oSIST prEN 1918-3:2025
(Main)Gas infrastructure - Underground gas storage - Part 3: Functional recommendations for storage in solution-mined salt caverns
Gas infrastructure - Underground gas storage - Part 3: Functional recommendations for storage in solution-mined salt caverns
This document covers the functional recommendations for design, construction, testing, commissioning, operation, maintenance and abandonment of underground gas storage (UGS) facilities in solution-mined salt caverns up to and including the wellhead.
It specifies practices which are safe and environmentally acceptable.
For necessary surface facilities for underground gas storage, EN 1918 5 applies.
In this context “gas” refers to flammable gas:
— which is in a gaseous state at a temperature of 15 °C and under a pressure of 0,1 MPa (the stored product is also named fluid);
— which meets specific quality requirements in order to maintain underground storage integrity, performance, environmental compatibility and fulfils contractual requirements.
This comprises:
— gas not in liquid phase under subsurface conditions;
— methane-rich gases;
— natural gas;
— biomethane;
— synthetic methane;
— hydrogen of various purities;
— any mixtures of the gases above;
— hydrocarbon gas in liquid phase under subsurface conditions such as;
— ethylene;
— liquified petroleum gas (LPG).
NOTE 1 Correspondingly the EN 1918 series can be considered where applicable for underground storage of any other fluid e.g. helium, carbon dioxide, compressed air, rDME (renewable dimethyl ether) and hydrogen transport fluids (such as ammonia and LOHC).
This document is not intended to be applied retrospectively to existing facilities.
NOTE 2 Correspondingly this document can be considered for major conversions in case of significant change of gas composition.
Gasinfrastruktur - Untertagespeicherung von Gas - Teil 3: Funktionale Empfehlungen für die Speicherung in gesolten Salzkavernen
Dieses Dokument legt die funktionalen Empfehlungen für Planung, Errichtung, Prüfung, Inbetriebnahme, Betrieb, Instandhaltung und Stilllegung von Untertagegasspeicheranlagen in gesolten Salzkavernen bis einschließlich des Bohrlochkopfes fest.
Es legt sichere und umweltverträgliche Verfahrensweisen fest.
Für erforderliche Übertageanlagen für einen Untertagegasspeicher gilt EN 1918 5.
In diesem Zusammenhang bezieht sich „Gas“ auf brennbares Gas, das
sich bei einer Temperatur von 15 °C und unter einem Druck von 0,1 MPa in gasförmigem Zustand befindet (das gespeicherte Produkt wird auch als Medium bezeichnet);
bestimmte Qualitätsanforderungen erfüllt, um die Integrität des Untertagespeichers, die Leistung und die Umweltverträglichkeit aufrechtzuerhalten, und vertragliche Anforderungen erfüllt.
Dies umfasst:
Gas, das sich bei Untertagebedingungen nicht in flüssiger Phase befindet;
methanreiche Gase;
Erdgas;
Biomethan;
synthetisches Methan;
Wasserstoff in verschiedenen Reinheitsgraden;
jegliche Gemische der oben genannten Gase;
Kohlenwasserstoffgas, das sich bei Untertagebedingungen in flüssiger Phase befindet, z. B.:
Ethylen;
Flüssiggas (LPG, liquefied petroleum gas).
ANMERKUNG 1 Entsprechend kann die Normenreihe EN 1918, soweit anwendbar, für die Untertagespeicherung von jedem anderen flüssigen oder gasförmigen Medium berücksichtigt werden, z. B. Helium, Kohlenstoffdioxid, Druckluft, erneuerbarer Dimethylether (en: renewable dimethyl ether, rDME) und Wasserstoff-Trägerfluide (wie Ammoniak und LOHC).
Es ist nicht vorgesehen, dieses Dokument rückwirkend auf bestehende Anlagen anzuwenden.
ANMERKUNG 2 Entsprechend kann dieses Dokument für umfassende Umstellungen bei wesentlichen Änderungen der Gaszusammensetzung berücksichtigt werden.
Infrastructure gazières - Stockage souterrain de gaz - Partie 3 : Recommandations fonctionnelles pour le stockage en cavités salines creusées par dissolution
Le présent document décrit les recommandations fonctionnelles pour la conception, la construction, les essais, la mise en service, l’exploitation, la maintenance et l’abandon des stockages souterrains de gaz en cavités salines creusées par dissolution jusqu’à la tête de puits incluse.
Il spécifie les pratiques acceptables pour la sécurité et l’environnement.
Pour les installations de surface nécessaires pour le stockage souterrain de gaz, l’EN 1918 5 s’applique.
Dans ce contexte, le terme « gaz » désigne un gaz inflammable :
— qui est à l’état gazeux à une température de 15 °C et sous une pression de 0,1 MPa (le produit stocké est également appelé « fluide ») ;
— qui satisfait à des prescriptions spécifiques en matière de qualité afin de maintenir l’intégrité, les performances, la compatibilité environnementale du stockage souterrain et qui respecte les prescriptions contractuelles.
Sont inclus :
— les gaz qui ne sont pas en phase liquide dans des conditions de subsurface :
— les gaz riches en méthane :
— le gaz naturel ;
— le biométhane ;
— le méthane de synthèse ;
— l’hydrogène de différentes puretés ;
— tout mélange des gaz précités ;
— les gaz d’hydrocarbure en phase liquide dans des conditions de subsurface, tels que :
— l’éthylène ;
— les gaz de pétrole liquéfiés (GPL).
NOTE 1 Par conséquent, une utilisation de la série EN 1918 peut être envisagée, lorsque cela est applicable, pour le stockage souterrain de tout autre fluide tel que l’hélium, le dioxyde de carbone, l’air comprimé, le diméthyl éther renouvelable (rDME) et les fluides de transport de l’hydrogène (ammoniac et LOHC, par exemple).
Le présent document n’est pas destiné à être appliqué de façon rétroactive aux installations existantes.
NOTE 2 Par conséquent, le présent document peut être utilisé pour des conversions majeures en cas de changement significatif de la composition du gaz.
Infrastruktura za plin - Podzemna plinska skladišča - 3. del: Funkcionalna priporočila za skladiščenje v solnih kavernah
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-junij-2025
Infrastruktura za plin - Podzemna plinska skladišča - 3. del: Funkcionalna
priporočila za skladiščenje v solnih kavernah
Gas infrastructure - Underground gas storage - Part 3: Functional recommendations for
storage in solution-mined salt caverns
Gasinfrastruktur - Untertagespeicherung von Gas - Teil 3: Funktionale Empfehlungen für
die Speicherung in gesolten Salzkavernen
Infrastructure gazières - Stockage souterrain de gaz - Partie 3 : Recommandations
fonctionnelles pour le stockage en cavités salines creusées par dissolution
Ta slovenski standard je istoveten z: prEN 1918-3
ICS:
75.200 Oprema za skladiščenje Petroleum products and
nafte, naftnih proizvodov in natural gas handling
zemeljskega plina equipment
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
DRAFT
EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM
May 2025
ICS 75.200 Will supersede EN 1918-3:2016
English Version
Gas infrastructure - Underground gas storage - Part 3:
Functional recommendations for storage in solution-
mined salt caverns
Infrastructure gazières - Stockage souterrain de gaz - Gasinfrastruktur - Untertagespeicherung von Gas - Teil
Partie 3 : Recommandations fonctionnelles pour le 3: Funktionale Empfehlungen für die Speicherung in
stockage en cavités salines creusées par dissolution gesolten Salzkavernen
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 234.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 1918-3:2025 E
worldwide for CEN national Members.
Contents Page
European foreword . 4
Introduction . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 7
3.1 Terms and definitions common to EN 1918 (all parts) . 7
3.2 Terms and definitions specific to this document . 11
4 Requirements for underground gas storage . 13
4.1 General . 13
4.2 Underground gas storage . 13
4.2.1 Overview and functionality . 13
4.2.2 Types . 14
4.2.3 General characterization . 15
4.2.4 Storage in salt caverns. 15
4.3 Long-term containment of stored fluids . 17
4.4 Environmental conservation . 17
4.4.1 General . 17
4.4.2 Methane emissions . 18
4.4.3 Other gas emissions . 19
4.5 Safety . 19
4.6 Monitoring . 19
5 Design . 20
5.1 Design principles . 20
5.2 Geological exploration . 20
5.3 Caverns . 21
5.4 Wells . 23
5.4.1 General . 23
5.4.2 Location . 23
5.4.3 Casings . 24
5.4.4 Completions . 24
5.5 Monitoring systems . 28
5.6 Neighbouring subsurface activities . 28
5.7 Solution mining . 28
5.7.1 General . 28
5.7.2 Brine discharge . 30
6 Construction . 30
6.1 General . 30
6.2 Wells . 30
6.3 Completions . 31
6.4 Solution mining . 31
6.4.1 General . 31
6.4.2 Leaching wellheads . 32
6.4.3 Cavern volume calculation . 32
6.5 Wellheads . 33
6.6 First gas fill . 33
6.6.1 General . 33
6.6.2 Monitoring the first gas fill . 34
6.7 Recompletion after the first gas fill . 34
6.8 First gas filling (gas in liquid phase) . 34
7 Testing and commissioning . 34
8 Operation, monitoring and maintenance . 35
8.1 Operating principles . 35
8.2 Cavern monitoring and maintenance . 35
8.3 Injection and withdrawal operations . 36
8.4 Maintenance of wells . 36
8.5 Health, safety and environment . 36
8.5.1 Safety management and health and environment . 36
8.5.2 Emergency procedures . 37
8.6 Adaptation to climate change . 37
8.6.1 Climate change effects . 37
8.6.2 Climate change impacts . 38
9 Abandonment. 39
9.1 General . 39
9.2 Withdrawal of the gas and pressure monitoring . 39
9.3 Plugging and abandonment of wells . 40
9.4 Surface facilities . 40
Annex A (informative) Significant technical changes between this document and
EN 1918-3:2016 . 41
Bibliography . 42
European foreword
This document (prEN 1918-3:2025) has been prepared by Technical Committee CEN/TC 234 “Gas
infrastructure”, the secretariat of which is held by DIN.
This document will supersede EN 1918-3:2016.
This document has been prepared under a standardization request addressed to CEN by the European
Commission. The Standing Committee of the EFTA States subsequently approves these requests for its
Member States.
For a list of significant technical changes between this document and EN 1918-3:2016, see Annex A.
This document is Part 3 of a European Standard on “Gas infrastructure - Underground gas storage”,
which includes the following five parts:
— Part 1: Functional recommendations for storage in aquifers;
— Part 2: Functional recommendations for storage in oil and gas fields;
— Part 3: Functional recommendations for storage in solution-mined salt caverns;
— Part 4: Functional recommendations for storage in rock caverns;
— Part 5: Functional recommendations for surface facilities.
Introduction
This document specifies common basic principles for underground gas storage facilities. Users of this
document are expected be to be aware that more detailed standards and/or codes of practice exist. This
document is intended to be applied in association with these national standards and/or codes of
practice and does not replace them. Furthermore, a non-exhaustive list of related European and
international standards can be found in the Bibliography.
NOTE Directive 2009/73/EC concerning common rules for the internal market in natural gas and the related
Regulation (EC) No 715/2009 on conditions for access to the natural gas transmission networks also aim at
technical safety including technical reliability of the European gas system. These aspects are also in the scope of
CEN/TC 234 standardization. In this respect, CEN/TC 234 evaluated the indicated EU legislation and amended this
technical standard accordingly, where required and appropriate.
1 Scope
This document covers the functional recommendations for design, construction, testing, commissioning,
operation, maintenance and abandonment of underground gas storage (UGS) facilities in solution-
mined salt caverns up to and including the wellhead.
It specifies practices which are safe and environmentally acceptable.
For necessary surface facilities for underground gas storage, EN 1918-5 applies.
In this context “gas” refers to flammable gas:
— which is in a gaseous state at a temperature of 15 °C and under a pressure of 0,1 MPa (the stored
product is also named fluid);
— which meets specific quality requirements in order to maintain underground storage integrity,
performance, environmental compatibility and fulfils contractual requirements.
This comprises:
— gas not in liquid phase under subsurface conditions;
— methane-rich gases;
— natural gas;
— biomethane;
— synthetic methane;
— hydrogen of various purities;
— any mixtures of the gases above;
— hydrocarbon gas in liquid phase under subsurface conditions such as;
— ethylene;
— liquified petroleum gas (LPG).
NOTE 1 Correspondingly the EN 1918 series can be considered where applicable for underground storage of
any other fluid e.g. helium, carbon dioxide, compressed air, rDME (renewable dimethyl ether) and hydrogen
transport fluids (such as ammonia and LOHC).
This document is not intended to be applied retrospectively to existing facilities.
NOTE 2 Correspondingly this document can be considered for major conversions in case of significant change
of gas composition.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
EN 1918-5, Gas infrastructure - Underground gas storage - Part 5: Functional recommendations for
surface facilities
ISO 1663, Rigid cellular plastics - Determination of water vapour transmission properties
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply. The definitions common
to EN 1918 (all parts) are listed in 3.1; those specific for this document are listed in 3.2.
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 Terms and definitions common to EN 1918 (all parts)
3.1.1
abandoned well
well permanently out of operation and permanently plugged including removed surface facilities
3.1.2
annulus
space between two strings of pipes or between the casing and the borehole
3.1.3
aquifer
reservoir, group of reservoirs, or a part thereof that is fully water-bearing and displaying differing
permeability/porosity
3.1.4
auxiliary well
well completed for other purposes than gas injection/withdrawal, e.g. water disposal
3.1.5
casing
pipe or set of pipes that are screwed or welded together to form a string which is placed in the borehole
for the purpose of supporting the borehole and to act as a barrier preventing subsurface migration of
fluids when the annulus between it and the borehole has been cemented and to connect the storage
reservoir respectively cavern to surface
3.1.6
casing shoe
bottom end of a casing
3.1.7
cementing
operation whereby usually a cement slurry is pumped and circulated down a cementation string within
the casing and then upwards into the annulus between the casing and the open or cased hole
3.1.8
completion
technical equipment inside the last cemented casing of a well
3.1.9
containment
capability of the storage reservoir or cavern and the storage wells to resist leakage or migration of the
fluids contained therein
Note 1 to entry: This is also known as the integrity of a storage facility.
3.1.10
core sample
sample of rock taken during coring operation in order, e.g. to determine various parameters by
laboratory testing and/or for a geological description
3.1.11
cushion gas volume
gas volume required in a storage for reservoir management purpose and to maintain an adequate
minimum storage pressure for meeting working gas volume delivery with a required withdrawal profile
and in addition in caverns also for stability reasons
Note 1 to entry: The cushion gas volume of storages in oil and gas fields can consist of recoverable and non-
recoverable in situ gas volumes and/or injected gas volumes.
3.1.12
drilling
all technical activities connected with the construction of a well
3.1.13
exploration
all technical activities connected with the investigation of potential storage locations for the assessment
of storage feasibility and derivation of design parameters
3.1.14
formation (horizon)
body of rock mass characterized by a degree of homogeneous lithology which forms an identifiable
geologic unit
3.1.15
gas injection
gas delivery from gas transport system into the reservoir/cavern through surface facilities and wells
3.1.16
gas inventory
total of working and cushion gas volumes contained in UGS
3.1.17
gas withdrawal
gas delivery from the reservoir or cavern through wells and surface facilities to a gas transport system
3.1.18
geological modelling
generating the image of a structure from the information gathered
3.1.19
hydrogen
H
lightest and most represented chemical element, found in nature as a diatomic molecule (H , or
dihydrogen), gaseous under standard conditions, colourless, odourless and highly flammable
3.1.20
indicator horizon
horizon overlying the caprock in the storage area and used for monitoring
3.1.21
landing nipple
device in a tubing string with an internal profile to provide for latching and sealing various types of
plugs or valves
3.1.22
liner
casing installed within last cemented casing in the lowermost section of the well without extension to
surface
3.1.23
lithology
characteristics of rocks based on description of colour, rock fabrics, mineral composition, grain
characteristics, and crystallization
3.1.24
logging
measurement of physical parameters versus depth in a well
3.1.25
master valve
valve at the wellhead designed to close off the well for operational reasons and in case of emergency or
maintenance
3.1.26
maximum operating pressure
MOP
maximum pressure of the storage reservoir or cavern, normally at maximum inventory of gas in
storage, which has not to be exceeded in order to ensure the integrity of the UGS and is based on the
outcome of geological/technical engineering and is approved by authorities
Note 1 to entry: The maximum operating pressure is related to a datum depth and in caverns usually to the
casing shoe of the last cemented casing.
3.1.27
minimum operating pressure
minimum pressure of the storage reservoir or cavern, normally reached at the end of the decline phase
of the withdrawal profile and is for caverns based on geomechanical investigations to ensure stability
and to limit the effect of subsidence
Note 1 to entry: The minimum pressure is related to a datum depth and in caverns usually to the casing shoe of
the last cemented casing.
3.1.28
monitoring well
observation well
well for purposes of monitoring the storage horizon and/or overlying or underlying horizons for
subsurface phenomena such as pressure fluctuation, fluid flow and qualities, temperature, etc
3.1.29
operating well
well used for gas withdrawal and/or injection
3.1.30
overburden
all sediments or rock that overlie a geological formation
3.1.31
permeability
capacity of a rock to allow fluids to flow through its pores
Note 1 to entry: Permeability is usually expressed in Darcy. In the SI Unit system permeability is measured in
m .
3.1.32
plug
tool designed to isolate different zones within a well, usually set to prevent fluid migration, especially
for well abandonment operations
3.1.33
porosity
volume of the pore space (voids) within a rock formation expressed as a percentage of its total volume
3.1.34
reservoir
porous and permeable (in some cases naturally fractured) formation having area- and depth-related
boundaries based on physical and geological factors
Note 1 to entry: It contains fluids which are internally in pressure communication.
3.1.35
saturation
percentages of pore space occupied by fluids
3.1.36
seismic technology
technology to characterize the subsurface image with respect to extent, geometry, fault pattern and
fluid content applying acoustic waves, impressed by sources near to surface in the subsurface strata,
which pass through strata with different seismic responses and filtering effects back to surface, where
they are recorded and analysed
3.1.37
string
entity of casing or tubing plus additional equipment, screwed or welded together as parts of a well
respectively completion
3.1.38
subsurface safety valve
valve installed in casing and/or tubing beneath the wellhead or the lower end of the tubing for the
purpose of stopping the flow of gas in case of emergency
3.1.39
tubing
pipe or set of pipes that are screwed or welded together to form a string, through which fluids are
injected or withdrawn or which can be used for monitoring
3.1.40
well
borehole and its technical equipment including the wellhead
3.1.41
well integrity
well condition without uncontrolled release of fluids throughout the life cycle
3.1.42
well integrity management
complete system necessary to ensure well integrity at all times throughout the life cycle of the well,
which comprises dedicated personnel, assets, including subsurface and surface installations, and
processes provided by the operator to monitor and assess well integrity
3.1.43
wellhead
equipment supported by the top of the casing including tubing hanger, shut off and flow valves, flanges
and auxiliary equipment which provides the control and closing-off of the well at the upper end of the
well at the surface
3.1.44
working gas volume
volume of gas in the storage above the designed level of cushion gas volume, which can be
withdrawn/injected with installed subsurface and surface facilities (wells, flow lines, etc.) subject to
legal and technical limitations (pressures, gas velocities, flowrates, etc.)
Note 1 to entry: Depending on local site conditions (injection/withdrawal rates, utilization hours, etc.) the
working gas volume can be cycled more than once a year.
3.1.45
workover
well intervention to restore or increase production, repair or change the completion of a well or the
leaching equipment of a cavern
3.2 Terms and definitions specific to this document
3.2.1
blanket
liquid or gaseous medium in the annulus between the last cemented casing string and the outer
leaching string used during the whole leaching period in order to ensure that the planned cavern shape
and the protection of cavern roof and casing shoe is achieved
3.2.2
cavern
developed volume in a salt formation by drilling and leaching, including the cavern sump
3.2.3
convergence
reduction in the cavern volume by salt creeping
3.2.4
cavern free volume
volume of the cavern that is available for the storage of gas
3.2.5
cavern height
distance between the bottom of the neck and the lowest point of the cavern, including the cavern sump
3.2.6
pillar
salt body surrounding the cavern required for stability reason and gas tightness
3.2.7
cavern roof
upper part of the cavern located between the bottom of the neck and the vertical wall of the cavern
3.2.8
cavern neck
well segment below the shoe of the last cemented casing string and above the cavern roof
3.2.9
cavern sump
bottom part of the cavern filled with sedimented, mostly insoluble materials and residual brine
3.2.10
hanger
device for supporting the weight of pipes and to assure the pressure tightness of the annulus
3.2.11
leaching step
period between two rearrangements of the leaching completion
3.2.12
solution mining
controlled leaching of the cavern to its desired shape and size
3.2.13
sonar survey
logging method to determine shape and volume of a cavern
4 Requirements for underground gas storage
4.1 General
This clause gives general requirements for underground gas storage. More specific requirements for
underground gas storage in solution-mined salt caverns are given in Clauses 5, 6, 7, 8 and 9.
4.2 Underground gas storage
4.2.1 Overview and functionality
EN 1918 series covers storage of gas as defined in scope. Because of the relevance of underground gas
storage of methane rich gases and hydrogen, the major part of this introduction is related to the storage
of these.
The underground gas storage (UGS) is an efficient proven common technology and is in use since 1915.
UGS became an essential indispensable link in the gas supply chain for adjusting supply to meet short-
term and seasonal changes in demand.
Gas produced from natural resources or synthesized by various production methods is increasingly
being used to supply energy requirements. As the gas supply from various sources does not match with
the variable market demand, gas is injected into underground gas storage reservoirs when market
demand falls below the level of gas delivery or if there is an economic incentive. Gas is withdrawn from
storage facilities to supplement the supply if demand exceeds that supply or withdrawal is economically
attractive.
The primary function of UGS is to ensure that supply is adjusted for peak and seasonal demand. Apart
from this, the storage facilities can provide stand-by reserves in case of interruption of the planned
supply or for regulatory and/or technical requirements. Typically, UGS is applied for commercial
storage services.
Thus, in summary underground gas storage facilities can be used for:
— security of supply;
— providing flexibilities;
— balancing of seasonal demand variabilities;
— structuring of gas supply;
— provision of balancing energy for the optimization of transport grids;
— trading and arbitrage purpose;
— stand-by provisions and strategic reserves;
— structuring renewable energy sources – power to gas;
— storage of associated gas as service for production optimization and resultant environmental
conservation;
— blending different types of gas quality.
4.2.2 Types
For storage of gas several types of underground gas storage facilities can be used, which differ by
storage formation and storage mechanism (see the following and Figure 1):
— pore storage;
— storage in aquifers;
— storage in former gas fields;
— storage in former oil fields;
— caverns;
— storage in salt caverns;
— storage in rock caverns (including lined rock caverns);
— storage in abandoned mines.
Key
1 operating wells
2 monitoring wells
3 indicator horizon
4 caprock
5 storage reservoir and stored gas
6 salt dome (valid also for salt layer)
7 cavern
Figure 1 — Storage in aquifers, oil and gas fields and solution mined salt caverns
For LPG storage only salt or rock caverns can be used.
The UGS type applied is dependent on the geological conditions and prerequisites as well as on the
designed capacity layout.
4.2.3 General characterization
UGS are naturally or artificially developed reservoirs respectively artificially developed caverns in
subsurface geological formations used for the storage of gas (or in case of LPG in a liquid phase under
subsurface conditions). An UGS consists of all subsurface and surface facilities required for the storage
and for the withdrawal and injection of gas. Several subsurface storage reservoirs or caverns may be
connected to one or several common surface facilities.
The suitability of subsurface geological formations shall be investigated individually for each location,
in order to operate the storage facilities in an efficient, safe and environmentally compatible manner.
In order to construct a storage facility, wells are used to establish a controlled connection between the
reservoir or cavern and the surface facilities at the wellhead. The wells used for cycling the storage gas
are called operating wells. In addition to the operating wells, specially assigned observation wells may
be used to monitor the storage performance with respect to pressures and saturations and the quality
of reservoir water as well as to monitor any interference in adjacent formations.
For the handling of gas withdrawal and gas injection the surface facilities are the link between the
subsurface facilities and the transport system, typically comprising facilities for gas
dehydration/treatment, hydrogen/natural gas separation, blending, compression, heating and cooling,
and measurement, e.g. for process control and safety systems and inhibition.
Gas is injected via the operating wells into the pores of a reservoir or into a cavern, thus building up a
reservoir of compressed gas.
Gas is withdrawn using the operating wells. With progressing gas withdrawal the reservoir or cavern
pressure declines according to the storage characteristic. For withdrawal re-compression may be
needed.
The working gas volume can be withdrawn and injected within the pressure range between the
maximum and minimum operating pressure. In order to maintain the minimum operating pressure it is
inevitable that a significant quantity of gas, known as cushion gas volume, remains in the reservoir or
cavern.
The storage facility comprises the following storage capacities:
— working gas volume;
— withdrawal rates;
— injection rates.
The technical storage performance is given by withdrawal and injection rate profiles versus working
gas volume.
Recommendations for the design, construction, testing and commissioning, operation and
abandonment of underground storage facilities are described in Clauses 5, 6, 7, 8 and 9.
Construction of a storage facility begins after the design and exploration phase and should be carried
out in accordance with the storage design, which applies proven experience from the oil and gas
industry.
For specific elements of an underground gas storage facility, e.g. wells and surface installations, existing
standards should be applied.
4.2.4 Storage in salt caverns
Underground storage of compressed gases in solution-mined salt caverns is a proven technology for
providing storage capacities on a short-term and seasonal basis.
Storages of gas in salt caverns are artificially developed containments in salt rock usually to provide
high withdrawal capacities but may as well be used for the storage of large gas volumes in case
numerous caverns are tied into one storage facility.
Salt caverns (see Figure 2) are constructed in suitable salt layers or salt domes by drilling a well into a
salt deposit with adequate protection for the underlaying, overlaying and lateral surrounding strata, i.e.
mainly by thickness of the salt and completion of the well.
NOTE Some salt caverns can have more than one well, so in this document the term “well” can also mean
“wells”.
It is known that suitable salt layers and salt domes provide gas tightness up to certain pressures. In
addition, cracks and faults in the salt are healed by the viscoplastic behaviour of the salt under the
geostatic pressure.
After drilling, salt caverns are leached by the controlled circulation of water or not saturated brine
down the wellbore into the salt zone and back as brine to the surface (see Figure 5). Once the
geometrical design volume is reached, the brine is displaced from the cavern by the controlled injection
of gas.
The pressure in a cavern can be cycled between the minimum and the maximum operating pressure of
the cavern while considering approved pressure change rates and maximum permissible time periods
for certain operating pressures.
Concerning caverns for liquid petroleum gas (LPG), the displaced brine is normally collected in a pond,
which has the geometrical volume of the cavern as minimum volume. When it is necessary to withdraw
the LPG from the cavern, the brine stored in the pond will be injected into the cavern. An LPG cavern, in
this case, does not require any downhole pumping equipment.
This is the most common method for constructing and operating an LPG cavern in salt. With shallow
salt caverns, however, the operation may be similar to the operation of a rock cavern for LPG (see
EN 1918-4).
There are more than 50 years of experience of storage of natural gas, hydrogen and LPG in solution-
mined salt caverns in Europe and the technique is well known and highly developed for natural gas and
LPG.
To guarantee a high level of safety, sophisticated techniques are available for:
— evaluation of the suitability of the geological salt formation for storage;
— testing and simulation of the salt behaviour under in situ stress conditions;
— simulation of the local stresses around the salt caverns and the demonstration of its mechanical
stability;
— drilling, cementing and completion of wells to prevent external gas migration from the cavern
towards the surface or upper geological formations;
— controlled leaching of the cavern to its designed shape and size;
— first gas filling under controlled conditions;
— monitoring relevant parameters of the caverns in the operation phase;
— abandonment procedures.
Key
1 operating wells
2 salt dome
3 cavern
Figure 2 — Cavern storage
4.3 Long-term containment of stored fluids
The storage facility shall be designed, constructed and operated to ensure the continuing long-term
containment of the stored or after abandonment remaining fluids.
This presupposes:
— adequate prior knowledge of the geological formation in which the storage is to be developed and
of its geological environment;
— acquisition of all relevant information needed for specifying parameter limits for construction and
operation;
— demonstration that the storage is capable of ensuring long-term containment of the stored or after
abandonment remaining gas through its hydraulic and mechanical integrity.
The facility shall be constructed and operated so as to maintain the integrity of the containment.
All operations adjacent to a storage facility shall be compatible with the storage activity and shall not
endanger its integrity.
All new storage projects shall take into account existing adjacent activities and potential impact of
climate change.
4.4 Environmental conservation
4.4.1 General
The storage facility shall be designed, constructed, operated and abandoned in order to have the lowest
reasonably practicable impact on ground movement at the surface and on the environment.
Emissions of stored gas or other fluids used or created in operation shall be minimized according to
applicable emission targets. Appropriate measures to minimize leakages shall be implemented.
NOTE Regarding emissions of stored gas or other fluids, local regulations or requirements can apply.
This presupposes for the subsurface that the surrounding formations have been identified and their
relevant characteristics determined and that they are adequately protected.
4.4.2 Methane emissions
4.4.2.1 General
As methane is a powerful greenhouse gas, methane emission management should be integrated in the
design, construction, commissioning, operation, maintenance and abandonment phase to minimize
emissions.
The methane emissions can be divided into 3 main categories:
— fugitives;
— vented;
— incomplete combustion.
As part of the methane emission management operators shall have procedures for monitoring and
reporting of methane emissions on a regular basis. Active measures shall be taken to prevent leakage by
implementing leak detection and repair programs.
For quantification of methane emissions, CEN/TS 17874 can be used.
4.4.2.2 Design phase
During the technical design phase, the following technical examples to reduce emissions should be
taken into account:
— use electric, air pneumatic or mechanical (e.g. valve actuators, valve controllers) rather than gas
operated (gas pneumatic) components or equipment;
— consider how fugitive emissions can be quantified to assist in maintenance scheduling.
4.4.2.3 Construction and abandonment/decommissioning
For all phases including the conversion to another gas, means shall be applied to minimize emissions.
For example, the recompression or the flaring of that gas or other uses should be considered as an
alternative to venting. See Clause 6 for more information on construction.
During abandonment, means shall be applied to minimize emissions. For example, the recompression
or the flaring of that gas or other uses should be considered as an alternative to venting. See Clause 9 for
more information on abandonment.
4.4.2.4 Operation and maintenance
Leak detection and repair (LDAR) shall be applied regularly to the well heads.
During maintenance or well interventions, emissions shall be minimized.
Ideally, monitoring devices are installed at crucial modules within the facility, that constantly monitor
possible emissions.
If leak detection and repair campaigns should be carried out on regular basis; this campaign should
consist of 4 phases:
— potential leak identification, based on site characteristics and/or previous experiences;
— detection and potential quantification of the leakages;
— repair or setting of a repair program and the associated reporting / tracing of the leakages and the
repairs;
— leakage and repair validation.
NOTE This can be applied to existing storages too.
Diverse emission quantification strategies can be applied depending on the type of asset (see CEN/TS
17874). Concerning fugitive emissions, the requirements of EN 15446 should be applied or any other
reference methodology, if available.
Adaption of common procedures should be considered to minimize methane emissions (e.g.
maintenance related vent reinjection).
Wells should be built with remote controlled pressure monitoring devices. Monitoring of wells shall be
done according to well integrity management systems in accordance with ISO 1663.
See Clause 8 for more information on operation.
4.4.3 Other gas emissions
The requirements of 4.4.2 can be applied to hydrogen and GHG emissions.
4.5 Safety
The storage facility shall be designed, constructed, operated, maintained and abandoned to get the
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