ISO/PRF 24835-2

Natural gas upstream area — Determination and calculation of shale brittleness index — Part 2: Determination of shale mechanical characteristics based on triaxial testing method

This document establishes the principles, instruments, sampling, experimental condition design, mechanical testing, calculation, and result reporting requirements and procedures for calculation of shale brittleness index based on the triaxial method for determining shale mechanical characteristics. This document is applicable to the testing and calculation of shale brittleness index in the assessment of reservoirs and selection of areas for marine shale gas exploration and development.

Titre manque — Partie 2: Titre manque

General Information

• Status: Not Published
• 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: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 30-Sep-2025
• Completion Date: 16-Oct-2025

Standards Content (Sample)

International
Standard
ISO 24835-2
First edition
Natural gas upstream area —
Determination and calculation of
shale brittleness index —
Part 2:
Determination of shale mechanical
characteristics based on triaxial
testing method
PROOF/ÉPREUVE
Reference number
ISO 24835-2:2025(en) © ISO 2025

ISO 24835-2:2025(en)
© ISO 2025
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
PROOF/ÉPREUVE
ii
ISO 24835-2:2025(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
5 Apparatus, tools and materials . 2
5.1 Apparatus .2
5.2 Tools and materials .5
6 Sample preparation . 5
7 Triaxial testing . 6
7.1 Procedure .6
7.2 Stress and strain calculation .6
7.3 Calculation of Young’s modulus and Poisson’s ratio .7
8 Calculation of shale mechanical brittleness index . 9
9 Precision . 9
9.1 Repeatability .9
9.2 Reproducibility .9
10 Test report . 10
Bibliography .11
PROOF/ÉPREUVE
iii
ISO 24835-2:2025(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
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 document 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.
A list of all parts in the ISO 24835 series can be found on the ISO website.
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.
PROOF/ÉPREUVE
iv
ISO 24835-2:2025(en)
Introduction
This document has been developed to address the need for evaluating shale brittleness in the production
of shale gas. In order to boost shale gas production, it is essential to create flow paths within shale gas
reservoirs through hydraulic fracturing. The shale brittleness index, which serves as a crucial parameter for
forecasting the complexity of hydraulic fracturing cracks, is a vital benchmark for identifying high-quality
shale gas reservoirs. Therefore, a standardized approach for calculating the shale brittleness index can help
stakeholders (e.g. oil companies, oilfield service providers, investment firms and governments) to accurately
pinpoint “sweet spots” within shale gas reservoirs, select promising areas with development potential and
facilitate efficient shale gas development.
In the global shale gas production industry, two types of optimal methods for characterizing shale
brittleness index are preferred: the mineral composition method and the rock mechanics method. This is
due to their theoretical effectiveness, operational generality and result reliability. The ISO 24835 series has
been developed on the basis of these two methods.
PROOF/ÉPREUVE
v
International Standard ISO 24835-2:2025(en)
Natural gas upstream area — Determination and calculation
of shale brittleness index —
Part 2:
Determination of shale mechanical characteristics based on
triaxial testing method
1 Scope
This document specifies the principles, instruments, materials and experimental conditions for testing
Young’s modulus and Poisson’s ratio using triaxial testing method. It also specifies the sampling and
mechanical testing procedures, as well as the method and precision requirements for calculating shale
mechanical brittleness index based on Young’s modulus and Poisson’s ratio.
This document is applicable to reservoir quality evaluation and sweet spot identification in shale gas
production.
2 Normative references
The following documents are essential to the application of the standard. For dated references, only the
edition cited applies. For undated references, their 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 5725-6, Accuracy (trueness and precision) of measurement methods and results — Part 6: Use in practice of
accuracy values
ISO 14532, Natural gas — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions in ISO 14532 and the following 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
triaxial testing
method of conducting rock mechanics tests by continuously applying axial stress (3.4) on a sample under
equal lateral confining pressure
PROOF/ÉPREUVE
ISO 24835-2:2025(en)
3.2
Young’s modulus
ratio of change in axial stress (3.4) to the corresponding axial strain (3.5) in the linear portion of the
compression stress – compression strain curve of the experimental sample
Note 1 to entry: The Young’s modulus in this document refers to the average Young’s modulus, i.e. the average slope of
the linear portion of the axial stress-strain curve.
Note 2 to entry: Adapted from ISO 22932-3:2023 3.4.1.
3.3
Poisson’s ratio
ratio of the shortening in the transverse direction to the elongation in the direction of an applied force in a
body under tension below the proportional limit
[SOURCE: ISO 22932-3:2023, 3.4.8]
3.4
axial stress
stress component in the direction of the applied force
[SOURCE: ISO 23718:2007, 1.1.10.3]
3.5
axial strain
linear strain in the direction of the applied force
[SOURCE: ISO 23718:2007, 1.1.10.2]
3.6
radial strain
relative deformation of a cylindrical experimental sample under axial load along the diameter direction
3.7
shale brittleness index
index derived from a particular brittleness characterization principle and algorithm among various options,
used for comparing the brittleness levels of different shales
3.8
shale mechanical brittleness index
shale brittleness index (3.8) obtained through calculation based on the shale mechanical parameters
4 Principles
Based on the principle that Young’s modulus and Poisson’s ratio can characterize the tendency of rocks to
fracture under stress without significant plastic deformation, use a triaxial method to test the stress – strain
curves of shale samples under formation temperature and pressure. Then, calculate Young’s modulus and
Poisson’s ratio. Obtain the shale brittleness index through normalized calculation.
NOTE References [3], [4] and [5] elaborate on the methods for calculating shale mechanical brittleness index
using Young’s modulus and Poisson’s ratio.
5 Apparatus, tools and materials
5.1 Apparatus
5.1.1 Triaxial rock mechanics tester (see Figure 1), which shall meet the following requirements:
a) The stiffness of the loading frame shall be higher than 5 × 10 N/m.
PROOF/ÉPREUVE
ISO 24835-2:2025(en)
b) The load capacity of the instrument shall be higher than 500 kN.
c) The accuracy of the load sensor shall be better than 0,001 kN.
d) The accuracy in confining pressure control shall be within ±0,5 MPa.
e) The temperature control accuracy shall be within ±0,5 K.
f) For axial deformation sensors, a resistance extensometer (see Figure 2) or a linear variable differential
transformer (see Figure 3) may be adopted. The resolution shall be better than 0,002 5 %.
g) For radial deformation sensors, a resistance extensometer (see Figure 2) or a linear variable differential
transformer (see Figure 3) may be adopted. The resolution shall be better than 0,002 5 %.

Key
1 axial loading system
2 confining pressure chamber
3 computer system
4 shale sample
5 confining pressure loading system
6 servo control system
7 hydraulic system
Figure 1 — Schematic diagram of triaxial rock mechanics experiment
PROOF/ÉPREUVE
ISO 24835-2:2025(en)
Key
1 upper platen
2 radial strain gauge
3 lower platen
4 axial strain
...

ISO/DISPRF 24835-2:2025(en)
ISO/TC 193/SC 3/WG 9
Secretariat: SAC
Date: 2025-07-3109-29
Natural gas upstream area — Determination and calculation of shale
brittleness index —
Part 2:
Determination of shale mechanical characteristics based on triaxial
testing method
PROOF
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: + 41 22 749 01 11
E-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles . 2
5 Apparatus, tools and materials . 2
5.1 Apparatus . 2
5.2 Tools and materials. 4
6 Sample preparation . 4
7 Triaxial testing . 5
7.1 Procedure . 5
7.2 Stress and strain calculation . 6
7.3 Calculation of Young’s modulus and Poisson’s ratio . 6
8 Calculation of shale mechanical brittleness index . 9
9 Precision . 9
9.1 Repeatability . 9
9.2 Reproducibility . 9
10 Test report . 9
Bibliography . 11

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 document 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.
A list of all parts in the ISO 24835 series can be found on the ISO website.
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
This document has been developed to address the need for evaluating shale brittleness in the production of
shale gas. In order to boost shale gas production, it is essential to create flow paths within shale gas reservoirs
through hydraulic fracturing. The shale brittleness index, which serves as a crucial parameter for forecasting
the complexity of hydraulic fracturing cracks, is a vital benchmark for identifying high-quality shale gas
reservoirs. Therefore, a standardized approach for calculating the shale brittleness index can help
stakeholders (e.g. oil companies, oilfield service providers, investment firms and governments) to accurately
pinpoint “sweet spots” within shale gas reservoirs, select promising areas with development potential and
facilitate efficient shale gas development.
In the global shale gas production industry, two types of optimal methods for characterizing shale brittleness
index are preferred: the mineral composition method and the rock mechanics method. This is due to their
theoretical effectiveness, operational generality and result reliability. The ISO 24835 series has been
developed on the basis of these two methods.
v
Natural gas upstream area — Determination and calculation of shale
brittleness index —
Part 2:
Determination of shale mechanical characteristics based on triaxial
testing method
1 Scope
This document specifies the principles, instruments, materials and experimental conditions for testing
Young’s modulus and Poisson’s ratio using triaxial testing method. It also specifies the sampling and
mechanical testing procedures, as well as the method and precision requirements for calculating shale
mechanical brittleness index based on Young’s modulus and Poisson’s ratio.
This document is applicable to reservoir quality evaluation and sweet spot identification in shale gas
production.
2 Normative references
The following documents are essential to the application of the standard. For dated references, only the edition
cited applies. For undated references, their 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 5725-6, Accuracy (trueness and precision) of measurement methods and results — Part 6: Use in practice of
accuracy values
ISO 14532, Natural gas — Vocabulary
3 Terms and definitions
For the purposes of this document, the terms and definitions in ISO 14532 and the following 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
triaxial testing
method of conducting rock mechanics tests by continuously applying axial stress (3.4) on a sample under equal
lateral confining pressure
3.2
Young’s modulus
ratio of change in axial stress (3.4) to the corresponding axial strain (3.5) in the linear portion of the
compression stress – compression strain curve of the experimental sample
Note 1 to entry: The Young’s modulus in this document refers to the average Young’s modulus, i.e.,. the average slope of
the linear portion of the axial stress-strain curve.

Note 2 to entry: Adapted from ISO 22932-3:2023 3.4.1.
3.3
Poisson’s ratio
ratio of the shortening in the transverse direction to the elongation in the direction of an applied force in a
body under tension below the proportional limit
[SOURCE: ISO 22932-3:2023, 3.4.8]
3.4
axial stress
stress component in the direction of the applied force
[SOURCE: ISO 23718:2007, 1.1.10.3]
3.5
axial strain
linear strain in the direction of the applied force
[SOURCE: ISO 23718:2007, 1.1.10.2]
3.6
radial strain
relative deformation of a cylindrical experimental sample under axial load along the diameter direction
3.7
shale brittleness index
index derived from a particular brittleness characterization principle and algorithm among various options,
used for comparing the brittleness levels of different shales
3.8
shale mechanical brittleness index
shale brittleness index (3.8) obtained through calculation based on the shale mechanical parameters
4 Principles
Based on the principle that Young’s modulus and Poisson’s ratio can characterize the tendency of rocks to
fracture under stress without significant plastic deformation, use a triaxial method to test the stress – strain
curves of shale samples under formation temperature and pressure. Then, calculate Young’s modulus and
Poisson’s ratio. Obtain the shale brittleness index through normalized calculation.
NOTE References [3], [4] and [5 [3] to [5]] elaborate on the methods for calculating shale mechanical brittleness
index using Young’s modulus and Poisson’s ratio.
5 Apparatus, tools and materials
5.1 Apparatus
5.1.1 Triaxial rock mechanics tester (see Figure 1), which shall meet the following requirements:
a) The stiffness of the loading frame shall be higher than 5× × 10 N/m.
b) The load capacity of the instrument shall be higher than 500 kN.
c) The accuracy of the load sensor shall be better than 0,001 kN.
d) The accuracy in confining pressure control shall be within ±0,5 MPa.
e) The temperature control accuracy shall be within ±0,5 K.
f) For axial deformation sensors, a resistance extensometer (see Figure 2) or a linear variable differential
transformer (see Figure 3) may be adopted. The resolution shall be better than 0,002 5 %;.
g) For radial deformation sensors, a resistance extensometer (see Figure 2) or a linear variable differential
transformer (see Figure 3) may be adopted. The resolution shall be better than 0,002 5 %;.

Key
1 axial loading system
2 confining pressure chamber
3 computer system
4 shale sample
5 confining pressure loading system
6 servo control system
7 hydraulic system
Figure 1 — Schematic diagram of triaxial rock mechanics experiment

Key
1 upper platen
2 radial strain gauge
3 lower platen
4 axial strain gauge
5 shale sample
Figure 2 — Schematic diagram of installation of resistive strain gauge

Key
1 upper platen
2 radial strain gauge
3 lower platen
4 axial strain gauge
5 shale sample
Figure 3 — Schematic diagram of installation of linear variable differential transformer strain gauge
5.1.2 The temperatureConvection oven, with a temperature control accuracy of convection ovenwhich
shall be better than ±2 K.
5.1.3 Rock coring machine, which can be equipped with a diamond core drill bit with an inner diameter of
25,4 mm.
5.1.4 Rock grinding machine, which should have a diameter of grinding area larger than 50 mm.
5.1.5 Parallelism measuring device, which shall have a resolution that can be read to the nearest
0,001 mm.
5.1.6 Verticality measuring device, which shall have a resolution that can be read to the nearest 0,001°.
5.2 Tools and materials
5.2.1 Vernier calliper, which shall be accurate to 0,02 mm.
5.2.2 Heat-shrink tubing, which shall have a diameter of 27 mm to 30 mm, and a shrink ratio greater than
2.
6 Sample preparation
6.1 Obtain a downhole full-hole shale core at a certain depth of the shale core. Drill at least three vertical
bedding samples using a rock coring machine
...