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

Overview

ISO 23335: Natural gas - Upstream area - Determination of hydrate equilibrium temperature is an international standard developed by the International Organization for Standardization (ISO). The document establishes general requirements and test methods for determining the equilibrium temperature at which natural gas hydrates form or decompose under constant volume laboratory conditions. Accurate measurement of hydrate equilibrium temperature is critical in the upstream sector of the natural gas industry for safe, efficient exploration, production, and handling of natural gas resources.

Key Topics

• Scope and Purpose: The standard outlines procedures for the preparation of test solutions and gases, selection and calibration of apparatus, step-by-step experimental procedure, data acquisition, and analysis for identifying hydrate equilibrium temperature.
• Laboratory Methods: Emphasis on laboratory-based simulations designed to replicate field conditions, supporting both formation and decomposition studies of natural gas hydrates.
• Test Materials: Details on the use and selection of test solutions (including distilled and formation water) and test gases in accordance with supporting standards such as ISO 10715 for gas sampling.
• Apparatus and Instrumentation:
  • Material supply and pressurizing units
  • Mixing unit for adequate agitation
  • Precise temperature control system
  • Sealed, pressure-resistant reaction devices
  • Data acquisition systems for real-time monitoring
• Experimental Procedure:
  • Calibration and air-tightness checks
  • Purging and sample loading
  • Stepwise cooling and controlled heating methods
  • Real-time recording and analysis of temperature and pressure data
• Data Processing and Reporting: Guidance on data interpretation (using pressure-temperature diagrams) and standard report generation for test results.

Applications

ISO 23335 offers practical value for stakeholders in the natural gas industry, including:

• Research Laboratories: Provides a standardized approach for measuring hydrate phase equilibrium, supporting scientific investigations and innovation.
• Field Development and Production: Essential for designing and operating pipelines and processing equipment, where hydrate formation can pose flow assurance challenges. Accurate understanding of hydrate equilibrium temperature helps prevent blockages and ensures uninterrupted gas flow.
• Hydrate Risk Management: Supports engineers in establishing operational boundaries to avoid hydrate formation, which is crucial for safety and reliability in upstream operations.
• Product Development: Useful for those creating inhibitors, anti-hydrate additives, or developing advanced materials and technologies for natural gas production.

Related Standards

ISO 23335 refers to and complements several other international standards relevant to natural gas sampling, terminology, and measurement methods, including:

• ISO 5725-2: Accuracy (trueness and precision) of measurement methods and results - Basic method for determination of repeatability and reproducibility.
• ISO 10715: Natural gas - Gas sampling.
• ISO 14532: Natural gas - Vocabulary.
• ISO 15156-1 (referenced for material selection in sour gas environments).

Organizations working within the upstream natural gas sector will benefit from aligning with ISO 23335 to support consistent laboratory practices, promote data comparability, and ensure compliance with global industry standards related to natural gas hydrate management.

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Keywords: ISO 23335, natural gas hydrate equilibrium, hydrate formation, phase equilibrium temperature, laboratory methods, upstream natural gas, gas hydrate standard, test procedures, gas sampling, pipeline flow assurance, hydrate risk management.

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
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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
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This document was prepared by Technical Committee ISO/TC 193, Natural gas, Subcommittee SC 3, Upstream
area.
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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
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