Fishing is the technology of removing any undesirable object from the wellbore.
The " fish " may include all or a part of the drill string, smaller pieces of equipment such as bit cones, pieces of tools, or any materials accidentally dropped into the wellbore.

There are many different types of fishing jobs and tools. Which tool to be used depends on the type of fish in the hole; whether the fish is stuck or free, whether it is in a cased hole or an open hole, the condition of the top of the fish and the condition of the hole at the top of the fish should be taken into considerations. At the beginning, we must show the fishing steps to get the junk:

  1. Determine the fish; its size, inner and outer diameter,material ... etc.
  2. Design or select a suitable fishing tool.
  3. Use the tool selected to locate, latch and retrieve fish.
  4. Finally, as with fishing, trial is required.

We have 2 types of fishing :

  • Open Hole Fishing.

  • Cased Hole Fishing.

Let’s dig in details to get more advanced information about open hole fishing.
This section discusses fishing in open hole environments. Open hole environments present unique challenges for successful fishing jobs. A free point and back-off must be made first; they will be illustrated later. After thefree point has been located, study the well plot carefully and check for true dog legs before deciding where to make the back-off. The back-off must be made at the depth that offers the best chance of connecting to the fish.

NOTE : Do not back-off just below a dogleg or in the hole curve if possible.

Develop a plan for the most effective bottom hole configuration and jar placement prior to the back-off.
Do not place the jars in the curve (Figur 1). Place them either in the vertical section or in the straight section below the curvature.
Now we discuss some specialized tools used in Open Hole Fishing.

  1. Pin tab or Screw-in-sub (Fig 2) provides an economical means to retrieve a tabular fish that restrained from rotation.
    Pin tabs are designed to mate with the boxup tool joint, allowing wireline tools to be run through the tab. Screwing in is the only option when down jars will be used due to the releasing mechanism of most engaging tools.

    NOTE : Pin tab is run in conjunction with a safety joint.

  2. Overshot (Fig 3) is frequently used in fishing recoveries because of its versatility. Normally the overshot or screw in sub is connected to the bottom of the bumper sub and fishing jars on the fishing assembly . As the tool is lowered over the fish, the top of the fish passes through the tool into the bowl. When the assembly is raised, grapples (Fig 4) engage the fish at a lower point and it is worked free and pulled upward. If the top of a fish is bent, twisted or broken, it should be dressed-off to provide a clean top so, the grapple can secure it firmly.
    Dressing off is achieved with a spirit or a hollow mill . Several types of mills are available for this purpose and other jobs.
    A grapple or similar slip mechanism on the overshot grips the fish, allowing the application of tensile force and jarring action.

    Figure 4

  3. The preferred tool for fishing inside drill-pipe, tubing or casing is the releasing spear (Fig 5).
    When the fish is stuck off bottom, a wash pipe spear may be run in conjunction with the wash pipe and screwed into the fish prior to the wash-over procedure.
    This prevents a fish freed by the wash over operation from dropping to the well bottom and damaging the wellbore drilling string or drilling bit.
    While running the spear, check to ensure that it is the correct size for the pipe to be caught and is properly assembled.
    A packer assembly may be run on the bottom of the spear to pack off. A bumper jar is also required to help in releasing the spear. To engage and pull the fish, perform the following steps.

    • Lower the string slowly until the spear has entered into the fish to the desired depth or the stop sub.
    • If the fish is not caught, rotate one full turn to the left to make sure the grapple is still in the catch position. This turns the mandrel down through the grapple, putting the grapple in the catch position. A straight pull will then wedge the grapple into positive engagement with the fish.

    Figure 5

  4. Fishing magnets (Fig 6) are especially cost-effective for retrieving smaller ferrous objects such as bit cones and slips and mill cuttings.
    Permanent magnets have circulating ports around the outer edge so that fill and cuttings can be washed away exposing the junk for proper magnetic contact. Fishing magnets are primarily used to pick up the bit cones, but they can pick up all types of small objects with magnetic properties. Magnets should be run on a pipe instead of a wireline, because of the capability to circulate and rotate. It is necessary to wash the top of the fish sufficiently so that enough surface area will contact the magnetic pole plate. Run a boot basket directly above the magnet to catch any loose bearings or small pieces of junk.

    Figure 6

    NOTE : Most, but not all tungsten carbide bit inserts are nonmagnetic.

    Check to see if the magnet is charged properly by attaching a 20-lb. sledge hammer to the face of it. If the magnet will support the weight of a sledge hammer, it will likely hold any small pieces of junk in the wellbore. The lifting capacity is a function of size, area, and charge.The OD of the magnet should be 1/4- to 1/2-in. less than hole size.
    Procedure run Magnet:

    1. Trip in the hole with the magnet and stop about 30-ft from the bottom.
    2. Circulate and ream to the bottom a couple times to rake any loose junk off the wall and top of the fish.
    3. Circulate at a good rate a couple inches off the fish to clean off any cuttings.
    4. Rotate a couple rounds while slacking off to help roll the fish into the face of the magnet.
    5. Stop rotating and set 3,000 to 4,000 pounds of weightdown on the junk, then shut the pump off.
    6. Pull out of the hole slowly without rotating. The guide should prevent the fish from being rubbed off while pulling out of the hole.
    CAUTION : Be careful, the magnet is not a drilling tool.

  5. Free Point Indicator

    A free point indicator is an electronic strain gauge running on a single conductor cable. It is used to measure the amount of torque or stretch in a string of stuck pipe. The tool is anchored to the pipe ID at two fixed points by either bow springs, dogs or magnets.
    When stretch or torque is applied at the surface, the gauge electronically transmits this information to the surface via the wireline. Following are the five requirements for a successful back-off:

    1. The connection to be backed off must be free.
    2. The correct amount of left-hand torque is needed.
    3. The connection being shot must be at the neutral weight.
    4. The proper size string shot is required.
    5. The shot must be across the tool joint when fired.

    A string shot can be run in a combination with the free point tool and usually will not damage the connection. It can be run in a smaller ID than cutters.
    A string shot can be used for an outside back-off as a jar shot, to jar a test plug out of a well head, to knock nozzles out of a bit, or to knock plastic and corrosion off the ID of pipe prior to making a chemical cut.
    An inside-outside back-off collar is run above an overshot. The overshot is attached to a plugged fish. A 35-ft. long string shot is deflected to the outside of the fish and fired.
    The plugged fish is retrieved. Avoid pulling pipe into the wall/free point with torque (Fig 7).

String Shot Back-off

A string shot back-off is the process of unscrewing a tool joint or coupling with explosives at a predetermined depth. The string shot the blasting cap that is used to perform the back-off. The string shot back-off method uses the following steps to loosen the stuck pipe.

  1. Apply left hand torque to a string of stuck pipe.
  2. Run the collar locator above the shot to locate the connection to be backed off.
  3. Detonate the shot across a free connection to create a back-off.

Where to Back-off ?

It is important to figure out where to back-off. Performing a back-off in the wrong location can be a costly mistake.
- Do you intend to fish the well?
* No - Back-off as deep as possible.
* Yes – At least one free joint above stuck point. Two free joints above a key seat. 100 feet below a casing seat.

Now, it’s your turn to get specialized details about this subject, you just need to read more and more, then you will get technically-improved.
So, you have learnt:

  1. What back-off is and when it is applied.
  2. How screw-in-sub works.
  3. How overshot works and its limitations.
  4. Spear precautions and when it is preferred.
  5. The running procedure of the fishing magnet.
  6. Free point measurement and tool requirements.
  7. How a sting shot back-off works.

Try not to stop your passion anymore and don’t deprive yourself of being a well-educated student with high level of petroleum technicality.
You just need to improve your research skill and you will reach your target.