What Does Borehole Narrowing Mean?
Borehole narrowing is a narrowing of the diameter of a borehole that results from the passage of a drill bit through areas of shale or, sometimes, salt.
The shale/salt undergoes a change in its makeup when the stress change of drilling exceeds the shale's strength. The salt/shale is so fragile it becomes a plastic flow within the drilling mud and fills in (narrows) the bore somewhat gradually, as opposed to the immediate nature of an outright borehole collapse.
Trenchlesspedia Explains Borehole Narrowing
Borehole narrowing is called "creep under the overburden pressure" because it's time-dependent and creeps into place over time, and the overburden pressure is the stress to which the rock is subjected as the drill and drilling fluid passes through it.
As the borehole narrows, the driller will notice an increase in the drag and torque on the drill. Ultimately, borehole narrowing can lead to pipe sticking (and the cost of a wireline unit and operator to remove the stuck pipe) and greater difficulty in installing pipes or conduits within a bore that's narrowed so it's too small to accommodate them.