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Line 30: − − [[file:fishing_fig1.png|thumb|{{figure number|1}}Differential sticking. © Exploration Logging, 1979.]] − − [[file:fishing_fig2.png|thumb|{{figure number|2}}Key seating. © Short, 1981; courtesy of PennWell Books.]]
→Reasons for fishing
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==Reasons for fishing==
==Reasons for fishing==
[[file:fishing_fig1.png|left|thumb|{{figure number|1}}Differential sticking. © Exploration Logging, 1979.]]
[[file:fishing_fig2.png|thumb|{{figure number|2}}Key seating. © Short, 1981; courtesy of PennWell Books.]]
''Fishing'' (in the oilfield sense) refers to the recovery of unwanted material left in the wellbore. Numerous situations can occur that require fishing:
''Fishing'' (in the oilfield sense) refers to the recovery of unwanted material left in the wellbore. Numerous situations can occur that require fishing:
* Items dropped into the hole from the rig floor.
* Items dropped into the hole from the rig floor.
* Failure of surface equipment, especially pumps, which allows the hole to cave in and stick the drill string.
* Failure of surface equipment, especially pumps, which allows the hole to cave in and stick the drill string.
* Differential sticking to a permeable formation (Figure 1).
* Differential sticking to a permeable formation ([[:file:fishing_fig1.png|Figure 1]]).
* Key seating, where a slot is worn into the wall of the well (Figure 2).
* Key seating, where a slot is worn into the wall of the well ([[:file:fishing_fig2.png|Figure 2]]).
* Twist-offs resulting from a stuck drill string being rotated until the pipe shears above the sticking point.
* Twist-offs resulting from a stuck drill string being rotated until the pipe shears above the sticking point.
* Loss of portions of downhole equipment, such as stabilizer fragments and bit cones.
* Loss of portions of downhole equipment, such as stabilizer fragments and bit cones.
* Pulling the drill string apart when trying to free stuck pipe.
* Pulling the drill string apart when trying to free stuck pipe.
* Bridging off by swelling formations or a collapsed hole.
* Bridging off by swelling formations or a collapsed hole.
When material is left in the hole due to these or other factors, a decision must be made whether to try to recover the material, to sidetrack around the original material, to abandon the well, or to attempt to complete in a shallower zone. Because all but the first of these options are costly, and the last is often undesirable, an attempt to recover the fish is almost always made.
When material is left in the hole due to these or other factors, a decision must be made whether to try to recover the material, to sidetrack around the original material, to abandon the well, or to attempt to complete in a shallower zone. Because all but the first of these options are costly, and the last is often undesirable, an attempt to recover the fish is almost always made.