There are a number of problems that can occur while drilling a well. Whether a drillstring breaks and falls to the bottom of the wellbore or a bit breaks, accidents happen. Even pipe or a tool can fall from the rig floor into the bottom of the well.
This stray equipment that has fallen into the well is referred to as fish or junk, and regular drill bits cannot drill through it. Should a fish fall into a well, fishing is required to remove it.
In order to perform fishing on a well, drilling must be shelved and special fishing tools employed. Each tool is specially crafted to perform a specific function, or retrieve a certain type of fish. Most fishing tools are screwed into the end of a fishing string, similar to drillpipe, and lowered into the well.
There are two options to recover lost pipe. The first is a spear, which fits within the pipe and then grips the pipe from the inside. On the other hand, an overshoot may be employed, and this tool surrounds the pipe and grips it from the outside to carry it up the wellbore.
When a fish is difficult to grip, a washover pipe or washpipe is used. Made of large-diameter pipe with a cutting surface at the tip, washpipe is run in the well and then the cutting edge grinds the fish to a smooth surface. Then drilling fluids are pumped into the well to remove debris, and another tool is used to retrieve the remaining fish.
Sometimes, a junk mill and boot basket are used to retrieve fish from the wellbore. In this instance, a junk mill is lowered into the well and rotated to grind the fish into smaller pieces. A boot basket, also known as a junk basket, is then lowered into the well. Drilling fluid is pumped into the well, and the ground parts of the fish are raised into the basket and then to the surface by the boot basket.
In order to recover casing that has collapsed within the well or irregularly shaped fish, a tapered mill reamer can be used. Permanent and magnetic magnets are employed to reclaim magnetic fish, and a wireline spear uses hooks and barb to clasp broken wireline. Additionally, an explosive might be detonated within the well to break the fish up into smaller pieces, and then a tool such as a junk bucket is used to retrieve the smaller items.
When a fishing professional is unable to determine which fishing tool might work best to retrieve the fish, an impression block is used to get an impression of the fish and allow the professional to know with what exactly he or she is dealing.
Fishing a well may take days to complete, and during this time, drilling cannot occur, although the operator is still responsible for drilling fees. Some drilling contractors offer fishing insurance, making operators not responsible for rig fees during fishing operations.