Fluids Sampling and Analysis - Expro Group

Fluids Sampling and Analysis
Gas condensate services
Expro is a market leader in the optimisation of welltest data accrued from the testing of gas
condensate wells. We also offer a complete spread of surface welltesting equipment and highly
experienced personnel.
In standard format a test separator is ill equipped to deal with the demands of testing lean wells
flowing at high rates. Without the use of specialised sampling techniques uncertainties exist in
all areas such as: Liquid rate measurement, Gas rate measurement, shrinkage, and BS&W
measurements.
Gas/Condensate and volatile oil reservoirs
Well fluid sampled after the choke manifold has been subjected to severe conditional changes,
due to high pressure drop (Joule Thompson effect). This cooling results in a large volume of liquid
drop out, and also possible wax and asphaltene drop out. Heating in a heat exchanger up to
120°C plus can follow. These excessive changes in conditions are not conducive to obtaining
good equilibrium and representative sampling of the produced fluids.
In the test separator there is also a temperature and pressure change which influences the
equilibrium composition of the gas and liquid phases. In this sequence of phase changes (choke,
heat exchanger and separator) volatile oil and gas systems create droplets smaller than 2
microns which are too small for separation by standard test separators.
Expro has developed a number of approaches to dealing with these challenges ranging from the
isokinetic and tracer techniques for saturated reservoirs through to single phase sampling in
downhole carriers for heavily undersaturated reservoirs where high drawdowns are needed to
ensure fluid lifting.
Overview of Advanced Gas Condensate Testing Equipment
a) Test Separator Shrinkage Tester
b) Isokinetic Separator Efficiency Measurements
c) Wellhead probe , Mixer and Mini Separator for accurate BS&W measurements
d) High Shrinkage Single Phase Downhole Samplers
e) Downhole Sampling with Carriers
f) Tracer Measurement Systems
g) Isokinetic Wellhead Sampling
h) SmartLab On-site Analysis/Turbo PVT
i) TurboPVT transfer services
j) Iso-Split Wellhead sampling which enables well sampling upstream of the choke manifold.
k) Iso-Split Separator Sampling used on the test separator.
exprogroup.com

Fluids Sampling and Analysis
Gas condensate services
TEST SEPARATOR ENHANCEMENTS
Shrinkage Tester
This Pressure and temperature controlled shrinkage tester is designed for
measurements at separator temperature or standard temperature (15 deg C) to
give on-line shrinkage measurement. High accuracy, fast glycol refill and sample
pump out.
ISO-SPLIT® SEPARATOR SAMPLING
The techniques presently used in the oil and gas industry for the recombination
of surface samples to obtain representative well stream compositions rely on
correct measurement of oil and gas rates.
The efficiency of the test separator deteriorates with high gas loads. This is a
consequence of the carry over of dispersed liquid in the gas phase. This carry
over is not normally accounted for in the Gas/Oil Ratio (GOR) measured for the
gas and liquid samples.
Experience gathered during the testing of over 400 gas condensate wells world-wide has shown that in the most extreme cases, separator
efficiency was less than 50%.The consequence of such errors in the compositions used for the prediction of reservoir, process and pipeline
performance is apparent and should clearly justify a more considered approach to the fluid sampling.
Injectable sampling probes inserted into the gas outlet via NPT threads enable wet and dry gas sampling and measurement of separator
efficiency. This is achieved by sampling into 20 litre bottles for onshore analysis or offshore efficiency determination via volumetric carry over
measurements. Normally, two pressurised samples of the separator gas are taken simultaneously via the injectable probe in two 20 litre
standard PVT gas sample vessels. One sample stream is taken upstream (isokinetically) as representative of the process stream containing
liquid carry over. The second sample stream is taken downstream as representative of a dry gas stream containing no carry over.
Determinaton of carry over and test separator efficiency is performed using the composition of the upstream and the downstream gas
samples and the recombined separator liquid.
OPERATIONAL CONDITIONS
Prior to performing isokinetic sampling from the test separator the gas flow rate must be determined under stable flowing conditions.
Once sampling has commenced there must be no significant changes in conditions at which the test separator is operating. In addition, it is
strongly recommended that accurate meter factors are performed prior to, during, and after the test using water.
The meters should be checked at rates covering the range of expected production during the test.
exprogroup.com

Fluids Sampling and Analysis
Gas condensate services
WELLHEAD BS&W MEASUREMENTS
Accurate BS & W measurements are crucial when the test separator is run as a two phase rather than
a three phase separator. Currently, a non-representative sample of wellfluid is usually flashed directly
from a pipe into a measuring cylinder. The volatile hydrocarbons tend to flash off resulting in an
incorrectly high water cut .
One solution is to install a mixing device downstream of the choke manifold which enables mixing of the
phases and sampling of the fluid via a probe located in the pipe. The fluid is then passed through a 20
litre sample bottle where the increased residence time allows the liquid to settle and segregate and an
accurate BS&W measurement to be obtained. The separator can also be partially filled with glycol to
absorb the saturated and the free water giving an “all inclusive water cut”.
Sample
Carrier
exprogroup.com
BOTTOM HOLE CARRIER SYSTEMS FOR UNDER
SATURATED GAS & VOLATILE OIL RESERVOIRS
The combination of downhole sampling carriers and Expro’s
20 K PCS samplers provide a powerful sampling tool for
undersaturated gas condensate wells.
The single-phase fluid sampler carrier is for running on
standard drillstem tests (DST), shoot and pulls, or anytime a
tubing-conveyed fluid sampling system is required.
The 25K nitrogen pressure compensation system on the PCS
single phase samplers keeps the sample in monophasic
condition from the reservoir to the surface. The tool can also
be run on slickline, electric-line or coiled tubing.
The PCS provides service in the most hostile well
environments, even with extreme H2S levels. Positive
displacement operation, a slow positive displacement of the
floating piston ensures no pressure differential across the
sample entry ports.
The non-reactive material makes the tool excellent for trace element sampling and sampling in highly corrosive hydrocarbon fluids
containing CO2, H2S and brines. The material of the tool provides insignificant loss of H2S from the sample.
Unlike conventional bottom hole samplers the PCS will maintain the sample in single-phase from bottom hole to surface. With no
requirement for re-establishing single-phase at surface sample transfer is performed rapidly and without jeopardising the integrity
of the sample. Single-phase reservoir sampling has widely been accepted as the industry standard for reservoir PVT sampling.
By keeping the sample in single-phase, errors during reconditioning of the sample are avoided. For Gas-condensates it is
particularly important to maintain the sample above the dew point pressure as asphaltene precipitation may be irreversible.
High pressures of the N2 charge are essential in order to secure single phase at surface. The high pressure rating of the PCS tool
is sufficient to achieve this for all reservoir conditions and reservoir fluids.

Fluids Sampling and Analysis
Gas condensate services
SINGLE PHASE FLUID SAMPLER CARRIER
The main objective on any DST is to gather data, including reservoir fluid data. Pipeconveyed
downhole sampling systems provide a cost-effective solution for drillstem
tests where running wire may be an issue, including deep water, horizontal, HPHT,
and H2S applications.
Not only does this result in a safer rig operation, but since wireline sampling runs
can now be eliminated, it can translate into substantial cost savings. Other
applications can include shoot and pull jobs where data acquisition is often
compromisedfor the sake of speed. By including pipe-conveyed sampler carriers in
the shoot and pull string, downhole fluid samples can be easily obtained without
complicating the test string design and without running wireline.
Pipe-conveyed sampling systems also play an important role in the new generation
of reduced emission testing.
CVU / MULTI-TRACE MULTIPHASE METERING BY TRACERS
Gas and oil wells produce a mixture of phases; water, gas and oil or condensate.
Accurate volumetric measurement of each phase produced is essential for reservoir
management and production optimization.
The Multitrace technique is non-intrusive and accurately measures the production of
water, gas and oil/condensate at flowing conditions with the use of chemical tracers.
The uncertainty in the measurement has been determined to be < ±10% of the true
value for individual liquid phases, and < ±4% for the gas phase.
In single phase flow, uncertainties of < ±2% are achieved. In principle, the technique
is capable of measuring water, gas, condensate or crude oil in all sorts of multiphase
flows. Wells with liquid fractions up to 100% have been tested successfully.
The multi-trace principle of operation is that the tracers are selective to the gas, water,
oil or condensate phases. The selected tracers are injected into the production
stream at a specific concentration and precisely metered rate. A sample of the produced fluids is then taken downstream of the injection point.
The actual production rate is calculated from the dilution factor of the tracer in the collected sample.All required equipment for the Multitrace
technique is portable and can be hand carried to the testing site. The tracers are non-radioactive and meet the most strict environmental
regulations.
During recent years, the oil industry has invested to develop on-line multiphase flow meter systems that can replace separators. The performance
of the developed multiphase meters are continuously improving, but their optimum performance is limited to operation within specific flow regimes,
velocities, or liquid fractions. Due to these variables and complexity in prediction and measurement of multiphase flow, there is a need for on-site
calibration of the meters and verification of their performance. The Multitrace
technique has been developed to meet these special requirements.
The Multitrace technique can also be used to determine the performance of existing
test separators. The gas, water and oil flow rates can be compared directly to the
separator readings and incorrect readings identified.
The Multitrace gas tracers are chosen to be specific to the gas phase with little
partitioning to the liquid phases. As such the gas tracer technique can be applied to
both single and multiphase applications. Typical applications include checking or
verification of installed (test separator) flow meters, MultiVent® wet gas venturi and
multiphase meters, and measurement of flow in systems where no flow meters are
installed.
Advantages of the technique include: in-situ measurements, installation effects
accounted for, measurement in irregular shaped conduits, and multiphase operation.
exprogroup.com
Features & Benefits:
Sampling verification with the Acoustic Telemetry System (ATS)
Reduces sample contamination
Full flow 2.25” ID for wireline applications
Multiple firing devices interchangeable to achieve acoustic, annular
pressure, pressure pulse, mechanical and wireline-fired systems
10,000 psi differential pressure rating,
Mercury-free sampling,
H2S service
Eliminates the high cost and safety issues associated with wireline
sampling to obtain single phase sample

Fluids Sampling and Analysis
Gas condensate services
NET - OIL AND WATERCUT MEASUREMENT
The Multitrace technique has the advantage that it measures the flow rate of the oil and
water phases directly and independently.
Because the uncertainty of the technique is expressed as a percentage of the actual net-oil
flow rate the application is most attractive for wells having relatively high watercuts.
Alternative techniques usually rely on measurement of the gross liquid production rate and
a separate watercut measurement. But as the oil rate in high watercut wells is only a small
fraction of the gross liquid rate, this often leads to high levels of uncertainty in the net-oil rate.
The Multitrace technique can also be used to study properties of process equipment. Some
examples include; determination of liquid retention times, and liquid carry over from the gas
outlet of production separators. As such the Multitrace technique is a versatile tool for
process control optimisation.
OVERVIEW OF THE MULTITRACE APPLICATIONS
• Gas, water and oil/condensate flow rate measurement
• Wet gas metering
• Net-oil measurement
• Watercut measurement
• Performance verification of test separators
• Field verification of Multiphase meters
• Process equipment diagnostics
ISO-SPLIT WELLHEAD SAMPLING
Expro’s Minilab and manifold are situated upstream of the choke manifold. The seperation
vessels in the Mini lab, though of a small scale provide for a longer residence time than the
test serperator (10 - 16 times longer).
The Mini lab also allows manipulation of pressure and temperature of the sample stream,
thus giving better equilibrium conditions. Therefore, we recommend that PVT compositional
sampling is carried out from the ISO-SPLIT® Mini lab as it can provide improved equilibrium
conditions and higher quality samples than the test separator.
The possibility of introducing errors during physical recombination in the PVT laboratory due
to differences in the conditioning procedures are also reduced.
Mini lab sampling provides an independent measurement of the Condensate Gas Ratio
(CGR) that is not influenced by inherent errors in the gas and liquid phase rate
measurements of the test separator. The test separator CGR is also influenced by reduced
efficiency at higher loads.
For testing of gas/condensate or volatile oil zones Expro recommend that ISO-SPLIT®
isokinetic split phase sampling be performed both upstream of the choke manifold and
down stream of the test separator in the gas outlet.
Isokinetic split phase sampling at the well head provides representative sampling of well fluid
that is closest to the reservoir conditions, dew point and is least affected by phase changes.
The sampled fluid will also be in good equilibrium at the wellhead conditions. The properties
of the stabilised liquid from the Mini lab can also be used to track the well stabilisation.
Monitoring the density of produced liquid can provide an indication of the relative amounts
of retrograde condensation lost in the well. Gas/condensate reservoirs are saturated with
water which drops out during conditional changes.
Water accumulated in the test separator can introduce errors in rate measurements. The
Mini lab uses glycol to absorb the produced water hence is not affecting rates. Expro has
equipment that can collect samples to determine separator efficiency on site.
exprogroup.com

Fluids Sampling and Analysis
Gas condensate services
SMARTLAB SERVICE
In order to obtain the highest quality of wellsite data from surface or downhole
PVT samples Expro has designed and constructed a mobile offshore laboratory
called “SmartLab”.
The aim of the SmartLab is to determine the quality of samples retrieved and also
measure as many parameters as possible with regard to hydrocarbons, water
and trace elements. As Expro is not operating the WFS-tools, we will offer our
technical knowledge and experience on site to assist the tool operator in securing
the best possible fluid sample. The current trend of obtaining reservoir
information by the use of WFS, thereby reducing the need for full production
testing, is likely to increase. With this in mind we have worked towards being able
to offer our clients a full package of services based upon the accurate analysis of
the oil, water and gas retrieved via the use of WFS.
TURBO PVT
A major advantage with the service is the continuous monitoring of the fluid properties during the sample transfer process. By measuring
permittivity, density and viscosity in the low volume (