oSIST prEN 1918-4:2025
(Main)Gas infrastructure - Underground gas storage - Part 4: Functional recommendations for storage in rock caverns
Gas infrastructure - Underground gas storage - Part 4: Functional recommendations for storage in rock caverns
This document covers the functional recommendations for design, construction, testing, commissioning, operation, maintenance and abandonment of underground gas storage (UGS) facilities in mined rock caverns up to and including the wellhead.
This document 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).
Gases that are liquid in subsurface conditions are not considered in this document.
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 4: Funktionale Empfehlungen für die Speicherung in Felskavernen
Dieses Dokument legt die funktionalen Empfehlungen für Planung, Errichtung, Prüfung, Inbetriebnahme, Betrieb, Instandhaltung und Stilllegung von Untertagegasspeicheranlagen in bergmännisch hergestellten Felskavernen bis einschließlich des Bohrlochkopfes fest.
Dieses Dokument 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 die vertraglichen 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).
Gase, die bei Untertagebedingungen flüssig sind, werden in diesem Dokument nicht behandelt.
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.
Infrastructures gazières - Stockage de gaz souterrain - Partie 4 : Recommandations fonctionnelles pour le stockage en cavités
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 minées jusqu’à la tête de puits incluse.
Le présent document 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).
Les gaz qui sont liquides dans des conditions de subsurface ne sont pas traités dans le présent document.
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 - 4. del: Funkcionalna priporočila za skladiščenje v skalnih kavernah
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-junij-2025
Infrastruktura za plin - Podzemna plinska skladišča - 4. del: Funkcionalna
priporočila za skladiščenje v skalnih kavernah
Gas infrastructure - Underground gas storage - Part 4: Functional recommendations for
storage in rock caverns
Gasinfrastruktur - Untertagespeicherung von Gas - Teil 4: Funktionale Empfehlungen für
die Speicherung in Felskavernen
Infrastructures gazières - Stockage de gaz souterrain - Partie 4 : Recommandations
fonctionnelles pour le stockage en cavités
Ta slovenski standard je istoveten z: prEN 1918-4
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 1918-4:2016
English Version
Gas infrastructure - Underground gas storage - Part 4:
Functional recommendations for storage in rock caverns
Infrastructures gazières - Stockage de gaz souterrain - Gasinfrastruktur - Untertagespeicherung von Gas - Teil
Partie 4 : Recommandations fonctionnelles pour le 4: Funktionale Empfehlungen für die Speicherung in
stockage en cavités Felskavernen
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 1918-4:2025 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 . 12
4.1 General. 12
4.2 Underground gas storage . 13
4.2.1 Overview and functionality . 13
4.2.2 Types . 13
4.2.3 General characterization . 14
4.2.4 Storage in mined caverns . 15
4.3 Long-term containment of stored products . 18
4.4 Environmental conservation . 18
4.4.1 General. 18
4.4.2 Methane emissions . 19
4.4.3 Other stored gas emissions . 20
4.5 Safety . 20
4.6 Monitoring . 20
5 Design . 20
5.1 Design principles . 20
5.1.1 General. 20
5.1.2 Unlined caverns . 21
5.1.3 Lined rock caverns . 22
5.2 Geological exploration . 22
5.3 Stored product containment . 24
5.3.1 Unlined caverns . 24
5.3.2 Lined rock caverns . 24
5.4 Determination of the maximum operating pressure . 25
5.4.1 General. 25
5.4.2 CNG storage . 25
5.4.3 Lined rock caverns . 26
5.5 Cavern stability . 26
5.6 Construction parameters . 26
5.7 Concrete plug specifications . 26
5.8 Connecting caverns to surface . 26
5.9 Monitoring systems . 29
5.10 Neighbouring subsurface activities . 29
6 Construction . 30
7 Testing and commissioning . 31
7.1 General. 31
7.2 First gas filling . 32
8 Operation, monitoring and maintenance . 32
8.1 Operating principles . 32
8.2 Monitoring. 32
8.2.1 Operating parameters. 32
8.2.2 Inventory . 32
8.2.3 Cavern stability, product containment, corrosion monitoring . 32
8.3 Maintenance . 33
8.4 Health, safety and environment . 33
8.4.1 Safety management and health and environment . 33
8.4.2 Emergency procedures . 34
8.5 Adaptation to climate change . 34
8.5.1 Climate change effects . 34
8.5.2 Climate change impacts . 35
9 Abandonment. 36
9.1 General . 36
9.2 Withdrawing the fluid and pressure monitoring . 36
9.3 Plugging and abandonment of wells and accesses . 37
9.4 Surface facilities . 37
Annex A (informative) Significant technical changes between this document and EN 1918-4:2016
............................................................................................................................................................................. 38
Bibliography . 39
European foreword
This document (prEN 1918-4:2025) has been prepared by Technical Committee CEN/TC 234 “Gas
infrastructure”, the secretariat of which is held by DIN.
This document is currently submitted to the CEN Enquiry.
This document will supersede EN 1918-4:2016.
For a list of significant technical changes with respect to EN 1918-4:2016, see Annex A.
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.
This document is Part 4 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 cavities;
— 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 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 mined rock
caverns up to and including the wellhead.
This document 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).
Gases that are liquid in subsurface conditions are not considered in this document.
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 / cavern through wells and surface facilities to 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 (H2, 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 limit the effect of subsidence
Note 1 to entry: The minimum pressure is related to a datum depth.
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, 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
capillary threshold pressure
pressure needed to overcome the property of a porous rock saturated with a wetting phase (water) to
block the flow of a non-wetting phase (gas)
3.2.2
cavern lining system
man-made system of different parts installed around the cavern (including the roof, walls, and invert) to
ensure gas tightness during storage operation
3.2.3
concrete plugs
concrete structures constructed at end of excavation works for tightly closing off at cavern level all
temporary drives to the cavern units and operation shafts
Note 1 to entry: Concrete plugs are gas tight. Water ingress towards the cavern remains possible but is limited.
3.2.4
drainage system
wells in the rock mass or pipes embedded within the cavern lining system, intended to protect the
tightness liner from high water table pressure
3.2.5
gas tightness
adherence to a maximum leakage rate in an approved test procedure
3.2.6
numerical simulation
computer simulation of a system
Note 1 to entry: Applied for stability analysis, hydraulic flow pattern around an excavation.
3.2.7
operating shafts
vertical shafts connecting cavern to surface facilities, designed for setting all necessary equipment to
operate and monitor the storage cavern
3.2.8
rock cavern roof
highest part in a rock cavern average cross section
3.2.9
sliding component
part of the cavern lining system that protects the tightness liner from large deformations
3.2.10
tightness liner
component of the cavern lining system that ensures gas tightness during storage operation
Note 1 to entry: Often referred to as the steel liner, as steel is typically the preferred material for this purpose.
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 mined rock 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 the Scope. Because of the relevance of underground
gas storage of methane rich gases and hydrogen, the major part of this introduction is related to these.
The underground gas storage is an efficient proven common technology and is in use since 1915.
Underground gas storage (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 applicable.
The UGS type applied is dependent on the geological conditions and prerequisites as well on the designed
capacity layout.
4.2.3 General characterization
UGS are naturally or artificially developed reservoirs respectively and/or 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 has to 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 well head. 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 the gas withdrawal and gas injection, the surface facilities are the link between the
subsurface facilities and the transport connection point, typically comprising facilities for gas
dehydration/treatment, hydrogen/natural gas separation, blending, gas 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 or LPG.
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 pressures. 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 mined caverns
4.2.4.1 Unlined rock caverns
Generally spoken, a mined cavern storage facility comprises subsurface facilities of access works by
tunnel or/and shaft, one or several galleries, excavated in hard rock, operation shaft and related surface
facilities for handling of the stored product.
Unlined mined cavern technology is widely used in the field of underground storage for:
— liquids (crude oil, distillates, etc.);
— liquefied petroleum gas (LPG).
Within a limited scope, this technology is adapted for underground storage of compressed natural gas
(CNG) in lined or unlined rock caverns.
Recently lined cavern technologies have been developed extending the field of application of the
underground storage technics to compressed natural gas (LRC CNG) and membrane lined rock cavern for
liquefied natural gas (MLRC LNG). For both concepts (LRC CNG and MLRC LNG), product tightness is
provided by a steel liner. For MLRC, the steel liner is completed by a thermal insulation and a water
drainage system when necessary. This functional recommendation focuses on the design, construction
and operation principles of underground storage in unlined mined caverns for LPG and CNG products
(see Figure 2).
Underground storage in mined caverns is an alternative to underground storage in salt leached caverns
especially where the local geological conditions do not provide salt or where the salt does not offer
suitable characteristics for solution mining.
The main advantage of this technology relies on its adaptability to several geological conditions allowing
implementing the storage capacity close to needs.
Most favourable geological conditions for implementing an unlined mined cavern are typically igneous,
metamorphic or hard sedimentary rocks such as granite, gneiss, andesites, shales, limestones, rock salt
or cemented sandstones.
Other host rocks such as chalks and marls can be also envisaged with adapted layout, cavern shapes and
rock mass supports.
Main prerequisites for any type of unlined rock caverns are geological and geomechanical rock mass
quality adapted to develop the requested storage capacity, ensuring the long-term stability of the cavern
and shafts, assuming the installation of adapted structural reinforcements, for reasonable construction
costs.
Another compulsory prerequisite for storage in unlined mined caverns is the presence of a favourable
hydrogeological context to ensure the hydrodynamic containment conditions of the stored product.
Water curtain systems and grouting works could be developed in order to improve the natural conditions
and control the hydraulic containment of the stored product during the lifetime of the storage facility.
The specific complementary prerequisite for CNG storage in mined caverns is a proven gas tightness of
the rock mass. The proof of the tightness has to take into account the presence of fractures.
Underground storage in mined caverns is a safe way to create reserves of oil and gas products in the
immediate vicinity of producing, importing or consuming centres such as:
— refineries, import or export terminals;
— petrochemical complexes for which LPG constitutes a feedstock;
— local storage for seasonal peak shaving;
— regional feedstock for resale;
— stockpiling.
The hydraulic containment principle in unlined mined rock caverns relies on the groundwater pressure
prevailing in the adjacent rock mass. The cavern is located at such a depth that the water naturally present
in the surrounding rock flows everywhere towards the cavern preventing the stored product from
migrating into the rock mass. The favourable effects of the capillary threshold pressure are considered
as an additional safety term.
The product, lighter than and hardly miscible with water, is in this way hydraulically contained within
the storage space.
The water which is collected in the cavern during operation is removed by pumping, treated and disposed
of or recycled.
Furthermore, depending on the required commercial product specifications, coalescers and/or dryers
are implemented at the surface if necessary for the product during withdrawal. Stripping units are
implemented before disposal or recycling if necessary for the seepage water.
The control of the gas containment conditions for CNG storage in unlined mined rock caverns is provided
by the capillary pressures and/or hydraulic potentials of the host rock mass.
Key
1 LPG outlet 10 concrete plug (shaft)
2 LPG inlet 11 LPG, vapor phase
3 seepage water outlet 12 rock mass
4 ground level 13 concrete plug (tunnel)
5 water inlet for water curtain 14 access tunnel
6 operation shaft 15 LPG, liquid phase
7 water table 16 water
8 water curtain boreholes 17 LPG pump
9 water curtain gallery 18 water pump
Figure 2 — Cross section of a typical unlined rock cavern for LPG
4.2.4.2 Lined rock caverns for compressed hydrogen or natural gas storage
This document also covers the technology of lined rock caverns specifically designed for compressed
hydrogen or natural gas storage. Key features of lined rock caverns are highlighted as necessary.
While lined rock cavern technology can also be applied to the storage of liquefied hydrogen (cryogenic
storage), other applications such as hydrogen carriers (e.g. LOHC, ammonia), helium, carbon dioxide, and
compressed air are not covered in this document.
Gaseous hydrogen or natural gas must be stored at high pressure due to its low volumetric density. High
storage pressures make the implementation of hydrodynamic containment more challenging. Indeed,
this containment principle requires balancing the storage pressure with a hydraulic potential, which
might necessitate placing the storage at very great depths. This is why the lined rock cavern technology
can be applied alternatively.
The lined rock cavern relies on the implementation of a cavern lining system composed of various
components. These components together ensure the storage’s tightness. The functions are divided as
follows: a tightness liner (generally made of steel) ensures tightness, and the rock mass provides
mechanical strength. Other components of the cavern lining system (e.g. concrete and drainage system)
help the tightness liner playing its function.
This principle of function separation typically makes pressure vessel regulations for surface vessels not
suitable.
This division of functions, in which the tightness liner has not to withstand the internal pressure, allows
for the use of a relatively thin liner. Consequently, the tightness liner does not act as a mechanical barrier
to the thrust forces exerted by the hydrogen stored under high pressure, forces that are entirely
transmitted to the rock mass. As a result, the tightness liner undergoes the cyclic deformations of the rock
during the storage operation cycles. The tightness liner shall therefore withstand significant cyclic
deformations in operation. It will be necessary to demonstrate through calculations and tests that the
choice of material is compatible with the expected deformations, especially in the presence of hydrogen.
Design of the whole cavern lining system (see 5.3.2) shall contribute to minimize these deformations.
4.3 Long-term containment of stored products
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 fluids through its hydraulic and mechanical integrity.
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 presup
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