Technical Explanation & Examples

Here is a detailed explanation and example of the API numbering structure:

Example: 42-501-20130-03-00

State Code:

The first two digits of the API number represent the surface location of the state the well is located in. In this example, the surface location of this well is located in state code 42 or Texas. Keep in mind that the bottom hole location of this well may be in a contiguous state such as Louisiana, Oklahoma, or New Mexico, but the API number is based on the surface location. Please refer to a State Code List for a complete listing of state and pseudo-state codes.

County Code:

The third through fifth digits of the API number represent the surface location of the county where the well is located. In this example the well is located in county code 501 or Yoakum County. Again, keep in mind that the bottom hole location may be in a contiguous county, but the API number is based on the surface location.

Please note that all county codes are odd numbers except for certain counties in Arizona and New Mexico and Kern County, California. This was designed for expansion. Kern County's well population has exceeded 99,999 so we use 029 for the first 99,999 wells and 030 for the wells drilled after that.

County codes are also used to designate offshore areas for both state and federal waters. Please refer to a County Code Listing for a complete listing of county and pseudo-county codes.

Unique Well Identifier:

In most states, the sixth through tenth digit of the API number is assigned as a unique number within the county the well is drilled. In a few states, the unique well identifier is based on the permit number and may only be unique within the state.

In most cases the series 00000 has been used for historical wells (i.e. wells drilled prior to the issuance of API numbers). The series 10000-50000 are used by most states for current numbers.

Directional Sidetrack Codes:

The sidetrack code is the eleventh and twelfth digits of the API number. It is used to identify each sidetrack uniquely for the well. A good rule of thumb is to increment the sidetrack code for each unique bottom hole location of the well. In the example above, this is the third sidetrack off the original wellbore.

Event Sequence Code:

The thirteenth and fourteenth digits of the API number indicate how many operations there have been on a single borehole. It is incremented only when part of the borehole identified in the 11th and 12th positions is deepened, recompleted, or worked over. Since the example above shows a code of 00, we know that this is the original drill of sidetrack #3 even though the original and either or both of the previous sidetracks may have been recompleted. (It is important to remember that this code may vary between data vendors based on their database recompletion criteria.)

Original Wellbores

Please note that the sidetrack code (11th and 12th digits) in each instance is 00, whether it is a straight, directional or horizontal hole. It is the original wellbore for this well. The subsequent operation code (13th and 14th digits) is 00, as this is the first operation for each well.

Sidetracks

Please note, the sidetrack code is incremented by 01 for the first sidetrack in each hole, by 02 for the second sidetrack, and so on. However, the subsequent operation code remains at 00 since none of the boreholes have been reworked. A good rule of thumb is any unique bottom hole location will have a new sidetrack code.

Event Sequence Code

Figure 5 shows a vertical wellbore that has been plugged back twice. Therefore, the API numbers associated with this well are:

Figure 6 shows a vertical wellbore that has been plugged back once, sidetracked once, and the sidetrack plugged back once. Therefore, the API numbers associated with this well are (the sequence would be the same if original had been directional):

Please note that even if the plug back to the sidetrack had occurred prior to the plug back of the vertical hole, the numbering would be as above.