Definition - What does Borehole Collapse mean?
Borehole collapse is the opposite of borehole stability. It begins when the drilling fluid's ability to stabilize the walls of the bore is compromised by one or more of several factors related to the drilling fluid. This means the drilling fluid must be the right type, the right consistency and introduced into the bore at the right pressure.
Trenchlesspedia explains Borehole Collapse
Drilling fluid, or mud, is at the heart of borehole stability. The mud is pumped into the borehole not only to lubricate the bit's passage but to stabilize the walls of the bore. Because it comes out of the drill string under pressure and because it's effectively denser than the surrounding earth when it does so, the mud packs itself into the soil surrounding the bore only to the extent necessary to stabilize the soil around the borehole.
If the mud is too dense or under too much pressure, it perforates the walls of the borehole, precipitating collapse or narrowing of the borehole. If it comes out of the drill string under just a little too much pressure or it's a little too thick, it might precipitate borehole enlargement or a fracture in the borehole walls.
With too little pressure, the borehole can narrow or collapse; an expensive proposition should the bottom hole tool (BHT) or drill pipe become entrapped in the borehole.