This is the action of enlarging a whole for drilling or for pipeline. One starts often with a small pilot whole which is easier and cheaper to do and makes it easier to ensure that the whole is directed properly. Then if a greater hole is needed, either in drilling operations because something went amiss and requires further and large casing, or simply because it is pre-planned to make small pilot whole when doing for instance horizontal drilling, the next step is called reaming and uses as ream to enlarge the size of the hole.
Pre-reaming operations are carried out to enlarge the drilled hole to a size suitable for accepting the product pipe. Pull-back pipe is added behind the reamer. Depending upon the pipe diameter to be installed several pre-reaming operations may be necessary, each progressively enlarging the hole.