The NSTA’s wellbore registration numbering system is a simple code system that provides an unambiguous reference for use in correspondence, cross-referencing between computer databases, and in other information and filing systems.

The NSTA recommends that operators carefully consider their internal wellbore-numbering systems for new projects and design them to avoid confusion with the NSTA’s wellbore registration numbers. Please remember that the full NSTA wellbore number must be quoted on all returns, well logs, reports and correspondence related to a wellbore. An operator’s development wellbore numbering system should be reviewed with the NSTA’s Licensing and Consents department at an early stage in development programme planning.

The terms of this notice cover both offshore and onshore wellbores, although land wellbores may also have an alternate name as a secondary identifier.

Throughout this notice the term ‘platform’ should be taken to include subsea templates and sites where development wellbores are clustered in close proximity, e.g. around a gathering manifold.

Each wellbore in the UK is assigned a unique reference number that has been assigned by the NSTA. The NSTA supports the Energistics (formerly POSC (Petroleum Open Software Corporation)) international Energy Standards initiatives.

Operators should apply for consent to carry out drilling activities and other wellbore operations by way of the Well Operations Notifications System (WONS). WONS is an NSTA software application accessed via the UK Energy Portal. Notifications can only be submitted against consented applications in WONS.

The operator must issue a Spud Notification to the NSTA via WONS. Once validated the system will assign the appropriate sequential wellbore number based on the details of the location, licence, consent and so on. The NSTA will inform both the operator and the HSE of the wellbore number via a standard notification email.

The Petroleum Production Reporting System (PPRS2000) number is derived from the well number elements of the NSTA wellbore number.

The NSTA defines a wellbore to be a borehole drilled into the earth from a single surface or subsurface location to a single subsurface location.

If the surface location changes but target location stays the same, a new wellbore is regarded as a re-spud of the first wellbore.

If the target location changes but surface location stays the same, the new wellbore is called a geological sidetrack.

If a sidetrack is made to bypass an obstruction while the surface and target locations remain the same, this is called a mechanical sidetrack.

Shallow boreholes drilled for the purpose of rig positioning or substrate investigation (not drilled primarily for the searching, boring or getting of petroleum) will not be included in the NSTA wellbore numbering scheme. These will not be given a Drilling Sequence Number but will be identified with the mnemonic EB (Exploratory Borehole) following the relevant quad and block number, to avoid confusion with other wellbores.

The NSTA is reviewing the definition of “Well”, “Wellbore”, “Well Identifier” etc. with reference to the definitions set out by the Professional Petroleum Data Management Association (PPDM).

The NSTA wellbore number comprises seven components (a)-(g), which uniquely define a wellbore, e.g. 211/30b-A21Z (this is a sidetrack of wellbore 21 on platform A in part block 211/30b). Country Quad Block Wellbore Number: The number is initially established when drilling commences towards a target by insertion of a number in component (f), replacing the target reference letter(s).

Definition of components of wellbore registration number

Country: one character code to define the country in whose waters the wellbore is drilled

UK offshore wellbores have a blank in this column

UK land wellbores have an L.

Wellbores drilled in licences issued by the Manx authorities have the prefix M

Quadrant: the number, or letter, of the quadrant in which the wellbore is drilled.

  • UK quadrants are areas enclosed by one degree of latitude and longitude.
  • UK offshore quadrants are identified by numbers, the number is up to three digits, right justified with no leading zero or zeros
  • UK onshore quadrants are identified by capital letters

The number or letter designation for the quadrant can be viewed on this NSTA map.

Block: the number of the block within the quadrant in which the wellbore is drilled.

  • Each UK quadrant is divided into 30 blocks measuring 12 minutes of longitude by 10 minutes of latitude
  • The number has two digits including a leading zero.

Block suffix: used if the block is subdivided, usually after partial surrender – otherwise it is omitted.

  • The retained part has the lower case suffix ‘a’
  • The part(s) surrendered, which may be re-licensed, are given lower case suffixes ‘b’, ‘c’ etc.

‘Platform’, subsea cluster designation or land site: for wellbores drilled from a fixed or floating platform, through a subsea template, or from a land grouping – one capital letter should be used for platform designation, otherwise leave blank.

  • The letter ‘S’ should not be used to avoid confusion with subsea wellbores in the PPRS

Wellbore Sequence: The Drilling Sequence Number (DSN) is the consecutive chronological number of the wellbore within the block or from the ‘platform’, consisting of up to two digits with no leading zero.

The chronological sequence number will be allocated to the wellbore only after it has commenced drilling to a specific bottom hole target location, as follows:

  • exploration, appraisal or single satellite subsea development wellbores will normally be numbered at the time a wellbore is spudded i.e. when drilling commences from surface
  • wellbores forming part of a multi-well development, in which casings are batch set prior to finalising the slot/target allocation, will be numbered only after drilling has commenced to a specific target location, normally at the time of drilling out of the 20” surface casing shoe or equivalent,.
  • Operators are required to submit a Spud Notification, via WONS, to advise the NSTA that drilling has commenced to a specific target location.
    • For most exploration and appraisal wellbores notification should be issued within two hours of spudding-in to the seabed/surface
    • For most development wellbores notification should be issued within two hours of spudding out of pre-set casing with the intention of drilling to the target.

Wellbore suffix: a single capital letter that distinguishes different related wellbores. There are several reasons for adding a wellbore suffix letter:

  • Re-spud: If a wellbore is re-spudded for any reason, it is distinguished by an upper case character ‘A’, ‘B’, ‘C’, etc. in this component. A wellbore is designated to be a re-spud when the original attempt to drill the wellbore fails for any reason at or near the surface and another wellbore must be started at a nearby surface location to drill the same target. First re-spud has capital A, second B, and so on.
  • Sidetrack: If a wellbore is sidetracked and the abandoned part of the hole has significant data (i.e. logs, MWD and/or core), the sidetrack is distinguished by ‘Z’, ‘Y’, ‘X’, etc. in this column. A suffix is not required if a sidetrack is for mechanical reasons only, and the abandoned part of the hole does not have significant data (i.e. logs, MWD and/or core) and is not through any known hydrocarbon bearing interval. Suffixes of re-spudded wellbores that are then sidetracked are replaced by ‘Z’, ‘Y’, ‘X’, etc.
  • Redrill: the letter 'R' is employed to indicate that a well has been redrilled, this should therefore never appear as a suffix on the wellbore number.
  • Exceptions: historically the letters 'L', 'M' and 'N' have been employed to indicate related wellbores, this practice was never widely adopted and is no longer applied.

More information

  • 1st attempt 211/03b- 21
  • Re-spud 211/03b- 21A
  • mechanical sidetrack 1 (logged) 211/03b- 21Z
  • mechanical sidetrack 2 (cored) 211/03b- 21Y
  • geological sidetrack 211/03b- 21X
  • Multilateral wellbores will also be numbered in this way, with a suffix for each lateral. The allocation of suffixes should be agreed with the NSTA Information Management team.
  • 10/01- 2: second wellbore drilled in block 10/01
  • 3/14a- 17: 17th wellbore on a subdivided block
  • 2/05-H1: Heather platform well, first wellbore drilled from platform
  • 2/05-H3Z: Heather platform wellbore re-drilled to a new bottom hole location
  • 211/28- 1A: re-spudded wellbore
  • 15/16- 3Z: sidetracked wellbore
  • 30/17b- 3: third wellbore in block 30/17 in re-licensed part block 30/17b
  • 30/17a- 4: fourth wellbore in block 30/17 in retained part block 30/17a
  • LA/07- 3: land wellbore in quadrant A
  • L11/27- 1: land wellbore in quadrant 11

(a) Relinquishment of part blocks - effect on wellbore numbers

  • Wellbores within a block will continue to be numbered consecutively even after the block has been subdivided for licensing purposes, e.g. following partial surrender and subsequent relicensing. After subdivision however, the block subdivision suffix (e.g. 24/04b) must be used.
  • The first wellbore drilled from a platform will take the suffix applicable to the block or part block in which the platform resides when drilling commences.
  • All wellbores subsequently drilled from that platform assume the same block suffix as the first wellbore.
  • Once drilling has commenced on a platform, relinquishment of part of a block will not require a change in platform wellbores numbers, i.e. there will be no need to add a block suffix to the wellbore registration number for existing or future wellbores drilled from that platform.

(b) Extension of ‘platform’ designation to subsea template, etc

  • All subsea structures or templates through which two or more development wellbores may be drilled will be given a ‘platform’ designation letter (component e) and wellbores will be numbered (component f) like platform wells.
  • A platform or other installation set above the template will adopt the template designation letter. Clusters of subsea development wellbores will be treated in the same way as templates.

(c) Designation of wellbores prior to spudding-in

  • Wellbores should not be designated by drilling sequence number before spudding-in and drilling has commenced to a specific target location.
  • Reference should be made to an identifying letter (or target location letter) for non-platform wells and for platform wells an identifying letter (following the platform letter) and slot number.
  • A maximum of three characters can be used for the identifying letter and four for the slot number.
  • Conductors and surface casing strings set, or planned to be set, in wellbores before drilling has commenced to a specific target location should be identified by the slot number following the platform or subsea cluster designation.

(d) Designation of development wellbores drilled out of sequence

  • During development drilling, batch setting of conductors may be carried out. Often reservoir targets are not allocated at this stage, in which case the wellbores should continue to be referred to by their slot numbers.
  • A notification must be submitted to the NSTA via the WONS system within two hours of the wellbore drilling out of the conductor, with the intention of proceeding to the reservoir target.
  • The unique sequential wellbore number will be automatically assigned and the operator will be notified.

(e) Changes in wellbore number after re-spudding

  • If after spudding-in an exploration, appraisal or single satellite development wellbore is abandoned on mechanical grounds and the rig moved for the purpose of drilling what is effectively the same wellbore, it can be considered in one of two ways subject to prior agreement by the the NSTA:
    • add a suffix to indicate a second attempt to drill the same wellbore (24/04- 2A).
    • call the mechanically failed attempt a wellbore because it has reached a sufficient depth and acquired geological data which justifies so doing. In this case the second attempt is a new wellbore and in our example would be 24/04-3 (unless 24/04-3 has already spudded elsewhere when it would be 24/04-4).
  • In all cases an application for consent to abandon this existing hole and drill from the new location is required.

(f) Changes in wellbore number after re-drilling

  • When a wellbore has reached its target horizon and is then drilled to a new bottom hole location, it will fall into one of the following categories:
    • the old wellbore has production (including EWT but excluding DST) or injection history.
    • the old wellbore has no production or injection history.

    If the first above applies, the new wellbore is termed a re-drill and re-numbered with the next consecutive chronological number. The effective date to determine the new number (spud date) will be the date the wellbore drilled out of the existing wellbore.

    If the second applies the wellbore is considered a sidetrack and a suffix (starting from the end of the alphabet) should be added to the wellbore number for the sidetracked well (24/04-A6Z) to aid log identification. This also applies to pilot wellbores and their subsequent horizontal sidetracks including multilaterals, and to a wellbore re-entered to deepen to a new target following suspension.
  • In all cases an application for consent to abandon the existing section of the hole and drill to the new location is required.

(g) Wellbore numbering – licence obligations

  • No aspect of (e) and (f) above implies acceptance of the wellbores as more than one wellbore for licence obligation purposes.

(h) Wellbore numbering – wellbores tied into platforms

  • Wellbores drilled from mobile rigs and subsequently completed and tied into a ‘platform’ (eg single subsea completions) retain their original wellbore registration number.

(i) Reference to wellbores in reports and correspondence with the NSTA and HSE

  • With the exception of references to PPRS, whenever a wellbore, wellbore notification, wellbore record, report, core or sample has to be identified to the NSTA or HSE, it must be referred to or labelled in the following manner:

Before allocation of the sequential wellbore registration number (before spudding)

  • Exploration, appraisal and single subsea development wellbores: quadrant/block - identifying letter(s) of target, eg 107/01-G
  • Wellbores drilled from platforms or subsea clusters: quadrant/block - platform letter, identifying letter(s) (slot no), eg 23/05-H, FE (slot no 15)

After allocation of sequential wellbore registration number

  • Exploration, appraisal and single subsea development wellbores: wellbore registration number, eg 107/01- 2
  • Wellbores drilled from ‘platforms or subsea clusters’: wellbore registration number (slot number), eg 23/05-H1 (slot no 15)

(j) Change of well number (synonymous well number)

  • Normally a wellbore number cannot be changed after it has been used for identifying wellbore records. In exceptional circumstances however, an operator may apply to have the wellbore renumbered to fit into a ‘platform’ site sequence, e.g to make the existing wellbore A1 if ‘A’ is chosen for the ‘platform’ site letter. Subsequent wellbores drilled on the ‘platform or subsea cluster’ will then be numbered A2, A3 etc. The operator must discuss potential re-numbering of wellbores at the earliest opportunity during development planning. Reference must be made in all communications to both the original and synonymous wellbore number, eg 10/07- 2, A1