Fishing is defined as the removal of unwanted objects from an open borehole. These objects, also referred to as junk or fish, can include tools, equipment or broken pieces of drilling bits.
Fishing requires the use of specialized tools, each designed for a specific function to remove various types of fish. These tools are connected to the end of a fishing string and lowered into the open drill well.
Trenchlesspedia Explains Fishing
No drilling operation is perfect and it is very common for broken pieces of equipment or tools to fall to the bottom of the drill well. In order to retrieve these objects, the drilling operation must be suspended in order to commence fishing.
Fishing can be done using a variety of methods. The retrieval of lost pipes can be accomplished using a spear or overshoot method. The spear consists of a tool that is inserted into the pipe which then grips the pipe from the inside. An overshot entails the use of a toll that envelops and grips the lost pipe from the outside to carry it up and out of the open borehole.
When a fish is difficult to remove due to soil or debris surrounding the object, a washover pipe is used. This consists of a large diameter pipe with a cutting edge that slips over the fish and grinds the surrounding debris. Drilling fluid is then pumped into the bore to flush out and remove the debris. With the surrounding soil cleared, another fishing tool can be used to recover the fish.
Other fishing methods include using boot baskets, permanent magnets, tapered mill reamers and wireline spears.