Measurement While Drilling - Retrievable Tools

Retrievable Tools

MWD tools may be semi-permanently mounted in a drill collar (only removable at servicing facilities), or they may be self-contained and wireline retrievable.

Retrievable tools, sometimes known as Slim Tools, can be retrieved and replaced using wireline through the drill string. This generally allows the tool to be replaced much faster in case of failure, and it allows the tool to be recovered if the drillstring becomes stuck. Retrievable tools must be much smaller, usually about 2 inches or less in diameter, though their length may be 20 feet or more. The small size is necessary for the tool to fit through the drillstring, however, it also limits the tool's capabilities. For example, slim tools are not capable of sending data at the same rates as collar mounted tools, and they are also more limited in their ability to communicate with and supply electrical power to other LWD tools.

Collar-mounted tools, also known as Fat Tools, cannot generally be removed from their drill collar at the wellsite. If the tool fails, the entire drillstring must be pulled out of the hole to replace it. However, without the need to fit through the drillstring, the tool can be larger and more capable.

The ability to retrieve the tool via wireline is often useful. For example, if the drillstring becomes stuck in the hole, then retrieving the tool via wireline will save a substantial amount of money compared to leaving it in the hole with the stuck portion of the drillstring. However, there are some limitations on the process.