Definition - What does Tail String mean?
A tail string is a section of piping that's added onto the drill string for extension purposes. The tail string is normally applied to the bottom hole assembly (BHA) to assist in reaming and back reaming or the retrieval of the drill string after boring and reaming is complete.
Trenchlesspedia explains Tail String
In some applications, such as tunneling through rock or reaming large distances, a tail string may be used to isolate the rig from from the drill string advancement. This protects the drilling rig from any unexpected surges and shock loads while drilling and provides increased control should surging occur. A variation of the tail string is the telescoping tail string, which offers variable lengths, extending automatically as needed.