This European Standard covers the functional recommendations for design, construction, testing, commissioning, operation, maintenance and abandonment of underground gas storage (UGS) facilities in aquifers 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.