1. n. [Formation Evaluation]

The downhole hardware needed to make a log. Measurements-while-drilling (MWD) logging tools, in some cases known as logging while drilling (LWD) tools, are drill collars into which the necessary sensors and electronics have been built. Wireline logging tools are typically cylinders from 1.5 to 5 in. [3.8 to 12.7 cm] in diameter. Since the total length is more than can be conveniently handled in one piece, the logging tool is divided into different sections that are assembled at the wellsite. These sections consist of cartridges and sondes. Different measurements can be combined to make up a tool string. The total length of a tool string may range from 10 to 100 ft [3 to 30 m] or more. Flexible joints are added in long tool strings to ease passage in the borehole, and to allow different sections to be centralized or eccentralized. If the total length is very long, it is often preferable to make two or more logging runs with shorter tool strings.

See related terms: bridle,  cable,  drill collar,  head,  log,  torpedo