ISO 24835-2:2025

International
Standard
ISO 24835-2
First edition
Natural gas upstream area —
2025-11
Determination and calculation of
shale brittleness index —
Part 2:
Determination of shale mechanical
characteristics based on triaxial
testing method
Reference number
© ISO 2025
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ii
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

iii
Foreword
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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
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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
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

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 followi
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