Gas infrastructure - Underground gas storage - Part 2: Functional recommendations for storage in oil and gas fields

This standard covers the functional recommendations for design, construction, testing, commissioning, operation, maintenance and abandonment of underground gas storage facilities in oil and gas fields up to and including the wellhead. It specifies practices which are safe and environmentally acceptable. The necessary surface facilities for underground gas storage are described in prEN 1918-5. In this context "gas" is any hydrocarbon fuel which is in a gaseous state at a temperature of 15°C and under a pressure of 1 bar. This includes natural gas (also called CNG) and LPG.

Gasinfrastruktur - Untertagespeicherung von Gas - Teil 2: Funktionale Empfehlungen für die Speicherung in Öl- und Gasfeldern

Diese Europäische Norm legt die funktionalen Empfehlungen für Planung, Errichtung, Prüfung, Inbetriebnahme, Betrieb, Instandhaltung und Stilllegung von Untertagegasspeicheranlagen in Öl- und Gasfeldern bis einschließlich des Bohrlochkopfes fest.
Sie legt sichere und umweltverträgliche Verfahrensweisen fest.
Für erforderliche Übertageanlagen für einen Untertagegasspeicher gilt EN 1918 5.
In diesem Zusammenhang ist „Gas“ jeder Brennstoff auf Kohlenwasserstoffbasis, der:
—   sich bei einer Temperatur von 15 °C und unter einem Druck von 0,1 MPa in gasförmigem Zustand befindet (dazu gehören Erdgas, komprimiertes Erdgas (en: compressed natural gas, CNG) und Flüssiggas (en: liquefied petroleum gas, LPG). 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.
Die vorliegende Europäische Norm legt allgemeine Grundsätze für Untertagegasspeicheranlagen fest. Die Anwender dieser Europäischen Norm sollten sich bewusst sein, dass detailliertere Normen und/oder Technische Regeln existieren. Eine Liste einschlägiger Normen, die allerdings nicht abschließend ist, findet sich in Anhang A.
Es ist vorgesehen, dass diese Europäische Norm in Verbindung mit nationalen Normen und/oder Technischen Regeln angewendet wird. Sie ersetzt diese nicht.
Beim Auftreten von Widersprüchen aufgrund restriktiverer Anforderungen in nationalen Gesetzen/Vor-schriften als in der vorliegenden Europäischen Norm gefordert haben nationale Gesetze/Vorschriften Vorrang, wie in CEN/TR 13737 (alle Teile) dargestellt.
ANMERKUNG   CEN/TR 13737 (alle Teile) enthält:
—   Darstellung der in den einzelnen Ländern zu beachtenden Gesetze/Vorschriften;
—   wenn notwendig, darin enthaltene restriktivere nationale Anforderungen;
—   eine nationale Kontaktstelle für die aktuellsten Informationen.
Es ist nicht vorgesehen, diese Europäische Norm rückwirkend auf bestehende Anlagen anzuwenden.

Infrastructures gazières - Stockage souterrain de gaz - Partie 2: Recommandations fonctionnelles pour le stockage en gisements de pétrole et de gaz

La présente Norme européenne 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 (UGS) en gisements de pétrole et de gaz jusqu'à la tête de puits incluse.
Elle spécifie les pratiques acceptables pour la sécurité et l'environnement.
Pour les installations de surface qui sont nécessaires pour un stockage souterrain de gaz, l'EN 1918-5 s’applique.
Dans ce contexte, le terme « gaz » désigne tout combustible hydrocarboné :
—   qui est à l'état gazeux à une température de 15 °C et sous une pression de 0,1 MPa (sont inclus le gaz naturel, le gaz naturel comprimé (GNC) et le gaz de pétrole liquéfié (GPL)). 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.
La présente Norme européenne spécifie les principes communs fondamentaux des stockages souterrains de gaz. Il convient que les utilisateurs de la présente Norme européenne gardent à l'esprit qu'il existe des normes et/ou des codes de bonne pratique plus détaillés. Une liste non exhaustive des normes pertinentes figure à l'Annexe A.
La présente Norme européenne est destinée à s'appliquer conjointement avec ces normes nationales et/ou codes de bonne pratique ; elle ne les remplace pas.
En cas de conflits en termes de prescriptions plus restrictives dans la réglementations/législation nationale vis-à-vis des prescriptions de la présente Norme européenne, la réglementation/législation nationale prévaut comme illustré dans le CEN/TR 13737 (toutes les parties).
NOTE   Le CEN/TR 13737 (toutes les parties) donne :
—   la clarification des législations/règlements applicables dans un pays ;
—   si approprié, des prescriptions nationales plus restrictives ;
—   un point de contact national pour une information ultérieure.
La présente Norme européenne n'est pas prévue pour être appliquée de façon rétroactive aux installations existantes.

Infrastruktura za plin - Podzemna plinska skladišča - 2. del: Funkcionalna priporočila za skladiščenje na naftnih in plinskih poljih

Ta standard zajema funkcionalna priporočila za načrtovanje, izgradnjo, preskušanje, delovanje, vzdrževanje in opustitev podzemnih plinskih skladišč na naftnih in plinskih poljih, vključno z ustjem vrtine. Določa prakse, ki so varne in okoljsko sprejemljive. Potrebni nadzemni objekti za podzemno skladiščenje plina so opisani v standardu prEN 1918-5. »Plin« v tem kontekstu pomeni vsako ogljikovodikovo gorivo, ki je v plinastem stanju pri temperaturi 15 °C in tlaku 1 bar. To vključuje zemeljski plin (imenovan tudi CNG) in LPG.

General Information

Status
Published
Public Enquiry End Date
14-Aug-2014
Publication Date
12-Oct-2016
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
06-Oct-2016
Due Date
11-Dec-2016
Completion Date
13-Oct-2016

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.NFLRQDOQDGasinfrastruktur - Untertagespeicherung von Gas - Teil 2: Funktionale Empfehlungen für die Speicherung in Öl- und GasfeldernInfrastructures gazières - Stockage souterrain de gaz - Partie 2: Recommandations fonctionnelles pour le stockage en gisements de pétrole et de gazGas infrastructure - Underground gas storage - Part 2: Functional recommendations for storage in oil and gas fields75.200Petroleum products and natural gas handling equipmentICS:Ta slovenski standard je istoveten z:EN 1918-2:2016SIST EN 1918-2:2016en,fr,de01-november-2016SIST EN 1918-2:2016SLOVENSKI

STANDARDSIST EN 1918-2:19991DGRPHãþD
SIST EN 1918-2:2016
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 1918-2
March
t r s x ICS
y wä t r r Supersedes EN
s { s zæ tã s { { zEnglish Version
Gas infrastructure æ Underground gas storage æ Part

tã Functional recommendations for storage in oil and gas fields Infrastructures gazières æ Stockage souterrain de gaz æ Partie

tã Recommandations fonctionnelles pour le stockage en gisements de pétrole et de gaz

Gasinfrastruktur æ Untertagespeicherung von Gas æ Teil

tã Funktionale Empfehlungen für die Speicherung in Ölæ und Gasfeldern This European Standard was approved by CEN on

s r January
t r s xä

egulations which stipulate the conditions for giving this European Standard the status of a national standard without any alterationä Upætoædate lists and bibliographical references concerning such national standards may be obtained on application to the CENæCENELEC Management Centre or to any CEN memberä

translation under the responsibility of a CEN member into its own language and notified to the CENæCENELEC Management Centre has the same status as the official versionsä

CEN members are the national standards bodies of Austriaá Belgiumá Bulgariaá Croatiaá Cyprusá Czech Republicá Denmarká Estoniaá Finlandá Former Yugoslav Republic of Macedoniaá Franceá Germanyá Greeceá Hungaryá Icelandá Irelandá Italyá Latviaá Lithuaniaá Luxembourgá Maltaá Netherlandsá Norwayá Polandá Portugalá Romaniaá Slovakiaá Sloveniaá Spainá Swedená Switzerlandá Turkey and United Kingdomä

EUROPEAN COMMITTEE FOR STANDARDIZATION COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre:
Avenue Marnix 17,
B-1000 Brussels

t r s x CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Membersä Refä Noä EN

s { s zæ tã t r s x ESIST EN 1918-2:2016

EN 1918-2:2016 (E) 2 Contents Page European foreword ....................................................................................................................................................... 3 1 Scope .................................................................................................................................................................... 4 2 Normative references .................................................................................................................................... 4 3 Terms and definitions ................................................................................................................................... 5 3.1 Terms and definitions common to parts 1 to 4 of EN 1918 .............................................................. 5 3.2 Terms and definitions not common to parts 1 to 4 of EN 1918 ...................................................... 9 4 Requirements for underground gas storage ...................................................................................... 10 4.1 General ............................................................................................................................................................. 10 4.2 Underground gas storage .......................................................................................................................... 11 4.3 Long-term containment of stored gas ................................................................................................... 14 4.4 Environmental conservation ................................................................................................................... 15 4.5 Safety ................................................................................................................................................................ 15 4.6 Monitoring ...................................................................................................................................................... 15 5 Design ............................................................................................................................................................... 15 5.1 Design principles .......................................................................................................................................... 15 5.2 Field description and storage behaviour ............................................................................................ 16 5.3 Determination of the maximum operating pressure ...................................................................... 17 5.4 Wells ................................................................................................................................................................. 18 5.5 Monitoring systems ..................................................................................................................................... 22 5.6 Neighbouring subsurface activities ....................................................................................................... 23 6 Construction ................................................................................................................................................... 23 6.1 General ............................................................................................................................................................. 23 6.2 Wells ................................................................................................................................................................. 23 6.3 Completions ................................................................................................................................................... 23 6.4 Wellheads ........................................................................................................................................................ 24 7 Testing and commissioning ...................................................................................................................... 24 8 Operation, monitoring and maintenance ............................................................................................ 24 8.1 Operating principles ................................................................................................................................... 24 8.2 Monitoring ...................................................................................................................................................... 24 8.3 Injection and withdrawal operations ................................................................................................... 25 8.4 Maintenance of wells .................................................................................................................................. 25 8.5 HSE ..................................................................................................................................................................... 25 9 Abandonment ................................................................................................................................................ 26 9.1 General ............................................................................................................................................................. 26 9.2 Withdrawal of the gas ................................................................................................................................. 27 9.3 Plugging and abandonment of wells ..................................................................................................... 27 9.4 Surface facilities ........................................................................................................................................... 27 9.5 Monitoring ...................................................................................................................................................... 27 Annex A (informative)

Non-exhaustive list of relevant standards ............................................................ 28 Annex B (informative)

Significant technical changes between this European Standard and the previous version EN 1918-2:1998 ......................................................................................................... 30

SIST EN 1918-2:2016

EN 1918-2:2016 (E) 3 European foreword This document (EN 1918-2:2016) has been prepared by Technical Committee CEN/TC 234 “Gas infrastructure”, the secretariat of which is held by DIN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by September 2016 and conflicting national standards shall be withdrawn at the latest by September 2016. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document supersedes EN 1918-2:1998. This document has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association. For a list of significant technical changes between this European Standard and EN 1918-2:1998, see Annex B. This document is Part 2 of a European Standard on “Gas infrastructure - Underground gas storage” which includes the five following 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. 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. According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. SIST EN 1918-2:2016

EN 1918-2:2016 (E) 4 1 Scope This European Standard covers the functional recommendations for design, construction, testing, commissioning, operation, maintenance and abandonment of underground gas storage (UGS) facilities in oil and gas fields 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" is any hydrocarbon fuel: — which is in a gaseous state at a temperature of 15 °C and under a pressure of 0,1 MPa (this includes natural gas, compressed natural gas (CNG) and liquefied petroleum gas (LPG). 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 European Standard specifies common basic principles for underground gas storage facilities. Users of this European Standard should be aware that more detailed standards and/or codes of practice exist. A non-exhaustive list of relevant standards can be found in Annex A. This European Standard is intended to be applied in association with these national standards and/or codes of practice and does not replace them. In the event of conflicts in terms of more restrictive requirements in the national legislation/regulation with the requirements of this European Standard, the national legislation/regulation takes precedence as illustrated in CEN/TR 13737 (all parts). NOTE CEN/TR 13737 (all parts) contains: — clarification of relevant legislation/regulations applicable in a country; — if appropriate, more restrictive national requirements; — national contact point for the latest information. This European Standard is not intended to be applied retrospectively to existing facilities. 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 SIST EN 1918-2:2016

EN 1918-2:2016 (E) 5 3 Terms and definitions 3.1 Terms and definitions common to parts 1 to 4 of EN 1918 For the purposes of this document, the following terms and definitions apply. They are common to parts 1 to 4 of EN 1918. 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. SIST EN 1918-2:2016

EN 1918-2:2016 (E) 6 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 may 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 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 indicator horizon horizon overlying the caprock in the storage area and used for monitoring 3.1.20 landing nipple device in a tubing string with an internal profile to provide for latching and sealing various types of plugs or valves SIST EN 1918-2:2016

EN 1918-2:2016 (E) 7 3.1.21 liner casing installed within last cemented casing in the lowermost section of the well without extension to surface 3.1.22 lithology characteristics of rocks based on description of colour, rock fabrics, mineral composition, grain characteristics, and crystallization 3.1.23 logging measurement of physical parameters versus depth in a well 3.1.24 master valve valve at the wellhead designed to close off the well for operational reasons and in case of emergency or maintenance 3.1.25 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. 3.1.26 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 for caverns is based on geomechanical investigations to ensure stability and to limit the effect of subsidence and normally has to be approved by authorities and has not to be underrun Note 1 to entry: The minimum pressure is related to a datum depth. 3.1.27 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. 3.1.28 operating well well used for gas withdrawal and/or injection 3.1.29 overburden all sediments or rock that overlie a geological formation SIST EN 1918-2:2016

EN 1918-2:2016 (E) 8 3.1.30 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 m2. 3.1.31 porosity volume of the pore space (voids) within a rock formation expressed as a percentage of its total volume 3.1.32 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. 3.1.33 saturation percentages of pore space occupied by fluids 3.1.34 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 3.1.35 string entity of casing or tubing plus additional equipment, screwed or welded together as parts of a well respectively completion 3.1.36 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 3.1.37 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 3.1.38 well borehole and its technical equipment including the wellhead 3.1.39 well integrity well condition without uncontrolled release of fluids throughout the life cycle SIST EN 1918-2:2016

EN 1918-2:2016 (E) 9 3.1.40 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 3.1.41 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 3.1.42 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 may be cycled more than once a year. 3.1.43 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 not common to parts 1 to 4 of EN 1918 For the purposes of this document, the following terms and definitions apply, which are common to part 2 of EN 1918 only. 3.2.1 boundary fault fault, which forms the physical border in some storage reservoirs 3.2.2 capillary pressure pressure difference between the non-wetting phase and the wetting phase in a porous rock 3.2.3 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) 3.2.4 caprock sealing barrier for fluids overlying the pore storage reservoir 3.2.5 closure vertical distance between the top of the structure and the spill point SIST EN 1918-2:2016

EN 1918-2:2016 (E) 10 3.2.6 gas oil contact interface between the gas and the oil phase in a reservoir 3.2.7 gas water contact interface between the gas and water in a reservoir 3.2.8 hanger device for supporting the weight of pipes and to assure the pressure tightness of the annulus 3.2.9 material balance calculation method based on the fluids withdrawn from or injected into a reservoir and the fluids remaining in the reservoir excluding the displacement process in the reservoir 3.2.10 initial reservoir pressure pressure existing in a reservoir before any change due to operation of the reservoir or due to operation in the surrounding area Note 1 to entry: The initial reservoir pressure is related to a datum depth. 3.2.11 reservoir simulation numerical modelling of a reservoir to predict or to monitor the behaviour and movement of the fluids in the formation and in general the reservoir behaviour with respect to rates, pressures and saturation distribution Note 1 to entry: A reservoir model may be calibrated against historical data through the history match process. 3.2.12 sand screen filters placed at the level of the storage formation in order to avoid the entrainment of sand particles and fines during withdrawal 3.2.13 spill point structural point within a reservoir, where hydrocarbons could leak and migrate out of the storage structure 3.2.14 well testing taking pressure and flow rate measurements during flowing and shut-in periods of operating wells to provide information about the characteristics of the storage and the capacity of the wells 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 oil and gas fields are given in Clauses 5, 6, 7, 8 and 9. SIST EN 1918-2:2016

EN 1918-2:2016 (E) 11 4.2 Underground gas storage 4.2.1 Overview and functionality of underground gas storage The EN 1918 covers storage of natural gas, Compressed Natural Gas (CNG) and Liquefied Petroleum Gas (LPG). Because of the relevance of underground gas storage of CNG the major part of this introduction is related to the storage of this. 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. Natural gas produced from oil and gas fields is increasingly being used to supply energy requirements. As the gas supply from these fields does not match with the variable market demand natural gas is injected into subsurface storage reservoirs when market demand falls below the level of gas delivery or if there is an economic incentive for injection. 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. Increasingly 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. 4.2.2 Types of UGS For storage of natural gas several types of underground gas storage facilities can be used, which differ by storage formation and storage mechanism (see Figure 1): — pore storage: — storage in aquifers; — storage in former gas fields; — storage in former oil fields. — caverns: SIST EN 1918-2:2016

EN 1918-2:2016 (E) 12 — 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 7 cavern Figure 1 — Storage in aquifers, oil and gas fields, solution mined salt caverns For LPG storage only salt or rock caverns can be applied. The UGS type applied is dependent on the geological conditions and prerequisites as well on the designed capacity layout. 4.2.3 General characterization of UGS UGS are naturally or artificially developed reservoirs respectively artifically developed caverns in subsurface geological formations used for the storage of natural gas (or LPG). A UGS consists of all subsurface and surface facilities required for the storage and for the withdrawal and injection of natural gas (or storage of LPG). Several subsurface storage reservoirs or caverns may be connected to one or several common surface facilities. The suitability of subsurface geological formations have 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 SIST EN 1918-2:2016

EN 1918-2:2016 (E) 13 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, comprising facilities for gas dehydration/treatment, compression, process control and measurement. Gas is injected via the operating wells into the pores of a reservoir or into a cavern, thus building up a reservoir of compressed natural 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 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, 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. It is based on 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 oil and gas fields Storage of gas in oil and gas fields i

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