Natural Gas — Wet gas flow measurement in natural gas operations

ISO/TR 12748:2015 describes production flow measurement of wet natural gas streams with WGFMs in surface and subsea facilities. Wet natural gas streams are gas-dominated flows with liquids like water and/or hydrocarbon liquids. ISO/TR 12748:2015 defines terms/symbols, explains the various concepts, and describes best practices of wet gas flow meter design and operation. It addresses metering techniques, testing, installation, commissioning, and operation practices such as maintenance, calibration, and verification. It also provides a theoretical background of this comprehensive, challenging and still evolving measurement technology.

Gaz naturel — Mesurage du débit de gaz humide dans les opérations de gaz naturel

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REPORT 12748
First edition
Natural Gas — Wet gas flow
measurement in natural gas operations
Gaz naturel — Mesurage du débit de gaz humide dans les opérations
de gaz naturel
Reference number
ISO/TR 12748:2015(E)
ISO 2015

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ISO/TR 12748:2015(E)

© ISO 2015, Published in Switzerland
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form
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ii © ISO 2015 – All rights reserved

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ISO/TR 12748:2015(E)

Contents Page
Foreword .vi
Introduction .vii
1 Scope . 1
2 Terms and Definitions . 1
3 Symbols .10
4 Objectives of wet gas flow measurement .13
4.1 Common production scenarios .14
4.2 Production allocation .15
4.3 Flow assurance aspects .16
4.4 WGFM considerations .16
4.5 Reliability in remote WGFM installations .16
5 Flow regimes .17
5.1 Horizontal wet gas flow regimes .17
5.1.1 Stratified flow .18
5.1.2 Slug flow .18
5.1.3 Annular mist flow . . .18
5.2 Vertical up wet gas flow regimes .18
5.2.1 Churn flow .18
5.2.2 Annular mist flow . . .19
5.3 Vertical down wet gas flow regimes .19
5.4 Inclined flow.19
5.5 Examples of wet gas flow regimes .19
5.6 Flow regime maps .20
5.7 Different wet gas flow parameters.21
5.8 Water in wet gas flow .21
6 Wet gas flow metering principles .22
6.1 General .22
6.2 In-Line wet gas flow meters .23
6.2.1 Single-phase gas flow meter with correction factor .23
6.2.2 Two-phase wet gas flow meter .24
6.2.3 Multiphase wet gas flow meter .24
6.3 Single-phase gas differential pressure meters with wet gas flow .24
6.3.1 DP Meter design influence on wet gas over-reading .25
6.3.2 Lockhart-Martinelli parameter influence on DP meter wet gas
flow over-reading .25
6.3.3 Gas to liquid density ratio influence on DP meter wet gas flow over-reading .25
6.3.4 Gas densiometric Froude number influence on DP meter wet gas
flow over-reading .26
6.3.5 DP meter orientation influence on DP meter wet gas flow over-reading .26
6.3.6 Influence of β on DP meter wet gas flow over-reading .28
6.3.7 Fluid property influence on DP meter wet gas flow over-reading.28
6.3.8 Meter size/diameter influence on DP meter wet gas flow over-reading .28
6.3.9 Applying DP meter wet gas flow correlations .28
6.4 General discussion on DP meter wet gas correlations .29
6.4.1 Wet gas flow performance characterization vs. published wet gas correlations .29
6.4.2 Horizontally-installed orifice plate meter .29
6.4.3 Horizontally-installed Venturi meter .31
6.4.4 Horizontally-installed cone meter .32
6.5 Generic two-phase wet gas meter designs .33
6.5.1 Multiple single-phase meters in series .33
6.5.2 Differential pressure meter classical DP/permanent pressure loss wet
gas meters .35
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ISO/TR 12748:2015(E)

6.5.3 Fast response sensor system .36
6.6 Multiphase wet gas flow meters .37
6.6.1 Trace water metering with multiphase wet gas flow meters .38
6.6.2 Multiphase wet gas flow meter subsystems .38
6.6.3 Phase fraction device choices .39
6.6.4 Gas volume fraction vs. gas void fraction measurement .41
6.6.5 Semi-empirical multiphase flow calculation — Slip model .41
6.6.6 PVT (pressure volume temperature) models .42
6.6.7 Multiphase wet gas flow meter required fluid property inputs .42
6.6.8 Multiphase wet gas flow meter phase fraction measurement .42
6.6.9 Measurement of water salinity .43
6.6.10 Multiphase wet gas flow meter redundant subsystems and diagnostics.43
6.6.11 Selection of multiphase wet gas flow meter technologies .44
6.7 Wet gas flow meter performance testing .44
6.8 Virtual metering system (VMS) .45
7 DP Meter Wet Gas Correlation Practical Issues .45
7.1 DP meter wet gas flow installation issues .46
7.1.1 Liquid flow rate estimation techniques .46
7.1.2 Monitoring wet gas liquid loading with a DP meter downstream port .47
8 Design and Installation Considerations .49
8.1 Design considerations.49
8.1.1 Meter orientation and fluid flow .49
8.1.2 Meter location relative to other piping components .50
8.1.3 Use of two-phase flow rate and composition maps .50
8.1.4 Fluid sampling.52
8.1.5 Redundancy and external environmental considerations .52
8.1.6 Security .53
8.1.7 Cost and project schedule implications .54
8.2 Performance specifications .54
8.3 Wet gas flow measurement uncertainty .55
8.3.1 Uncertainty evaluation methodologies .55
8.3.2 Additional factors affecting wet gas flow measurement uncertainty .55
8.3.3 Expressing uncertainty of wet gas flow rates .56
9 Testing, Verification and Calibration .56
9.1 Meter orientation .56
9.2 Comments on flow regimes and mixers .57
9.3 Installation requirements .57
9.4 Wet gas flow characterization tests — Single-phase DP meter baselines .57
9.5 Wet gas flow facility operational considerations .58
9.5.1 Test facility operational issues — Achieving thermodynamic equilibrium .58
9.5.2 Test facility operational issues — Phase flow rate stability .60
9.5.3 Test facility operational issues — Witnessing of tests .61
9.6 Meter testing in a wet gas flow facility .62
10 Operational and Field Verification Issues .65
10.1 Laboratory reference vs. field hydrocarbon flow composition estimates .66
10.2 Laboratory reference vs. field calibration of phase fractions .66
10.3 Comparisons of multiphase wet gas flow meter and single-phase meter requirements .66
10.3.1 The challenge of supplying multiphase wet gas flow fluid properties .66
10.3.2 Confidential slip models .67
10.4 The importance of correct fluid property predictions .67
10.4.1 The importance of gas properties when metering small liquid flow rates .70
10.4.2 Preparation for fluid property variations during meter service .71
10.4.3 Fluid property sensitivity investigation.71
10.5 The benefit of an initial wet gas flow facility test . .73
10.6 Line size limitations for some multiphase meters .73
10.7 In situ wet gas flow meter verification .73
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ISO/TR 12748:2015(E)

10.7.1 Reconciliation factors and meter output confidence .74
10.8 Operation and maintenance .74
10.8.1 System redundancy and diagnostics .74
10.8.2 Operating WGFM diagnostics .75
10.9 Miscellaneous operational issues .76
10.9.1 Wet gas flow and DP transmitters .76
10.9.2 Software and fluid property update procedures .77
10.9.3 Long term trending comparisons with test facility/factory characterization .77
11 Common Field Issues .77
11.1 Inefficient separator systems .77
11.2 Separator systems — An adverse environment for single-phase meters .78
11.2.1 Separator Outlet deployment .79
11.2.2 Gas Measurement at the separator outlet .79
11.2.3 Liquid Turbine Meter .80
11.2.4 Practical limitations of wet gas flow metering with separator technology .80
11.3 Wet gas flow meter practical problems .81
11.3.1 Considerations for wet gas flow metering .81
11.3.2 The adverse effects of contamination, hydrates, scale, and salts .82
11.3.3 Theoretical, laboratory and actual wet gas flow conditions .84
11.3.4 Undisclosed WGFM calculation procedures .84
11.3.5 Differential pressure measurement and wet gas flows .85
11.3.6 Problems due to lack of long time operating experience of WGFMs .86
Annex A (informative) WGFM design checklist.87
Annex B (informative) Wet gas parameters equations .89
Bibliography .90
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ISO/TR 12748:2015(E)

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
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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
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
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The committee responsible for this document is ISO/TC 193, Natural Gas, Subcommitte SC 3,
Upstream Area.
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ISO/TR 12748:2015(E)

Oil and gas companies started developing Wet Gas Flow Meters (WGFMs) and Multiphase Flow Meters
(MPFMs) through extensive R&D activities in the late 1980s. During this period, WGFMs and MPFMs
were typically perceived as two distinct technologies for different applications: MPFMs were designed
for liquid continuous flow conditions and WGFMs were designed for gas continuous flow conditions.
In recent years, however, the operating range of these two technologies has increasingly overlapped,
blurring the distinction between a WGFM and MPFM. As wet gas flow is presently considered a subset
of multiphase flow, a WGFM is an MPFM that specializes in gas-dominant multiphase flow conditions. In
this Technical Report, such technologies will be referred to as WGFMs.
There are many factors that contributed in the decision to replace a separator with a WGFM, with each
application warranting careful consideration. A well-designed and maintained separator working
within an appropriate flow condition range should produce accurate flow measurements. A primary
concern for oil and gas companies was to reduce costs by replacing complex and bulky test separators,
as well as to further simplify the upstream infrastructure, in particular for offshore and subsea
1) 2)
projects. WGFMs typically require lower capital and operational expenditures than fully equipped
test separators. More savings in CapEx may be achieved by omitting dedicated test lines in satellite
developments. In addition, there is a significant benefit for offshore developments, in terms of weight
and space conservation, by using the much smaller footprint of WGFMs.
Due to various operational problems, a conventional test separator does not continuously provide
accurate and reliable well test data, giving only relevant information when the well is switched to the
test separator. With the use of WGFMs testing well production more frequently or even continuously
becomes possible. WGFM developments and extensive testing over the last two decades have resulted in
WGFM technology that is a viable alternative to a test separator. Modern WGFMs now offer continuous
well monitoring (per installation on individual wells).
WGFM technology is an attractive option for multiphase wet gas flow measurement. Over the last two
decades, some WGFMs have been developed from prototypes into very mature, robust, advanced, and
field-proven measurement devices, increasing their application scope. Although originally intended for
use mainly in reservoir and well production allocations, WGFMs have evolved into a technology that
spans even fiscal product allocation. In the latter case, the output of a WGFM is used to determine money
transactions between operating companies or between an operating company and a host government.
This Technical Report focuses on the measurement of wet gas flow, i.e. terminology, models, and
principles, and the design, implementation, testing, and operation of WGFMs.
1)  Capital expenditure (CapEx) or costs for purchasing and installing a WGFM/MPFM includes all hardware to
operate the WGFM (data transmission, verification facilities, sampling arrangements, etc.)
2)  Operating expenditure (OpEx) or costs to operate a WGFM/MPFM (maintenance, verification processes,
sampling for fluid properties, etc.)
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Natural Gas — Wet gas flow measurement in natural gas
1 Scope
This Technical Report describes production flow measurement of wet natural gas streams with WGFMs
in surface and subsea facilities. Wet natural gas streams are gas-dominated flows with liquids like
water and/or hydrocarbon liquids (see 2.67 for a detailed definition). This Technical Report defines
terms/symbols, explains the various concepts, and describes best practices of wet gas flow meter design
and operation. It addresses metering techniques, testing, installation, commissioning, and operation
practices such as maintenance, calibration, and verification. It also provides a theoretical background
of this comprehensive, challenging and still evolving measurement technology.
There are four general methods in measuring wet natural gas flow. Each approach is detailed below.
— Single-phase gas flow meter with correction factor: Uses a single-phase gas flow meter (often
a conventional gas flow metering device) with a correction factor for the effect of liquid on the
metering system. In these cases, the liquid flow rate required to determine the correction factor,
should be estimated from an external source.
— Two-phase WGFM: The gas and liquid (both water and hydrocarbon liquid combined) flow rates
are predicted with no additional external information regarding the liquid flow rate required.
This is generally known as a two-phase WGFM and will be referred to in this Technical Report
simply as WGFM.
— WGFM: A flow meter that measures the gas and liquid flow rates, and also the gas, water and
hydrocarbon liquid ratios (or “phase fractions”) with no external information required regarding
the liquid flow rate.
— Phase separation/Measurement after phase separation: This traditional and conventional method
of wet gas flow metering uses a two- or three-phase separator with single-phase flow meters
measuring the outgoing single-phase flows.
The first three of these methods, which emerged in the last two decades, may be described as in-line
wet gas flow metering, i.e. wet gas flow measurement is executed with WGFMs without separating
the gas and liquid phases. This Technical Report discusses in detail these first three methods. Several
best practice documents have already been issued to describe, among other topics, wet gas flow
measurement .
The last method is more conventional and describes wet gas flow measurement after the gas and liquid
phases have been separated. Wet gas meters

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