Natural gas — Upstream area — Determination of composition by Laser Raman spectroscopy

This document describes a laser Raman spectroscopy method for the quantitative determination of chemical composition of natural gas in upstream area.

Gaz naturel - Zone amont - Détermination de la composition par spectroscopie Laser Raman

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ISO 23978:2020 - Natural gas -- Upstream area -- Determination of composition by Laser Raman spectroscopy
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Natural gas — Upstream area —
Determination of composition by
Laser Raman spectroscopy
Reference number
ISO 23978:2020(E)
ISO 2020

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ISO 23978:2020(E)

© ISO 2020
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ISO 23978:2020(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
4.1 Working principle of the laser Raman analyser . 2
4.2 Calculation . 3
5 Instruments . 3
5.1 Laser Raman gas analyser . 3
5.2 Laser specifications . . 4
5.3 Detection module . 4
5.4 Signal processing and user interface . 4
5.5 Sample filtration and probe . 4
5.6 Gas pressure regulator . 4
5.7 Sulfur compound absorber. 5
6 Reagents and materials . 5
6.1 Zero gas . 5
6.2 Base span calibration gases . 5
6.3 Working span calibration gases. 5
7 Measurement procedures . 5
7.1 preparation . 5
7.2 Calibration . 6
7.2.1 Calibration frequency . 6
7.2.2 Calibration procedure . 6
7.2.3 Zero calibration . 6
7.2.4 Base span calibration . 6
7.2.5 Working span calibration . 6
7.3 Sampling and sample analysis . 6
7.4 Data record . 7
8 Repeatability . 7
9 Uncertainty evaluation . 7
9.1 General . 7
9.2 Uncertainty of I and I . 7
i Ri
9.3 Uncertainty of C . 8
9.4 Uncertainty of result . 8
10 Test report . 9
Annex A (informative) Statistical procedure for estimation of the repeatability .10
Bibliography .17
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ISO 23978:2020(E)

ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
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For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 3,
Upstream area.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
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ISO 23978:2020(E)

Gas chromatography methods for determination of composition in natural gas already exist as
ISO 6974-1 to ISO 6974-6 or ISO/TR 14749.
Gas laser Raman spectrometry is a simpler and more direct analysis method than gas chromatography.
Gas laser Raman offers a faster and more convenient means of determining composition of upstream
area natural gas because it is an entirely optical method operating at the speed of light with no moving
parts. Natural gas exploration and development benefits from fast determination of gas composition
and real-time monitoring of gas composition better optimizes natural gas treatment processes.
Gas laser Raman spectrometry enables rapid and simultaneous analysis of multiple gas species
because each type of gas molecule emits unique light frequencies shifted from the frequency of laser
light striking it. This "Raman scattering" is instantaneous and directly proportional to the number of
molecules the light impacts. This simple principle allows continuous on-site real-time data monitoring
and control, it will bring tremendous improvements to gas exploration, well operations, transport, and
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Natural gas — Upstream area — Determination of
composition by Laser Raman spectroscopy
1 Scope
This document describes a laser Raman spectroscopy method for the quantitative determination of
chemical composition of natural gas in upstream area.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 6142-1, Gas analysis — Preparation of calibration gas mixtures — Part 1: Gravimetric method for Class
I mixtures
ISO 6144, Gas analysis — Preparation of calibration gas mixtures — Static volumetric method
ISO 6145, Gas analysis — Preparation of calibration gas mixtures using dynamic methods
ISO 10715, Natural gas — Sampling guidelines
ISO 11095, Linear calibration using reference materials
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
Raman effect
process, in which photons from a light source are absorbed by the electrons surrounding polyatomic
molecules and result in a new photons being emitted at wavelengths higher or lower than the source
photon wavelength
Note 1 to entry: The resulting wavelength changes are called Raman shifts. The Raman shifts are determined by
the vibrational and rotational frequencies of the atomic bonds within each molecule.
multichannel photodetector
photosensitive semiconductor device that transports electric charge from one capacitor to another,
allowing serial output of parallel data, typically used for digital image capture
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ISO 23978:2020(E)

avalanche photodiode
diode with an internal gain mechanism
Note 1 to entry: As in the case of standard diodes, photons generate electron-hole pairs, which are accelerated
by the applied external voltage such that further electrons are introduced to the conduction band by means of
impact ionization. These secondary electrons can in turn absorb sufficient energy to raise further electrons into
the conduction band.
signal intensity
amount of Raman-shifted photons reaching the detection module
external cavity
measuring gas with a spectrograph outside the laser
measuring gas inside the laser itself
sulfur compound absorber
container filled with basic solution for extraction of sulphur compounds
base span calibration
calibration process for eliminating cross-interference and Raman shift drift
working span calibration
calibration process for calculation of the concentration of sample
4 Principle
4.1 Working principle of the laser Raman analyser
The sketch of the measuring principle is shown in Figure 1. Monochromatic (single wavelength) light
from a laser goes through the sample, these source photons interact with the molecules of the gas
sample via the Raman effect to emit photons at a different wavelengths. These Raman shifted photons
are quantified by wavelength in a detection module. The detection module can be external cavity or
intracavity type, using a spectrograph or discrete detectors.
In external cavity, a spectrograph directs photons of different wavelengths to different pixels on
multichannel photodetector, it can be charge-coupled device (CCD), complementary metal oxide
semiconductor (CMOS) or other detectors. Each multichannel photodetector pixel stores the specific
wavelength photons striking them, periodically creating digital signal intensity proportional to the
number of photons collected. All pixels taken together generate a spectrum representing the number
of photons detected at each wavelength. The spectrum can be mathematically processed to yield the
molecular type and concentration in the gas sample.
In intracavity, a detector subsystem includes a focusing lens, an optical filter to select a specific Raman
line and an APD or photomultiplier to capture the filtered light. The APD or photomultiplier separately
collects and counts Raman photons emitted from individual gas species in millisecond time intervals.
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ISO 23978:2020(E)

Gas concentrations are calculated for a specified period. Because each discrete detector collects
photons at one wavelength, one detector position is typically needed to detect each natural gas species.
NOTE Currently, only analysers with 8 APD detector positions per module are available. 8 is sufficient for
some natural gas applications, however, if more than 8 gases need to be measured, two or more detectors can be
operated in parallel.
1 laser generator
2 mirror
3 gas chamber
4 lens
5 detection module
6 user interface
Figure 1 — Working principle of the laser Raman gas analyser
4.2 Calculation
Because the gas concentration is directly and linearly proportional to the signal intensity expressed by
photon counts, the concentration of components in the sample can be calculated by Formula (1):
C =×C (1)
i Ri
C is the concentration of component i in the sample;
I is the signal intensity of component i in the sample;
I is the signal intensity of component i in the reference gas;
C is the concentration of component i in the reference gas.
5 Instruments
5.1 Laser Raman gas analyser
The type of Raman gas analyser is selected according to the types of gases and the natural gas
concentration ranges of interest. The base analyser unit contains a laser, optical path control, gas
detection chamber, photon detection and processing, signal processing, and human/machine interface.
The analyser should have sensitivity sufficient to detect the components in the analytical range given
in Table 1. It should keep a stable state after zero calibration.
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ISO 23978:2020(E)

Table 1 — Natural gas components and range of composition covered
concentration (10 mol/mol)
min. max.
Nitrogen N 0,02 10
Carbon dioxide CO 0,02 30
Hydrogen sulfide H S 0,02 30
Methane CH 50 100
Ethane C H 0,02 20
2 6
Propane C H 0,02 10
3 8
NOTE 1  The ranges in this table do not indicate detection limits, but indicate that the specified
precision can be achieved. The range can be wider when precision is not limited.
NOTE 2  Other gases such as H , i-C H , n-C H can also be desired. For some gas Raman
2 4 10 4 10
technologies, multiple detector modules can be required.
5.2 Laser specifications
The laser shall have a narrow-enough line width with a stable-enough power output and wavelength so
as not to compromise the generation and analysis of the Raman spectra. Laser power should be high-
enough to ensure sufficient sensitivity.
— laser wavelength should be in the range of 500 nm to 800 nm, and a laser of wavelength 785 nm is
generally suggested;
— laser wavelength stability, initially less than ±0,005 nm, less than ±0,05 nm after 10 000 h of
— laser power is typically 0,5 W to 5,0 W depending on Raman photon collection and detection
— Laser power stability, short term (seconds) ±0,5 % average power, less than 20 % after 10 000 h of
5.3 Detection module
The detection module should be capable of sufficient resolution, high throughput and stability.
5.4 Signal processing and user interface
Raman spectroscopy software is included as a part of the analyser to process the detector data
and provide a user interface. Software should be able to indicate gases measured, calculate the gas
concentrations, track key diagnostic factors such as temperatures, spectral intensity, and laser power,
and maintain and check instrument calibration and drift.
5.5 Sample filtration and probe
Particulates and aerosols larger than 0,2 μm should be removed by filtration prior to entering the
detector. The filter housing and sample probe shall be made of a material which is inert, non-adsorptive
and non-permeable to components in the gas sample, stainless steel is preferred.
5.6 Gas pressure regulator
For sulfur-containing natural gas, hydrogen sulfide corrosion resistant gas pressure regulators should
be selected.
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ISO 23978:2020(E)

5.7 Sulfur compound absorber
Beakerflask, flask etc. filled with basic solution can be used as sulfur compound absorber in order
to eliminate sulfur compounds discharged to the atmosphere. Sodium hydroxide solution (200 g/l),
prepared by dissolving sodium hydroxide (chemically pure) in water is suggested as the basic solution.
6 Reagents and materials
6.1 Zero gas
Pure argon or other monatomic gas is used to set the zero level of the Raman analyser. The purity
should not be less than 99,999 %.
6.2 Base span calibration gases
Base span calibration gas used to set software parameters shall be working standard pure gases
or binary mixtures in accordance with ISO 6142-1, ISO 6144 or with ISO 6145. Binary gas mixtures
consisting of blending zero gas and a single gas component present in natural gas are preferred when
their typical concentrations in natural gas are below 75 %. For typical natural gas upstream area
samples, 1 % H S/Ar, 5 % CO /Ar, 3 % N /Ar, 3 % C H /Ar, 2 % C H /Ar, are suggested. As base span
2 2 2 3 8 2 6
calibration gases for all but CH which can be pure (100 %) or Ar diluted.
6.3 Working span calibration gases
Regular calibration shall perform using working standard gas mixtures in accordance with ISO 6142-1,
ISO 6144 or with ISO 6145. The working standard gas mixtures shall contain appropriate concentrations
and cover the analytic range of the analyser. All components in the reference standard shall be
homogenous in the vapour state at the time of use. The concentration of a component in the reference
standard gas shall be close to the actual sample gas concentration, shall be less than relatively 20 % of
the corresponding component in the test gas, the minimum concentration shall be greater than 0,02 %.
7 Measurement procedures
7.1 preparation
Make sure the analyser placed on a flat, horizontal surface, in a clean location that maintained at a
relatively constant environmental conditions:
— temperature: 15 °C to 35 °C;
— relative humidity: 10 % to 75 %.
Set up the laser Raman gas analyser according to the manufacturer’s instructions, connect zero gas,
base span calibration gas, measurement calibration gas, and sample in sequence to the instrument as
shown in Figure 2.
Figure 2 — Flowchart of the measurement procedures
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ISO 23978:2020(E)

7.2 Calibration
7.2.1 Calibration frequency
Calibration should be carried out when experimental conditions were changed. And it should be
recalibrated according to the needs of the user.
7.2.2 Calibration procedure
Connect the calibration gas with the analyser according to Figure 2. Adjust the input pressure and
temperature to a stable condition. Sweep the analyser with the gas flow for at least 2 min.
After the sulfur-containing calibration gas is injected, it should go through the absorption solution
before it is vented.
7.2.3 Zero calibration
According to 7.2.2, connect zero gas with the analyser, start calibration, update the background when
the signal values are stable.
7.2.4 Base span calibration
Before sample measurement, base span calibration should be done to overcome the instrument drift
caused by temperature, pressure and general electronic fluctuations, and the cross-interference
caused by the multiple components in the gas could be maximum eliminated by calibrating the cross-
interference factors. According to 7.2.2, connect base span calibration gas to the analyser, start
calibration, observe the signal values, adjust the cross-interference factors till the interference signal to
a reasonable value (usually in a range of -1 to 3).
7.2.5 Working span calibration
Using ISO 11095, the user shall choose the calibration method depending on the analytical accuracy
that is required. The following are candidate methods and their requirements:
— bracketing calibration: the difference between calibration gas and analytical result should be less
than relatively 20 %;
— response formula simulation: no less than three calibration gas should be used to cover the
analytical range;
— one point calibration: the difference between calibration gas and analytical result should be less
than relatively 20 %.
If the method is used to monitor fixed natural gas source, one point calibration or bracketing calibration
should be applied. If the method is used to analyse a non-constant natural gas source, response formula
simulation should be applied.
7.3 Sampling and sample analysis
Connect gas sample, gas pressure regulator, filters, and analyser in sequence adjust pressure and flow
to the experimental conditions, start sample analysis, the sample will be automatically measured.
Sampling shall be in accordance with ISO 10715.
Sulfur-containing sample should go through the absorption solution before it is vented.
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ISO 23978:2020(E)

7.4 Data record
In measurement mode, the results are automatically calculated out by Formula (1), the listed result is
the content of the analysis, and testing result should be read directly.
NOTE The reference conditions of the test results are the same as that of working span calibration gas.
Conversion between reference conditions is according to ISO 13443.
8 Repeatability
In the analytical range given by this document, under repeatability conditions, the difference of two
successive test results in a short time interval (10 s) should not exceed the values listed in Table 2 at
a 95 % level of confidence. Annex A gives an example of the statistical procedure of the repeatability
Table 2 — Repeatability of different concentration ranges
−2 −2
Component Content range (10 mol/mol) Repeatability limits (10 mol/mol)
0,02 to 1 0,01
2 6
above 1 to 20 0,07
0,02 to 1 0,01
3 8
above 1 to 10 0,03
0,02 to 1 0,01
above 1 to 15 0,04
0,02 to 1 0,02
above 1 to 30 0,10
0,02 to 1 0,02
above 1 to 30 0,05
CH above 50 0,18
9 Uncertainty evaluation
9.1 General
The component concentration is calculated using Formula (1). The uncertainty of the final result shall
be based on Formula (1) with some mathematical computing in accordance with ISO/IEC Guide 98-3.
9.2 Uncertainty of I and I
i Ri
The uncertainty of I is the standard deviation of the analytical data, I , and the relative uncertainty of I
i i i
is calculated by Formula (2):

ij i
u ()I = (2)
rel i
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ISO 23978:2020(E)

uI() is the relative standard uncertainty of the signal intensity of component i of sample gas;
is the signal intensity of component i of sample gas in each test;
is the mean of all the signal intensity values of component i of sample gas.
The uncertainty of I depends on the calibration procedure. In the one point calibration procedure, the
relative uncertainty of I is calculated by Formula (3):

RRij i
u ()I = (3)
is the relative standard uncertainty of the signal intensity of component i of reference gas
(working span calibration gas);
I is the signal intensity of component i of reference gas in each test;
is the mean of all the signal intensity values of component i of reference gas.
9.3 Uncertainty of C
The standard relative uncertainty of C is given by the producer of the calibration gas, which are listed
in the certificate of the calibration gas.
9.4 Uncertainty of result
As the test result is related to I , I and C as shown in Formula (1), according to ISO/IEC Guide 98-3,
i Ri Ri
the relative standard uncertainty of the result is calculated by Formula (4):
22 2
uu u
uC()= I + C + I +2ruI ,I I u I (4)
() () () () ( ))( )
relrel rel
rel i i Ri RRi i i rel i relRi
uC() is the relative standard uncertainty of the final analytical result;
rel i
uI() is the relative standard uncertainty of signal intensity of component i of the sample;
rel i
uI() is the relative standard uncertainty of signal intensity of component i of the reference gas;
is the relative standard uncertainty of component i of reference gas;
is correlation factor between I and I
i Ri.
NOTE r I ,I is estimated by experience, to simplify the calculation, the degree of correlation between I
i Ri
and I is set nearly independent, and then r I ,I = 0.
i Ri
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ISO 23978:2020(E)

10 Test report

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