ISO/FDIS 23335

DRAFT
International
Standard
ISO/DIS 23335
ISO/TC 193/SC 3
Natural gas — upstream area
Secretariat: SAC
— Determination of hydrate
Voting begins on:
equilibrium temperature
2025-07-23
Gaz naturel — zone en amont — Détermination de la
Voting terminates on:
température d’équilibre des hydrates
2025-10-15
ICS: 75.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
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Reference number
ISO/DIS 23335:2025(en)
DRAFT
ISO/DIS 23335:2025(en)
International
Standard
ISO/DIS 23335
ISO/TC 193/SC 3
Natural gas — upstream area
Secretariat: SAC
— Determination of hydrate
Voting begins on:
equilibrium temperature
Gaz naturel — zone en amont — Détermination de la
Voting terminates on:
température d’équilibre des hydrates
ICS: 75.060
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
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Published in Switzerland Reference number
ISO/DIS 23335:2025(en)
ii
ISO/DIS 23335:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Reagents and materials . 2
5.1 Test solution .2
5.2 Test gas .2
6 Apparatus . 2
6.1 General .2
6.2 Material supply unit .3
6.3 Pressurizing unit .3
6.4 Mixing device .3
6.5 Temperature control system . .3
6.6 Reaction device .4
6.7 Liquid feed device (optional) .4
6.8 Data acquisition system .4
7 Procedure . 4
7.1 Calibration and pressure test .4
7.2 Cleaning .4
7.3 Sample preparation .4
7.4 Parameter setup .4
7.5 Cooling process .5
7.6 Hydrate formation .5
7.7 Heating process .5
7.8 Ending the experiment .5
7.9 Overview .5
8 Expression of results . 6
9 Test report . 8
10 Precision . 8
Annex A (informative) Method of temperature setting of gas hydrate test . 9
Annex B (informative) The statistical analysis of precision experiments .11
Bibliography .18

iii
ISO/DIS 23335:2025(en)
Foreword
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
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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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
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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/DIS 23335:2025(en)
Introduction
Natural gas hydrate is a crystalline structure formed under specific conditions (high pressure and low
temperature), in which gas molecules (primarily methane) are surrounded by water molecules. Due to
its immense potential energy value and environmental significance, research on natural gas hydrates
has been ongoing internationally. The phase equilibrium point is a crucial parameter for the formation
and decomposition of natural gas hydrates, and its determination is of great importance for hydrate
management. Comm on methods for measuring the phase equilibrium point of hydrates are mainly divided
into observation method and PVT method.
The observation method requires a high-pressure resistant transparent material (such as sapphire) for
the reaction device to clearly observe the formation and decomposition process of the hydrate. With this
method, the formation and decomposition process of the hydrate are directly observed, and the results are
intuitive and reliable. However, this method is limited by the pressure resistance and transparency of the
reaction device.
The PVT method measures the phase equilibrium of hydrates by varying any two of the parameters of
pressure, volume and temperature (P, V, T) in the reaction system while keeping the another parameter
constant. Depending on the parameter that is kept constant, it can be categorized into constant pressure,
constant volume and constant temperature methods. The constant volume method is suitable for measuring
the phase equilibrium of hydrates in multi-component systems and under complex conditions. It provides
more comprehensive phase equilibrium information, but requires precise instrument control and data
analysis. This document is compiled to meet the demand for measuring the phase equilibrium point of
hydrates in natural gas hydrate research under constant volume method.

v
DRAFT International Standard ISO/DIS 23335:2025(en)
Natural gas — upstream area — Determination of hydrate
equilibrium temperature
1 Scope
This document specifies a method for determining the phase equilibrium point of natural gas hydrates in
a laboratory setting under constant volume conditions, including the principle, reagents and materials,
apparatus, procedure, expression of results, test report and precision.
This document applies to laboratory simulations of hydrate formation and decomposition processes, and
involves the analysis of collected temperature and pressure data to determine the phase equilibrium
temperature of hydrates.
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 5725-2, Accuracy (trueness and precision) of measurement methods and results —Part 2: Basic method for
the determination of repeatability and reproducibility of a standard measurement method
ISO 10715, Natural gas — Gas sampling
ISO 14532, Natural gas — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14532 and the following terms apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
hydrate
solid crystalline substance resembling ice formed by gas molecules and water molecules
[SOURCE: ISO/DIS 14532:2025, 3.46]
3.2
equilibrium
limit state of each phase change in a multiphase system, where all phases in the reaction system reach balance
3.3
phase equilibrium point
intersection of the cooling curve and the heating curve in the phase diagram formed by temperature and
pressure data
ISO/DIS 23335:2025(en)
4 Principle
The formation and decomposition of natural gas hydrates follow the basic principle of phase equilibrium.
For multiphase systems, the mutual transformation between phases, the formation of new phases, and the
disappearance of old phases are related to temperature, pressure, and composition. Under constant volume
conditions, the process of hydrate formation and decomposition is simulated. Throughout the entire testing
process, changes in temperature and pressure are recorded, After data processing, a pressure-temperature
[1]
(P-T) diagram is plotted to identify the phase equilibrium point
5 Reagents and materials
5.1 Test solution
5.1.1 Operator determines the best testing medium in accordance with the experimental design.
5.1.2 For tests designed in laboratory, reagents of recognized analytical grade and only distilled water or
water of equivalent purity should be used as reaction solution in the laboratory.
5.1.3 For simulated field tests, produced water samples should be taken as many as possible. If produced
water samples are unavailable, the composition of produced water shall be analyzed and prepared.
5.1.4 For buffer solution, If there is sour gas involved in the reaction, the sour gas should be absorbed.
5.2 Test gas
5.2.1 Operator determine the best testing medium in accordance with the experimental design.
5.2.2 For simulated field tests, produced gas samples shall be taken as the methods specified in ISO 10715.
If produced gas samples are unavailable, the composition of produced gas shall be analyzed and prepared.
5.2.3 For tests designed in laboratory, use pure gases or mixed gases, the following gases should be used,
with the specified purity levels:
— methane (purity ≥ 99,9 %);
— ethane (purity ≥ 99,9 %), if required;
— propane (purity ≥ 99,9 %), if required;
— carbon dioxide (purity ≥ 99,9 %), if required;
— nitrogen (purity ≥ 99,9 %), if required;
— hydrogen sulfide (purity ≥ 99,9 %) , if required.
6 Apparatus
6.1 General
The following usual laboratory apparatus shall be used (see Figure 1 for an example):
— material supply unit;
— pressurizing unit;
— mixing device;
ISO/DIS 23335:2025(en)
— temperature control system;
— reaction device;
— liquid feed device (optional);
— data acquisition system.
Key
1 material supply unit
2 reaction device
3 pressurizing unit
4 mixing device
5 temperature control system
6 liquid feed device (optional)
7 data acquisition system.
Figure 1 — Example illustrating the composition of apparatus
6.2 Material supply unit
It is a unit used to supply test gas and test solution.
6.3 Pressurizing unit
Pr
...