ISO 23978:2020

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

General Information

• Status: Published
• Publication Date: 10-Sep-2020
• ICS: 75.060 - Natural gas
• Technical Committee: ISO/TC 193/SC 3 - Upstream area
• Drafting Committee: ISO/TC 193/SC 3/WG 7 - Composition/Raman spectroscopy
• Current Stage: 9060 - Close of review
• Completion Date: 04-Mar-2031

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 23978
First edition
2020-09
Natural gas — Upstream area —
Determination of composition by
Laser Raman spectroscopy
Reference number
©
ISO 2020
© 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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ii © ISO 2020 – All rights reserved

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
Ri
9.4 Uncertainty of result . 8
10 Test report . 9
Annex A (informative) Statistical procedure for estimation of the repeatability .10
Bibliography .17
Foreword
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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
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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.
iv © ISO 2020 – All rights reserved

Introduction
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
processing.
INTERNATIONAL STANDARD ISO 23978:2020(E)
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/
3.1
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.
3.2
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
3.3
avalanche photodiode
APD
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.
3.4
signal intensity
amount of Raman-shifted photons reaching the detection module
3.5
external cavity
measuring gas with a spectrograph outside the laser
3.6
intracavity
measuring gas inside the laser itself
3.7
sulfur compound absorber
container filled with basic solution for extraction of sulphur compounds
3.8
base span calibration
calibration process for eliminating cross-interference and Raman shift drift
3.9
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.
2 © ISO 2020 – All rights reserved

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.
Key
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):
I
i
C =×C (1)
i Ri
I
Ri
where
C is the concentration of component i in the sample;
i
I is the signal intensity of component i in the sample;
i
I is the signal intensity of component i in the reference gas;
Ri
C is the concentration of component i in the reference gas.
Ri
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.
Table 1 — Natural gas components and range of composition covered
−2
concentration (10 mol/mol)
Component
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
operation;
— laser power is typically 0,5 W to 5,0 W depending on Raman photon collection and detection
efficiency;
— Laser power stability, short term (seconds) ±0,5 % average power, less than 20 % after 10 000 h of
operation.
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.
4 © ISO 2020 – All rights reserved

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 t
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