ISO/PRF 25195

• Abstract:

  This document specifies the work content, methods, requirements and results for evaluating the recoverability of coalbed methane (CBM). It is applicable to the phases of the exploration and appraisal, and production capacity building of CBM and provides certain references for the development and adjustment phases.

• Status: Not Published
• ICS: 73.020 - Mining and quarrying
• Technical Committee: ISO/TC 263 - Coalbed methane (CBM)
• Drafting Committee: ISO/TC 263 - Coalbed methane (CBM)
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 25-Sep-2026
• Completion Date: 26-Sep-2026

Overview

ISO/PRF 25195 is the ISO standard for the evaluation method of coalbed methane recoverability. It provides a structured framework for assessing the development feasibility of coalbed methane (CBM) under existing technical and economic conditions. The document is designed to support decision-making in the exploration, appraisal, and production capacity building phases, while also offering useful reference for development and adjustment stages.

This standard is especially valuable for organizations that need a consistent CBM recoverability evaluation process, from data collection and geological analysis to economic screening and ranking of recoverable units.

Key Topics

The standard focuses on a practical workflow for evaluating CBM recoverability, including:

• Data collection and collation
  • Geological, core, fluid property, production testing, stimulation, and economic input data
• Enrichment unit evaluation
  • CBM resource abundance
  • Hydrodynamic conditions
  • Fault development index
• High-yield unit evaluation
  • Structural evaluation
  • Reservoir rock evaluation
  • Gas reservoir evaluation
• Economic recoverability evaluation
  • Gas-bearing area determination
  • Geological reserve estimation
  • Recoverable reserve calculation
  • Production profile determination
  • Cash flow-based economic evaluation

The document also defines key terms such as enrichment unit, high-yield unit, and economically recoverable unit, supporting clearer communication across CBM projects and technical teams.

Applications

ISO/PRF 25195 supports a wide range of coalbed methane development activities, including:

• Project screening and block ranking
  • Identify promising CBM areas with stronger recovery potential
• Exploration and appraisal planning
  • Improve early-stage assessment of resource quality and development feasibility
• Production capacity building
  • Support planning for units with higher production potential
• Economic prioritization
  • Compare units and determine development priority based on recoverability and economic performance
• Technical evaluation
  • Align geological interpretation, reservoir analysis, and production data in one recoverability framework

For operators and consultants, this standard helps create a more consistent and transparent CBM resource evaluation process.

Related Standards

ISO/PRF 25195 should be considered alongside other standards and references relevant to coalbed methane, geological evaluation, and resource development. The document cites ISO 18875:2015 for the definition of coalbed methane.

Additional ISO and IEC terminology resources may also be useful for implementation and interpretation, including:

• ISO Online Browsing Platform
• IEC Electropedia

As a draft and proof-stage standard prepared by ISO/TC 263, Coalbed methane (CBM), ISO/PRF 25195 is intended to support standardized, repeatable, and economically informed CBM recoverability assessments.

International
Standard
ISO 25195
First edition
Evaluation method of coalbed
methane recoverability
PROOF/ÉPREUVE
Reference number
ISO 25195:2026(en) © ISO 2026
ISO 25195:2026(en)
© ISO 2026
All rights reserved.
This ISO publication is protected by copyright and is owned by ISO and/or its licensors.
The content of this ISO publication is provided under licence, not sold. Use is subject to the applicable licence terms issued by ISO,
an ISO member body, or an authorized third-party distributor.
Except as required for implementation or expressly permitted by a separate licence, no part of this ISO publication may be
reproduced, distributed, modified, used, or made available in any form or by any means – electronic or mechanical, including
photocopying, scanning, recording, or posting on internal or external digital platforms.
Any use beyond the scope of the granted rights is prohibited and may result in legal action.
ISO copyright office
CP 401 • CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
PROOF/ÉPREUVE
ii
ISO 25195:2026(en)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Procedure for the evaluation of CBM recoverability . 2
5 Data collection and collation . 2
6 Enrichment unit evaluation . 3
6.1 Abundance of CBM resources .3
6.2 Hydrodynamic conditions .3
6.3 Fault development index .4
7 Evaluation of high-yield units . 5
7.1 Structural evaluation . .5
7.1.1 Structural form .5
7.1.2 Fracture development .5
7.2 Reservoir rock evaluation .6
7.2.1 Coal-body structure .6
7.2.2 Effective stress .6
7.3 Gas reservoir evaluation .7
7.3.1 Gas saturation .7
7.3.2 Water content of surrounding rocks .7
8 Determination of enrichment and high-yield units. 8
8.1 Enrichment unit determination .8
8.2 Identification of high-yield units .8
8.3 Comprehensive evaluation .9
9 Content of CBM economic recoverability evaluation .10
9.1 Economic recoverability evaluation procedure .10
9.2 Determination of gas-bearing areas of high-yield units .10
9.3 Calculation of geological reserves of high-yield units .10
9.4 Factors for production profile determination .11
9.5 Determination of recoverable reserves . 12
9.6 Determination of production profiles . 12
9.7 Economic evaluation . 12
9.8 Determination of economically recoverable units . 13
10 Drawings of results . 14
11 Annexed tables . 14
Annex A (informative) Names, units, symbols, and effective digits of the CBM recoverability
evaluation parameters .15
Bibliography . 17
PROOF/ÉPREUVE
iii
ISO 25195:2026(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 263, Coalbed methane (CBM).
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 25195:2026(en)
Introduction
This document specifies the key evaluation parameters for economically recoverable coalbed methane
(CBM) reserves. It provides an assessment of CBM resource recoverability, in which units with high
production potential are identified based on resource enrichment levels and production capacity. The
recoverable reserves of these units are estimated, and their production profiles are determined based on
recovery factors, engineering technologies, and development indices of the target block and comparable
mature CBM development areas. The cash flow method is used to perform an economic evaluation of the
production profiles of high-yield units. The units are then ranked according to economic recoverability
evaluation parameters to determine the development priority of economically recoverable units.
PROOF/ÉPREUVE
v
International Standard ISO 25195:2026(en)
Evaluation method of coalbed methane recoverability
1 Scope
This document specifies methods, recommended technical approaches, and results for the evaluating of
coalbed methane (CBM) recoverability.
This document is applicable to the exploration, appraisal and production capacity building of CBM. It also
provides certain references for the development and adjustment phases.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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
coalbed methane
CBM
methane-rich gas naturally occurring in coal seams (and surrounding rock) typically comprising of 80 % to
95 % methane with lower proportions of ethane, propane, nitrogen and carbon dioxide
Note 1 to entry: In common international use, this term refers to methane recovered from un-mined coal seams using
surface boreholes.
[SOURCE: ISO 18875:2015, 2.1]
3.2
coalbed methane recoverability
CBM recoverability
development feasibility of coalbed methane (3.1) in coal reservoirs under existing technical and economic
conditions
3.3
enrichment unit
geological unit with certain coalbed methane (3.1) resources under existing technical conditions
3.4
high-yield unit
geological unit with high production potential under existing technical conditions
3.5
economically recoverable unit
geological unit that can be economically exploited under existing economic conditions
PROOF/ÉPREUVE
ISO 25195:2026(en)
4 Procedure for the evaluation of CBM recoverability
The detailed procedure for the evaluation of CBM recoverability is shown in Figure 1.
Figure 1 — Workflow of CBM recoverability evaluation
5 Data collection and collation
Data collection and collation should include the following:
a) basic geological data: seismic, drilling, mud logging and well logging data;
PROOF/ÉPREUVE
ISO 25195:2026(en)
b) core analysis data of coal: coal quality analysis, coal rock testing, rock mechanics, sensitivity and other
relevant parameters;
c) fluid property data should include as follows:
1) CBM content;
2) composition;
3) carbon and hydrogen isotopes;
4) isothermal adsorption/desorption;
5) calorific value, and other relevant test results;
6) ion composition and content;
7) salinity;
8) water type;
9) pH value, for example, of formation water;
10) other special analyses if required, e.g. sampling and testing of hydrogen sulfide;
d) production testing data: production scheme, gas production performance, water production
performance, bottomhole pressure, casing pressure, working fluid level, downhole service parameters,
etc;
e) production stimulation data: perforating and fracturing designs, fracturing operation summary (types
fracturing fluids and proppants, treatment pressure, injection rate, injected fracturing fluid volume,
injected proppant quantity, proppant concentration, etc.), fracture monitoring results, etc;
f) basic parameters for economic evaluation: capital cost, production cost, depreciation, tax rate, CBM
sales price, etc.
6 Enrichment unit evaluation
6.1 Abundance of CBM resources
6.1.1 In most cases, the following three parameters apply to the evaluation of CBM resources abundance:
— coal seam thickness interpreted by well logging, which can be taken as the net coal seam thickness;
— coal seam gas content, which can be calculated as the product of air-dried basis gas content and methane
percentage, as measured by core analysis of the coal seam;
— coal rock density, which can be calculated as the average value of bulk density measurements from core
analysis of the coal seam.
8 3 2
6.1.2 CBM abundance below 0,8 × 10 m /km represents low abundance and is considered unviable. CBM
8 3 2 8 3 2
resource abundance ranging from 0,8 × 10 m /km to 1,2 × 10 m /km represents medium abundance,
8 3 2
and that above 1,2 × 10 m /km represents high abundance.
6.2 Hydrodynamic conditions
6.2.1 Evaluation of regional hydrodynamic intensity is based on hydrogeological data of the study area,
including the regional distributions of stratigraphy, aquifers and aquicludes, and the distributions of rivers
and springs around the basin. Formation pressure data and analysis data of cores and produced water of
wells drilled in the study area are investigated comprehensively to evaluate the hydrodynamic intensity.
PROOF/ÉPREUVE
ISO 25195:2026(en)
6.2.2 The recommended classification parameters for hydrodynamic conditions are piezometric head
elevation, formation water salinity and ion concentrations, and methane carbon isotope ratio (δ C ).
Piezometric head elevation refers to the difference between the water level in the coal seam at the well
point and mean sea level. An isobaric map is drawn based on piezometric head elevations and used to
determine the flow direction and hydraulic pressure gradient of confined water. When used in combination
with regional data, this also helps to clarify the recharge and discharge relationships between confined
and phreatic water. Formation water salinity is determined by analysis of produced water, with test results
- 2- -
generally including the contents of seven major ions – bicarbonate (HCO ), sulfate (SO ), chloride (Cl ),
3 4
+ + 2+ 2+
potassium (K ), sodium (Na ), calcium (Ca ), and magnesium (Mg ) – as well as total salinity, pH value, and
water type. Changes in ion concentrations and salinity are correlated with the flow of confined water from
recharge areas to discharge areas, and therefore, indicate the flow direction and activity levels of confined
water, and the interference of external water. The carbon isotope ratio of methane (δ C ) is determined
from core desorption gas or produced gas. Active groundwater triggers flushing of the carbons in methane,
resulting in a lower carbon isotope ratio.
Based on these parameters, the study area can be divided into a strong runoff zone, a weak runoff zone,
and a confined zone. The strong runoff zone features strong hydrodynamics and low gas content in the coal
seams, and is therefore unviable. In contrast, the confined zone and the weak runoff zone have comparatively
favourable conditions for CBM preservation and higher gas contents. However, the limits of the parameters
for determining viability vary between different areas and coal ranks. For example, in the southern part of
the Qinshui Basin, located in the southeastern part of Shanxi Province, China, a high-rank coal area with a
piezometric head elevation lower than 650 m, salinity lower than 1 000 mg/l, and methane carbon isotope
13 1
ratio (δ C ) smaller than −38 ‰, is classified as a strong runoff zone with active hydrodynamics and low
gas content and is therefore considered unviable. Piezometric head elevation is calculated using Formula (1):
Hzh (1)
where
H is the piezometric head elevation, expressed in metres (m);
z is the distance from the depth of the coal seam at the well point to the mean sea level, expressed
in m;
h is the piezometric head, namely the elevation of the water level of the coal seam at the well
point, expressed in m.
6.3 Fault development index
6.3.1 Detailed analysis of fault morphology is carried out by seismic interpretation using regional 2D and
3D seismic data, which reveals the striking and dipping directions and the dip angle of the fault plane, as
well as fault types (normal, reverse, and strike-slip faults), and fault displacement (vertical and horizontal
displacement). To further improve the accuracy of fault interpretation, the attribute prediction method
(preferably adopting the coherence and curvature attributes) may be applied across the 3D seismic survey
area. Data from core analysis and production testing are incorporated to define the different effects of faults
on gas content and, hence, demarcate fault-affected zones.
6.3.2 It is recommended that the fault development index be used for evaluation, using Formula (2), to
calculate the index. The fewer the faults, the smaller the fault-affected zone for CBM, and the higher the CBM
recoverability.
The influence of faults varies between areas, which therefore should be accurately determined from actual
drilling and production testing data. The southern part of the Qinshui Basin, located in the south-eastern
part of Shanxi Province, China can be taken as an example. The normal faults in this basin are divided into
three grades according to fault displacement:
— grade-I normal faults, which have displacements greater than 300 m and affect the gas contents of coal
seams within 500 m to 600 m on either side of the fault;
PROOF/ÉPREUVE
ISO 25195:2026(en)
— grade-II normal faults, which have displacements of 100 m to 300 m, with the fault-affected zone
extending 200 m to 500 m on either side of the fault;
— grade-III normal faults, which have displacements less than 100 m and influence the gas contents of coal
seams within 150 m to 200 m on either side of the fault.
Reverse faults generally have displacements of less than 100 m and have better sealing than normal faults.
Their affected zones are 50 m to 100 m on either side of the fault. The fault development index is used to
compare the influence of faults. A fault development index of less than 0,3 indicates a highly favourable
area and is relatively little affected by faults, an index between 0,3 and 0,7 suggests comparatively well-
developed faults, and an index greater than 0,7 means severe fault damage to the area, thus being unviable.
Formula (2) is calculated as follows:
S
fm
F= (2)
S
m
where
F is the fault development index, dimensionless;
S is the area affected by faults within the target block, expressed in square kilometres (km );
fm
S is the area of the target block, expressed in km .
m
7 Evaluation of high-yield units
7.1 Structural evaluation
7.1.1 Structural form
7.1.1.1 Structural forms are analysed in detail by seismic interpretation using regional 2D and 3D seismic
data, which highlights the development status and types of folds and formation dip angles. In order to
further improve the accuracy of structural form characterization, methods such as the structural amplitude
difference attribute, curvature attribute and 3D visualization may be applied to the 3D seismic survey area.
7.1.1.2 It is recommended to broadly divide regional structures into two categories: monoclines and folds.
Monocline structures are further sub-divided according to amplitude undulation and dip angles. Areas
with fewer undulations and smaller formation dip angles are more favourable. Fold structures are further
sub-divided according to fold morphology and interlimb angles. There are two types of fold morphology:
antiforms and synforms. Folds are also divided into gentle folds (interlimb angles > 120°), open folds
(interlimb angles of 70°–120°), closed folds (interlimb angles of 30°–70°), tight folds (interlimb angles < 30°)
and isoclinal folds (interlimb angles of nearly 0°).
7.1.2 Fracture development
7.1.2.1 Natural fracture development is a key factor influencing the permeability of medium- and high-
rank coal reservoirs. Fracture development is assessed using direct and indirect methods. The direct
method uses macroscopic core description to determine relevant parameters. These include the lengths,
widths, heights and densities of major and minor fractures. The higher the number of core samples, the
greater the accuracy of the core descriptions. It is recommended that major fracture development density be
adopted as the primary classification index.
The indirect method uses 3D seismic attribute prediction. The early production system based on horizontal
drilling coherent attribute method and the pre-stack azimuthal anisotropy method are recommended, as
they are both able to qualitatively identify fracture development areas.
PROOF/ÉPREUVE
ISO 25195:2026(en)
7.1.2.2 Degrees of fracture development are likely to vary between different areas and coal ranks. For
instance, for the high-rank coal of the Qinshui Basin, located in the south-eastern part of Shanxi Province,
China, the following applies:
— a coal reservoir with a major fracture density of less than 4 fractures/cm is considered non-fractured
with underdeveloped natural fractures;
— a coal reservoir with a major fracture density of 4 fractures/cm to 6 fractures/cm, is considered relatively
fractured with relatively well-developed natural fractures;
— a coal reservoir with a major fracture density exceeding 6 fractures/cm, is considered fractured with
well-developed natural fractures.
7.2 Reservoir rock evaluation
7.2.1 Coal-body structure
7.2.1.1 Coal-body structures are classified into four types: the primary structure, cataclastic structure,
granulated structure and mylonitic structure. The methods for distinguishing coal-body structure types
include coal core identification, well logging response prediction, and geophysical prediction. Coal core
identification is carried out by direct observation of coal cores collected by drilling. The advantage of this
method is that it entails direct obse
...

ISO/DISPRF 25195:2026(en)
Date:2026-04-30
ISO/TC 263
Secretariat: SAC
Coalbed methane (CBM) — Date: 2026-09-24
Evaluation method of coalbed methane recoverability

© PROOF
ISO/DISPRF 25195:2026(en)
All rights reserved. Unless otherwise specified,
This ISO publication is protected by copyright and is owned by ISO and/or its licensors.
The content of this ISO publication is provided under licence, not sold. Use is subject to the applicable licence terms issued
by ISO, an ISO member body, or an authorized third-party distributor.
Except as required in the context of its for implementation or expressly permitted by a separate licence, no part of this
ISO publication may be reproduced or utilized otherwise, distributed, modified, used, or made available in any form or
by any means, – electronic or mechanical, including photocopying, scanning, recording, or posting on internal or external
digital platforms.
Any use beyond 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 countryscope of the requestergranted rights is prohibited and may
result in legal action.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
iii
ISO #####-#:####(X/PRF 25195:2026(en)
Contents
Foreword . v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Procedure for the evaluation of CBM recoverability . 2
5 Data collection and collation . 3
6 Enrichment unit evaluation . 4
6.1 Abundance of CBM resources . 4
6.2 Hydrodynamic conditions . 4
6.3 Fault development index . 5
7 Evaluation of high-yield units. 6
7.1 Structural evaluation . 6
7.2 Reservoir rock evaluation . 7
7.3 Gas reservoir evaluation . 8
8 Determination of enrichment and high-yield units. 9
8.1 Enrichment unit determination . 9
8.2 Identification of high-yield units . 9
8.3 Comprehensive evaluation . 11
9 Content of CBM economic recoverability evaluation . 12
9.1 Economic recoverability evaluation procedure . 12
9.2 Determination of gas-bearing areas of high-yield units . 12
9.3 Calculation of geological reserves of high-yield units . 12
9.4 Factors for production profile determination. 13
9.5 Determination of recoverable reserves . 13
9.6 Determination of production profiles . 13
9.7 Economic evaluation . 14
9.8 Determination of economically recoverable units . 15
10 Drawings of results . 15
11 Annexed tables . 16
Annex A (informative) Names, units, symbols, and effective digits of the CBM recoverability
evaluation parameters . 17
Bibliography . 19

© ISO #### 2026 – All rights reserved
iv
ISO/DISPRF 25195:2026(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 263, Coalbed methane (CBM).
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.

v
ISO #####-#:####(X/PRF 25195:2026(en)
Introduction
This document specifies the key evaluation parameters for economically recoverable coalbed methane (CBM)
reserves. It provides an assessment of CBM resource recoverability, in which units with high production
potential are identified based on resource enrichment levels and production capacity. The recoverable
reserves of these units are estimated, and their production profiles are determined based on recovery factors,
engineering technologies, and development indices of the target block and comparable mature CBM
development areas. The cash flow method is used to perform an economic evaluation of the production
profiles of high-yield units. The units are then ranked according to economic recoverability evaluation
parameters to determine the development priority of economically recoverable units.
© ISO #### 2026 – All rights reserved
vi
DRAFT International Standard ISO/DIS 25195:2026(en)

Coalbed methane (CBM) — Evaluation method of coalbed methane
recoverability
1 Scope
This document specifies methods, recommended technical requirementsapproaches, and results reporting for
the evaluating of coalbed methane (CBM) recoverability.
This document is applicable to the exploration, appraisal and production capacity building of CBM. It also
provides certain references for the development and adjustment phases.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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 3.1
coalbed methane
CBM
methane-rich gas naturally occurring in coal seams (and surrounding rock) typically comprising of 80 % to
95 % methane with lower proportions of ethane, propane, nitrogen and carbon dioxide
Note 1 to entry: In common international use, this term refers to methane recovered from un-mined coal seams using
surface boreholes.
[SOURCE: ISO 18875:2015, 2.1]
3.2 3.2
coalbed methane recoverability
CBM recoverability
development feasibility of coalbed methane (3.1(3.1)) in coal reservoirs under existing technical and economic
conditions
3.3 3.3
enrichment unit
geological unit with certain coalbed methane (3.1(3.1)) resources under existing technical conditions
3.4 3.4
high-yield unit
geological unit with high production potential under existing technical conditions
3.5 3.5
economically recoverable unit
geological unit that can be economically exploited under existing economic conditions
4 Procedure for the evaluation of CBM recoverability
The detailed procedure for the evaluation of CBM recoverability is shown in Figure 1Figure 1.
Evaluation
block
Resource Hydrodynamic Fracture
Resource
abundance conditions development
evaluation
Non-enrichment
Enrichment
units rejected for
unit
evaluation
Gas reservoir
Structural Reservoir rock
evaluation
evaluation evaluation
Structure Fault Coal body Effective Gas Water content of
High yield
potential
form development Structure stress saturation surrounding rock
evaluation
High-yield
unit
Economic Gas-bearing area Estimation of Estimation of
recoverability determination geological reserves recoverable reserve
evaluation
Production profile
determination of high-
yield units
Economic
evaluation
Economically
recoverable
Figure 1 — Workflow of CBM recoverability evaluation
5 Data collection and collation
Data collection and collation should include as followsthe following:
a) a) Basicbasic geological data: seismic, drilling, mud logging and well logging data.;
b) b) Corecore analysis data of coal: coal quality analysis, coal rock testing, rock mechanics,
sensitivity and other relevant parameters.;
c) c) Fluidfluid property data should include as follows:
1) CBM content;
2) composition;
3) carbon and hydrogen isotopes;
4) isothermal adsorption/desorption;
5) calorific value, and other relevant test results;
6) ion composition and content;
7) salinity;
8) water type;
9) pH value, etc.,for example, of formation water;
10) other special analyses if required, e.g. sampling and testing of hydrogen sulfide.;
d) d) Productionproduction testing data: production scheme, gas production performance, water
production performance, bottomhole pressure, casing pressure, working fluid level, downhole service
parameters, etc.;
e) e) Productionproduction stimulation data: perforating and fracturing designs, fracturing
operation summary (types fracturing fluids and proppants, treatment pressure, injection rate, injected
fracturing fluid volume, injected proppant quantity, proppant concentration, etc.), fracture monitoring
results, etc.;
f) f) Basicbasic parameters for economic evaluation: capital cost, production cost, depreciation, tax
rate, CBM sales price, etc.
6 Enrichment unit evaluation
6.1 Abundance of CBM resources
6.1.1 In most cases, the following three parameters apply to the evaluation of CBM resources abundance. :
— coal seam thickness interpreted by well logging, which can be taken as the net coal seam thickness;
— coal seam gas content, which can be calculated as the product of air-dried basis gas content and methane
percentage, as measured by core analysis of the coal seam;
— coal rock density, which can be calculated as the average value of bulk density measurements from core
analysis of the coal seam.
8 3 2
6.1.2 CBM abundance below 0,8× × 10 m /km represents low abundance, and is considered unviable. CBM
8 3 2 8 3 2
resource abundance ranging from 0,8 × × 10 m /km to 1,2 × × 10 m /km represents medium abundance,
8 3 2
and that above 1,2 × × 10 m /km represents high abundance.
6.2 Hydrodynamic conditions
6.2.1 Evaluation of regional hydrodynamic intensity is based on hydrogeological data of the study area,
including the regional distributions of stratigraphy, aquifers and aquicludes, and the distributions of rivers
and springs around the basin. Formation pressure data and analysis data of cores and produced water of wells
drilled in the study area are investigated comprehensively to evaluate the hydrodynamic intensity.
6.2.2 The recommended classification parameters for hydrodynamic conditions are piezometric head
elevation, formation water salinity and ion concentrations, and methane carbon isotope ratio (δ C ).
Piezometric head elevation refers to the difference between the water level in the coal seam at the well point
and mean sea level. An isobaric map is drawn based on piezometric head elevations and used to determine
the flow direction and hydraulic pressure gradient of confined water. When used in combination with regional
data, this also helps to clarify the recharge and discharge relationships between confined and phreatic water.
Formation water salinity is determined by analysis of produced water, with test results generally including
- 2- - +
the contents of seven major ions – bicarbonate (HCO ), sulfate (SO ), chloride (Cl ), potassium (K ), sodium
3 4
+ 2+ 2+
(Na ), calcium (Ca ), and magnesium (Mg ) – as well as total salinity, pH value, and water type. Changes in
ion concentrations and salinity are correlated with the flow of confined water from recharge areas to
discharge areas, and therefore, indicate the flow direction and activity levels of confined water, and the
interference of external water. The carbon isotope ratio of methane (δ C ) is determined from core
desorption gas or produced gas. Active groundwater triggers flushing of the carbons in methane, resulting in
a lower carbon isotope ratio.
Based on these parameters, the study area can be divided into a strong runoff zone, a weak runoff zone, and a
confined zone. The strong runoff zone features strong hydrodynamics and low gas content in the coal seams,
and is therefore unviable. In contrast, the confined zone and the weak runoff zone have comparatively
favourable conditions for CBM preservation and higher gas contents. However, the limits of the parameters
for determining viability vary between different areas and coal ranks. For example, in the southern part of the
Qinshui Basin, located in the southeastern part of Shanxi Province, China, a high-rank coal area with a
piezometric head elevation lower than 650 m, salinity lower than 1 000 mg/l, and methane carbon isotope
13 1
ratio (δ C ) smaller than -−38 ‰‰, is classified as a strong runoff zone with active hydrodynamics and low
gas content and is therefore considered unviable. Piezometric head elevation is calculated using
Formula (1)Formula (1)::
𝐻𝐻 =𝑧𝑧+ℎ (1)
where
H is the piezometric head elevation, expressed in metres (m);
z is the distance from the depth of the coal seam at the well point to the mean sea level, expressed in m;
h is the piezometric head, namely the elevation of the water level of the coal seam at the well point, expressed in m.
6.3 Fault development index
6.3.1 Detailed analysis of fault morphology is carried out by seismic interpretation using regional 2D and
3D seismic data, which reveals the striking and dipping directions and the dip angle of the fault plane, as well
as fault types (normal, reverse, and strike-slip faults), and fault displacement (vertical and horizontal
displacement). To further improve the accuracy of fault interpretation, the attribute prediction method
(preferably adopting the coherence and curvature attributes) may be applied across the 3D seismic survey
area. Data from core analysis and production testing are incorporated to define the different effects of faults
on gas content and, hence, demarcate fault-affected zones.
6.3.2 It is recommended that the fault development index be used for evaluation, using
Formula (2)Formula (2), to calculate the index. The fewer the faults, the smaller the fault-affected zone for
CBM, and the higher the CBM recoverability.
The influence of faults varies between areas, which therefore should be accurately determined from actual
drilling and production testing data. The southern part of the Qinshui Basin, located in the southeasternsouth-
eastern part of Shanxi Province, China can be taken as an example. The normal faults in this basin are divided
into three grades according to fault displacement. Grade:
— grade-I normal faults, which have displacements greater than 300 m and affect the gas contents of coal
seams within 500 m to 600 m on either side of the fault; Grade
— grade-II normal faults, which have displacements of 100m100 m to 300 m, with the fault-affected zone
extending 200m200 m to 500 m on either side of the fault; Grade
— grade-III normal faults, which have displacements less than 100 m and influence the gas contents of coal
seams within 150 m to 200 m on either side of the fault.
Reverse faults generally have displacements of less than 100 m and have better sealing than normal faults.
Their affected zones are 50 m to 100 m on either side of the fault. The fault development index is used to
compare the influence of faults. A fault development index of less than 0,3 indicates a highly favourable area,
and is relatively little affected by faults;, an index between 0,3 and 0,7 suggests comparatively well-developed
faults;, and an index greater than 0,7 means severe fault damage to the area, which thus being unviable.
Formula (2) is calculated as follows:
𝑆𝑆 𝑆𝑆
fm fm
𝐹𝐹 = (2)
𝑆𝑆 𝑆𝑆
m m
where
F is the fault development index, dimensionless;
S is the area affected by faults within the target block, expressed in square kilometres (km );
fm
S is the area of the target block, expressed in km .
m
7 Evaluation of high-yield units
7.1 Structural evaluation
7.1.1 Structural form
7.1.1.1 7.1.1.1 Structural forms are analysed in detail by seismic interpretation using regional
2D and 3D seismic data, which highlights the development status and types of folds and formation dip angles.
In order to further improve the accuracy of structural form characterization, methods such as the structural
amplitude difference attribute, curvature attribute, and 3D visualization may be applied to the 3D seismic
survey area.
7.1.1.2 7.1.1.2 It is recommended to broadly divide regional structures into two categories:
monoclines and folds. Monocline structures are further sub-divided according to amplitude undulation and
dip angles. Areas with fewer undulations and smaller formation dip angles are more favourable. Fold
structures are further sub-divided according to fold morphology and interlimb angles. There are two types of
fold morphology: antiforms and synforms. Folds are also divided into gentle folds (interlimb angles > > 120°),
open folds (interlimb angles of 70°–120°), closed folds (interlimb angles of 30°–70°), tight folds (interlimb
angles < < 30°),°) and isoclinal folds (interlimb angles of nearly 0°).
7.1.2 Fracture development
7.1.2.1 7.1.2.1 Natural fracture development is a key factor influencing the permeability of
medium- and high-rank coal reservoirs. Fracture development is assessed using direct and indirect methods.
The direct method uses macroscopic core description to determine relevant parameters. These include the
lengths, widths, heights and densities of major and minor fractures. The higher the number of core samples,
the greater the accuracy of the core descriptions. It is recommended that major fracture development density
be adopted as the primary classification index.
The indirect method uses 3D seismic attribute prediction. The early production system based on horizontal
drilling coherent attribute method and the pre-stack azimuthal anisotropy method are recommended, as they
are both able to qualitatively identify fracture development areas.
7.1.2.2 7.1.2.2 Degrees of fracture development are likely to vary between different areas and
coal ranks. For instance, for the high-rank coal of the Qinshui Basin, located in the southeasternsouth-eastern
part of Shanxi Province, China, the following applies:
— a coal reservoir with a major fracture density of less than 4 fractures/cm is considered non-fractured with
underdeveloped natural fractures; one with a major fracture density of 4 fractures/cm to 6 fractures/cm,
relatively fractured with relatively well-developed natural fractures; one with a major fracture density
exceeding 6 fractures/cm, fractured with well-developed natural fractures.
— a coal reservoir with a major fracture density of 4 fractures/cm to 6 fractures/cm, is considered relatively
fractured with relatively well-developed natural fractures;
— a coal reservoir with a major fracture density exceeding 6 fractures/cm, is considered fractured with well-
developed natural fractures.
7.2 Reservoir rock evaluation
7.2.1 Coal-body structure
7.2.1.1 7.2.1.1 Coal-body structures are classified into four types: the primary structure,
cataclastic structure, granulated structure, and mylonitic structure. The methods for distinguishing coal-body
structure types include coal core identification, well logging response prediction, and geophysical prediction.
Coal core identification is carried out by direct observation of coal cores collected by drilling. The advantage
of this method is that it entails direct observation. Nevertheless, most wells fail to deliver complete cores for
vertical identification due to limited numbers of cored intervals. Moreover, this method is only practicable if
the wells are cored, which is not the case in many regions. Well logging response prediction is based on well
logging parameters sensitive to tectonic coal, such as resistivity, well diameter, acoustic interval transit time
and density. With increased crushing of coal-body structures, resistivity and density decline, while acoustic
interval transit time and well diameters grow. Geophysical prediction integrates well logs with seismic data
and is used to predict the planar distributions of coal-body structures. Numerous techniques have been
developed for coal-body structure prediction, and, in practice, multiple methods are usually used in
combination to improve prediction accuracy. The amplitude attribute and waveform clustering attribute
techniques are recommended for this purpose.
7.2.1.2 7.2.1.2 With current engineering technology, the primary-structure and cataclastic-
structure coal reservoirs generally have good reservoir stimulation performance. The stimulation
performance of granulated-structure coal reservoirs is less satisfactory, and mylonitic coal reservoirs cannot
be effectively stimulated. The coal-body development index is recommended for quantitative description to
improve the vertical precision of coal-body structure identification. The coal-body development index is the
ratio of the vertical thickness of primary- and cataclastic-structure coal to the total thickness of the coal seam
and is calculated using Formula (3)Fo
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