If a field is unitized for enhanced oil recovery, the well number and lease name will change.
WHCS was first offered to the oil and gas industry at the 1956 annual meeting of the AAPG in Los Angeles.
The first recommendations of the subcommittee were published in 1966 as Appendix A of API Bulletin D12 (Well Data Glossary).
In April 1968, API published Bulletin D12A, which dealt solely with well numbering systems.
This publication was modified slightly in December 1970 and January 1979, and the 1979 edition (reissued in 1982 and 1985) is the most current version.
[4] In 1995, the Subcommittee drafted some modifications to the API numbering scheme, but the group was disbanded before the recommendations were published.
[6] One of the Subcommittee's original recommendations was that the unique numbers should be assigned by regulatory agencies as part of the oil and gas well permitting process.
Most public databases that use API numbers are maintained by the individual oil and gas commissions, Therefore, they only require the "County Code" and "Unique Well Identifier.
There are four types of unique well identifiers, and these are called historical, current, reserved, and exempt.
Historical API numbers were assigned by Petroleum Information or other service companies and cooperative groups.
Current API numbers are assigned by regulatory agencies, usually the oil and gas commission for the state where the well was drilled.
These numbers are proprietary, and may not be assigned by any regulatory agency or data vendor like Petroleum Information.
The thirteenth and fourteenth digits (00 in the example above) are to distinguish between separate operations in a single bore hole.
In 1995, the API Subcommittee on Well Data Retrieval Systems proposed adding the event sequence code to deal with re-entries, recompletions, and hole deepenings.
However, because of industry conditions (low oil prices), the subcommittee was disbanded before the recommendations were published and adopted by API[4] IHS Energy [2], the successor to Petroleum Information, adopted these unpublished recommendations in January 1999.
However, this event sequence code is assigned by IHS Energy, and is not found in most oil and gas databases.
Petroleum Information (now IHS Energy[11]) assigned API numbers for most wells drilled before January 1, 1967.
Most oil and gas commissions make API numbers (and well header data) available on-line and free of charge.