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==Drilling problems==
==Drilling problems==
Occasionally, something will go wrong and a piece of equipment is lost down the well. For example, the drill pipe may twist off somewhere along its length and fall to the bottom of the hole. The drilling operation will come to a halt unless the foreign object or fish, as it is known, is "fished" out; that is, physically removed from the hole.<ref name=Woods_1992>Woods, A. M., 1992, [[Fishing]], ''in'' D. Morton-Thompson and A. M. Woods, eds., Development geology reference manual: AAPG Methods in Exploration Series 10, p. 83-86.</ref> Specialist tools are available for fishing operations. Sometimes the fishing operation can last many days.
Occasionally, something will go wrong and a piece of equipment is lost down the well. For example, the drill pipe may twist off somewhere along its length and fall to the bottom of the hole. The drilling operation will come to a halt unless the foreign object or fish, as it is known, is "fished" out; that is, physically removed from the hole.<ref name=Woods_1992>Woods, A. M., 1992, [[Fishing]], ''in'' D. Morton-Thompson and A. M. Woods, eds., Development geology reference manual: AAPG Methods in Exploration Series 10, p. 83-86.</ref> Specialist tools are available for fishing operations. Sometimes the [[fishing]] operation can last many days.
Every now and again, the hole will collapse in on itself. This will happen where the earth stresses exceed the rock strength. Salt sections or shale sections at shallow depths containing water-sensitive clays are prone to this. Water-sensitive clays can expand by reacting with drilling fluids, particularly low salinity muds. This can cause the borehole wall to founder and bury the drill bit irretrievably. A decision may then be made to branch off from what hole is left, and this is called sidetracking.
Every now and again, the hole will collapse in on itself. This will happen where the earth stresses exceed the rock strength. Salt sections or shale sections at shallow depths containing water-sensitive clays are prone to this. Water-sensitive clays can expand by reacting with drilling fluids, particularly low salinity muds. This can cause the borehole wall to founder and bury the drill bit irretrievably. A decision may then be made to branch off from what hole is left, and this is called sidetracking.