18. Fishing

A “fish” is a piece of equipment, a tool, a part or all of the drill string that is lost or stuck in the hole. Small pieces, such as a bit cone, or any other relatively small nondrillable items, are called junk or “fish” in the hole. These must be removed or fished out so that drilling operations can continue.

A number of ingenious tools and techniques have been developed to retrieve a fish. Fishing tools are divides them into two groups:-

1) Tools used to fish junk .

2) Tools used to fish pipe.

1- Fishing for junk:

When a relatively small piece of equipment (junk) is lost in the hole, it may be retrieved using one of the following teals "Junk" or "boot" sub. This is run immediately above the bit to catch small junk thrown up by turbulence. It is normally run before running a diamond bit so that no fragments can damage the bit.

"Finger-type" or "poorboy" junk basket:

This cuts a small core, after which weight is applied to the tool and bends the bevelled fingers inward to trap the junk inside. This can be made "on the spot" from casing.

Core-type junk basket:.

This is essentially a finger-type junk basket but has a mill shoe. Instead of applying weight to contain the core, this tool has "catchers" which grip the core and junk on the trip out.

Fishing Magnet:

This is used for picking up steel fragments.

Jet bottomhole cutter:

This is used when junk is so large or oddly shaped that it cannot be readily retrieved with regular junk baskets. It breaks up the junk into small pieces by use of a shaped explosive charge. The junk may then be retrieved using one of the above tools.

Grapple or rope spear

This is used to retrieve wireline in the hole.

2- Fishing for pipe:

When the drillstring has actually parted or is stuck in the hole, the operation for correcting the situation is called "fishing." (If the fish cannot be recovered, it must be cemented off and the hole is side-tracked). Some of the tools used for fishing are described below.

Mill

Milling is sometimes necessary in order to dress the top of a fish so that the selected fishing tool is able to make a firmpositive catch.

Overshot

This is probably the first tool to be used when it is established that the top of the fish is fairly smooth. It can be a rotary taper tap or die or a more modern type which works like a set of "slips" in a core barrel to engage the top of the fish.

Wall-hook guide

This is used if the tap of the fish is in a washed-out section of hole, and it takes the place of the regular guide on the bottom of an overshot. It engages the fish and guides it into the overshot.

Jar

This is used when a drillstring is stuck or when a "fish," caught in an overshot, cannot be pulled from the hole. In a normal drillstring a jar may be included in the middle of the collars, whereas in a fishing string it is placed immediately above the fishing tool. Jarring provides a method for giving an upward jerk to free the pipe. It works similar to a trip-hammer.

Free-point indicator and string shot

When fishing has not been successful, this is used to determine at what point in the hole the fish is stuck. It is an electronic instrument that can sense torque or movement; it is lowered by wireline as far as possible into the hole and raised slowly while the string is stressed. Below where the pipe is stuck no torque will be sensed, but the instrument gives a positive indication as soon as the free point is reached.

The free point indicator is raised until the string shot is positioned opposite a tool joint, one or two joints above the stuck point. Left-hand torque is applied to the drillpipe, and the primacord string shot is exploded. Loss of torque in the drillpipe is a definite indication that the tool joint has been loosened. The "back-off" is completed by further left hand rotation and by picking the pipe up a few feet.

Washover

This is a large-diameter pipe with a rotary cutting shoe on the bottom. It is run over stuck pipe in order to free it before fishing.

Spotting

This is used when jarring alone will not free the fish. Oil or special chemicals are spotted around the fish in an attempt to penetrate the wall cake, causing it to deteriorate and make the pipe slick. Spotting with water when differentially stuck, and acid spotting when stuck in limestone, are often used in an attempt to free the pipe.

If spotting and jarring do not free the fish, the "free point" is located and the portion of the drillpipe above is "backed off." washover operations can then be carried out to retrieve the stuck portion of fish.

Safety joint

This is a coarse-threaded joint which may be easily released and run above a fishing tool in case it should happen that the fish cannot be freed and the fishing tool cannot be released.