine cleaning
of the laboratory equipment parts is
labor-intensive and time-consuming.
Online water-in-oil measurement.
Most commercially available online water-in-oil
measurement instruments are
designed and constructed based on four
techniques: capacitance, density, infrared
absorption and microwave (FIG. 3).
Capacitance-based technology prob
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