The API Number is dead – long live the US Well Number


I first want to apologize to all non-U.S. readers. This article is about US well numbers and the way we uniquely identify wells here in the US.

The common way to identify a well in the world is to give the well a unique well identifier (UWI). Here in the US we have a specific structure of a UWI called a US Well Number (formerly known as an API number). PPDM has officially changed the name from API Number to US Well Number – more on that later.

I also want to apologize for past sins. Many years ago when I was working in Canada I accidentally referred to their well numbering scheme as Canadian API. That is apparently the social equivalent of insulting hockey, beer and curling in a single statement.

Again to my Canadian friends, a sincere apology. They actually have an excellent and logical system of identifying/numbering wells – maybe we’ll talk about that in a future blog.

There are 2 variants of US well numbers – 12 digit and 14 digit. The 14 digit is an extension of the 12 digit so we will start there.

I like to refer to the first 10 digits as the surface location number because the stuff that goes on below the surface affects the 12 and 14 digits, not the first 10. The first 10 digits consists of a state code (2 digit), a county code (3 digit) and a unique county identifier (5 digits).

The first 10 digits refer only to the surface location. Even if the wellbore (under the surface) crosses state or county lines, it is always referred to (and numbered) by its surface location.

State Code

The first 2 digits of the API number is the state code. Many people believe the State codes are based on the US FIPS codes. This is inaccurate. Don’t use the current FIPS codes, or you will end up with the wrong state.

In general, the state codes are labeled 01-51 in alphabetical , with special codes for offshore. Alabama is 01, Wyoming is 49, and the District of Columbia is 08. Alaska is 50, Hawaii is 51 and the Gulf is 60. The codes were created before Alaska and Hawaii became states so they ended up at the end of the list.

County Code

The county code is a 3 digit number following the state code that uniquely identifies the county. If a well was drilled here in Travis county Texas it would have a designation of 42-453, where 42 is the code for Texas and 453 is the county code for Travis county. Each state has its own listing and numbering for county codes thus there is a unique way of identifying Washington county Texas (42-477) from Washington county Arkansas (03-143) from Washington county Minnesota (22-163) from Washington county Pennsylvania (37-125) from Washington county New York (31-115) from Washington county Alabama (01-129) – you get the idea…

In general, the county codes are odd numbers (again in alphabetical order) with the even numbers reserved for expansion. New counties were added in New Mexico and Arizona and they were given even numbers. Rhode Island, for example has 5 counties and they are numbered 1,3,5,7 and 9 (there are also 2 offshore codes Newport Offshore – 205 and Washington Offshore – 209)

Well Code


The wells are numbered uniquely in each county. However, because API numbers were not assigned when the first well was drilled, don’t assume the well numbered 00001 was the first well spudded in the county.

The numbers go to 99,999. There is no longer a reserved slot for proprietary numbers or industry vendor numbers. The numbers assigned by the assigning authority are the official number. However, numbers assigned under the previous guideline (API Bulletin D12A) should be retained.

In Kern County, California, they have actually used up all 99,999 numbers so they have assigned another county code for it. Kern County, California has county code 029 and 030.

Wellbore Code

The 11th and 12th digits refer to the wellbore code. The first wellbore, or the original wellbore from the surface location, is always referred to as 00.

A subsequent sidetrack or deepening or lengthening is given another number. The actual number that is assigned is the responsibility of the Assigning Authority.

14 Digits

The official standard published by PPDM reserves and defines 1-12 digits. There is no official standard for digits 13 and 14, except for general guidance.

They encourage the use of the digits for physical changes in the wellbore such as plugbacks, completions or any other well conversion that requires some kind of completion.

PPDM is specific about not using this number for wellbores, permits or other types of filings.

History of US Well Numbers (AKA API numbers)


A numbering scheme for wells in the US was first developed by Petroleum Information for their WHCS (Well History Control System) that was started in the late 1950’s, and first introduced in the early 1960’s. It was based on an 80 column computer punch card with the well identification number as the key to identify the well.

The American Petroleum Institute (API) formed a subcommittee to standardize the well identification numbers in 1962. The API essentially set the standard for the numbering scheme in the US until PPDM took it over in 2010 on the condition that it be updated. In 2013 they (PPDM) updated the first version and the most recent version in June 2014.

The recommendation of the API back in the 1960’s was to turn the numbering scheme over to the appropriate state agency that was responsible for permitting and overseeing well activities in the state. That transition happened on January 1, 1967 with most state regulatory bodies assigning numbers after that time.

The PPDM latest specification stipulates that the well number should come “from the primary assigning authority and it supersedes any other identifier in public use.” So be sure to use some caution when using an API with an old spud date – make sure the state is supporting that number and it is not one that is made up.

No US Well Numbers

Indiana does not issue US well numbers and Virginia just started issuing them in July of this year. Indiana does have ​unique numbers that help identify and locate individual wells within the state.

Final Words

US Well Numbers? I just know I’m going to do the social equivalent of insulting apple pie, football and hot dogs by calling them API Numbers.

Your Turn

What do you think? Leave a comment below.

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John Fierstien

is the Director of Data Inventory. He has worked as a geologist for several E&P companies and as someone who has been helping to create some of the best tools for geologists and geophysicists to help them find oil and gas. He received his Bachelor of Science in Biology and Geology from Central Michigan University and his Master of Science in Geology from the University of Pittsburgh.