ISO 23335:2026

• Abstract:

  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. It involves the analysis of collected temperature and pressure data to determine the phase equilibrium temperature of hydrates.

• Status: Published
• Publication Date: 05-Aug-2026
• ICS: 75.060 - Natural gas
• Technical Committee: ISO/TC 193/SC 3 - Upstream area
• Drafting Committee: ISO/TC 193/SC 3 - Upstream area
• Current Stage: 6060 - International Standard published
• Start Date: 06-Aug-2026
• Due Date: 28-Sep-2026
• Completion Date: 06-Aug-2026

ISO 23335:2026 is an ISO International Standard for the upstream area of natural gas. It specifies a laboratory method for determining the hydrate equilibrium temperature of natural gas under constant volume conditions, using temperature and pressure data from hydrate formation and decomposition tests.

The document is aimed at laboratories and technical teams that need a repeatable way to simulate hydrate behaviour and identify the phase equilibrium point. It covers the principle, test media, apparatus, procedure, result calculation, test report and precision.

What does ISO 23335:2026 specify?

ISO 23335:2026 specifies a constant-volume laboratory method for determining the phase equilibrium point of natural gas hydrates. In practice, it tells you how to run the test, what to use, how to read the data and how to report the result.

The scope is limited to laboratory simulations of hydrate formation and decomposition processes. The document is organized into clauses for scope, references, terms, principle, test solution and gas, apparatus, procedure, calculation, precision and test report, followed by two informative annexes.

Annex	What it covers
Annex A	Methods for setting or predicting the test temperature for gas hydrate work
Annex B	Statistical analysis of precision experiments for this method

What are the key requirements of ISO 23335:2026?

ISO 23335:2026 requires the test to be run under constant volume conditions while recording temperature and pressure through cooling and heating. The phase equilibrium temperature is then taken from the pressure-temperature relationship, so the quality of data collection and curve interpretation matters directly to the reported result.

Test media must match the application

Clause 5 distinguishes between test solution and test gas. For field simulation, the document prefers formation water and produced gas samples where available; if samples are not available, the corresponding compositions are to be analyzed and prepared. For laboratory work, distilled water is used for the test solution and gas components should have purity of at least 99.9%.

This matters because hydrate equilibrium temperature depends on composition. If the water or gas does not reflect the intended service condition, the measured temperature will not represent the system being assessed.

The apparatus must support pressure, temperature and mixing control

Clause 6 sets the apparatus around a material supply unit, pressurizing unit, mixing unit, temperature control unit, reaction device, optional liquid feed unit and data acquisition unit. The pressure system should meet the test range and achieve high pressure, with pressure accuracy of ±0.01 MPa; the temperature control system should reach the required low-temperature range, with accuracy at least ±0.1 K and overshoot below 0.1 K.

The reaction device must be sealed, pressure-resistant, able to exchange heat, and made of materials that do not react with the test solution or test gas. The data acquisition unit needs temperature accuracy of 0.01 K, pressure accuracy of at least ±0.1 % full scale, and real-time logging with export of raw data. In practice, this means the test bench has to be stable enough to capture small changes near equilibrium.

The procedure starts with calibration, tightness checking and purging

Clause 7 requires calibration of the apparatus according to international or national measurement standards, with the calibration basis kept as documented information. Before a new test, the reaction device is subjected to an air tightness test. The system is then purged with test gas by pressurizing and depressurizing at least three times, and residual air may be reduced further by vacuum or depressurization.

This step is important because trapped air or leakage can distort the pressure-temperature path and affect hydrate formation. If the system is not clean and tight, the result can be difficult to interpret.

Sample preparation and data collection are tightly controlled

Clause 7 specifies that the test solution should occupy 20 % to 50 % of the reaction device volume. After pressurizing to the initial test pressure, the system is allowed to stabilize before recording begins. Data acquisition shall be at least one reading per minute.

For the reader, this means the test is not a quick spot measurement. The method depends on stable starting conditions and enough recorded points to draw a reliable pressure-temperature curve.

Cooling and heating are used to bracket the equilibrium point

The cooling step should start at least 5 K below the predicted hydrate equilibrium temperature, with a cooling rate of 3 K/h to 5 K/h. Hydrate formation is then monitored through the data acquisition unit, and a pressure drop may accompany extensive hydrate formation. Heating can be done by ramp heating at 0.1 K/h or lower, or by stepwise heating with 0.1 K steps and a pressure-stability check before each next step.

This is the core of the method. The slow heating is used to locate the dissociation path accurately, while the cooling path shows the formation side, allowing the equilibrium point to be identified from both curves.

The result is taken from the pressure-temperature plot

Clause 8 states that the collected data are plotted with pressure on the vertical axis and temperature on the horizontal axis. The phase equilibrium point is the intersection of the cooling and heating curves, and the corresponding temperature is the hydrate equilibrium temperature. If the heating and cooling curves do not meet cleanly, the linear part of the heating curve may be regressed to find the intersection.

This matters because the standard does not rely on a single observed event. It uses the relationship between the two curves to define equilibrium in a way that is practical for laboratory data.

Precision is based on a limited studied range

Clause 9 ties precision to the basic ISO 5725-2 method. The document states precision data for distilled water and methane within an initial pressure range of 4 MPa to 10 MPa, and only for hydrate phase equilibrium temperatures from 277.92 K to 286.56 K. Annex B gives the supporting statistical analysis and reports repeatability and reproducibility standard deviations.

For users, this means the stated precision is supported for the tested range and conditions, and Annex B is the place to look if you need the statistical basis.

What terms does ISO 23335:2026 define?

• Hydrate - a solid crystalline substance like ice, formed by gas molecules and water molecules.
• Equilibrium - the limit state in a multiphase system where all phases are balanced.
• Phase equilibrium point - the intersection of the cooling curve and the heating curve on the pressure-temperature diagram.
• P-T diagram - the plot of pressure against temperature used to locate the equilibrium point.
• Constant volume method - the test approach used here, where the system volume stays fixed while temperature and pressure change.
• PVT method - the pressure, volume and temperature family of hydrate measurement methods described in the introduction.

Who uses ISO 23335:2026?

ISO 23335:2026 is used by laboratories and researchers working on natural gas hydrates in the upstream area. It is also relevant to oil and gas technical teams, especially where hydrate formation, dissociation or prevention needs to be studied under controlled conditions.

Buyers and quality managers can use it to define a test method for lab services or internal evaluation work. Engineers can use it when selecting apparatus, setting test conditions or comparing hydrate behaviour across different gas and water compositions.

Which standards are used with ISO 23335:2026?

• ISO 5725-2 - provides the basic method for repeatability and reproducibility analysis used in Clause 9 and Annex B.
• ISO 10715 - provides the gas sampling method referred to for produced gas samples.
• ISO 14532 - provides the natural gas vocabulary used for terms and definitions in Clause 3.
• ISO 15156-1 - is cited for guidance on cracking-resistant materials in H2S-containing environments when sour gas service is involved.
• ISO 9001 - is cited in the procedure as an example of the international or national standards used as the basis for calibration records.

What does the ISO 23335:2026 document contain?

ISO 23335:2026 contains a process flow for the full test sequence, an apparatus diagram, and a pressure-temperature schematic used to identify the hydrate equilibrium point. It also includes a test report checklist that tells the user what information to record.

Annex A collects empirical, theoretical and software-based ways to predict the starting temperature for the test. Annex B provides a worked precision study with tables, formulas and statistical tests, including cell means, standard deviations, Cochran’s test and Grubbs’ test. This makes the document useful both for running the method and for checking its statistical performance.

International
Standard
ISO 23335
First edition
Natural gas — Upstream area
2026-08
— Determination of hydrate
equilibrium temperature
Gaz naturel — Zone en amont — Détermination de la
température d’équilibre des hydrates
Reference number
© ISO 2026
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Test solution and gas . 2
5.1 Test solution .2
5.2 Test gas .2
6 Apparatus . 2
7 Procedure . 4
7.1 Calibration and air tightness test .4
7.2 Purging .5
7.3 Sample preparation .5
7.4 Data acquisition .5
7.5 Cooling process .5
7.6 Hydrate formation .5
7.7 Heating process .5
7.7.1 General .5
7.7.2 Ramp heating .5
7.7.3 Stepwise heating .6
7.8 End of the test .6
7.9 Overview .6
8 Calculation . 8
9 Precision . 8
10 Test report . 9
Annex A (informative) Method of temperature setting of gas hydrate test .10
Annex B (informative) Statistical analysis of precision experiments .11
Bibliography .20

iii
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
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
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.

iv
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.
Common methods for measuring the phase equilibrium point of hydrates are mainly divided into the
observation method and the pressure, volume and temperature (PVT) method.
The observation method requires a high-pressure resistant transparent material (e.g. 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 is 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 PVT
in the reaction system while keeping the other 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
the constant volume method.
v
International Standard ISO 23335:2026(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. It
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 14532:2026, 3.1.16, modified — The preferred term has been modified from "gas hydrate" to
"hydrate".]
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
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 Test solution and gas
5.1 Test solution
Determine the best testing medium in accordance with the experimental design. The test solution complies
with the following recommendations and requirements:
a) For formation water in field test simulation, the formation water samples should be taken whenever
possible.
b) For prepared water in field test simulation, when formation water samples are unavailable, the
composition of formation water shall be analysed and prepared.
c) Distilled water should be used in laboratory experiment.
d) Other reagents of analytical grade shall be used.
5.2 Test gas
Determine the best testing medium in accordance with the experimental design. The test gas complies with
the following recommendations and requirements:
a) For produced gas in field test simulation, the produced gas samples shall be taken as the methods
specified in ISO 10715.
b) For prepared gas in field test simulation, when produced gas samples are unavailable, the composition
of produced gas shall be analysed and prepared.
c) For prepared gas in laboratory experiment, all test gas component purity should be ≥ 99,9 %.
6 Apparatus
The apparatus for determining the hydrate equilibrium temperature should be constructed in accordance
with Figure 1.
Key
1 material supply unit
2 pressurizing unit
3 mixing unit
4 temperature control unit
5 reaction device
6 liquid feed unit
7 data acquisition unit
NOTE This diagram only shows the composition of the main apparatus. The specific pipeline connections and
valve settings are not shown here.
Figure 1 — Example illustrating the components of apparatus
6.1 Material supply unit, which shall be used to supply test materials.
6.2 Pressurizing unit, which meets the following recommendations and requirements:
a) The pressure range should meet the experimental requirements and achieve a high-pressure condition.
NOTE 1 High-pressure conditions are one of the essential requirements for hydrate formation. In field
production, the pressure ranges usually from several megapascals to several tens of megapascals.
b) The pressure accuracy shall be ±0,01 MPa.
c) Pressurizing devices and pipelines shall be corrosion resistant in a sour gas testing environment.
NOTE 2 For more information about hydrogen sulfide resistant materials, see ISO 15156-1.
6.3 Mixing unit, which meets the following recommendations and requirements:
a) The mixing unit shall be capable of providing effective mixing to facilitate both hydrate formation and
decomposition.
b) Mechanical stirring, magnetic stirring, rocking or gas disturbance techniques should be used depending
on the experimental conditions.
c) High agitation shall be used for mixing. The higher the agitation, the faster the equilibrium is reached.
NOTE 1 The primary functions of the mixing are to enhance mass transfer, intensify heat transfer, promote
nucleation and simulate flow conditions.

NOTE 2 For more information about high agitation, see Reference [3].
6.4 Temperature control unit, which meets the following recommendations and requirements:
a) The temperature range should meet the experimental requirements and achieve a low-temperature
environment, with a recommended range of 253,2 K to 323,2 K.
b) The temperature accuracy shall be at least ±0,1 K.
c) Water bath, air bath or other temperature control methods should be used depending on the
experimental conditions.
d) The maximum overshooting of temperature control shall be lower than 0,1 K.
6.5 Reaction device, which meets the following requirements:
a) The reaction device shall be capable of forming a sealed, pressure-resistant space and exchanging heat
with the external environment to achieve temperature control.
b) The materials of reaction device shall not react with the test solution (5.1) or test gas (5.2).
c) The reaction device shall meet the test conditions, including the pressure range (6.2), temperature
range (6.4) and the data acquisition (6.7) requirements.
d) The reaction device, whether in the form of a reactor or a rocking cell, shall be capable of connecting to
the data acquisition unit (6.7) to meet the data collection requirements during the testing process.
e) Reaction devices with or without viewports shall be both acceptable for use.
6.6 Liquid feed unit. If the reaction device is easy to seal and open, a liquid feed unit is not required and
the material supply unit (6.1) should be used, instead.
6.7 Data acquisition unit. The data acquisition unit complies with the following recommendations and
requirements:
a) The temperature sensors shall be accurate to 0,01 K.
b) The temperature probes should be positioned in the central or lower-central region of the reaction
device (6.5), within the core mixing and reaction zone of the fluid and should avoid placement directly
against the inner wall of the reaction device.
c) The pressure sensor shall be accurate to ±0,1 % full scale at least.
d) For data processing and recording, it shall use a recorder which is capable of real-time logging
temperature and pressure data and exporting raw data, or an equivalent electronic data recording
device, such as a computer-based data acquisition system with software that supports pressure-
temperature curve plotting.
7 Procedure
7.1 Calibration and air tightness test
7.1.1 Calibration of all apparatus shall comply with international or national measurement standar
...