Fishing Services are used on a contingency basis when drilling a well. The purpose of fishing services is to retrieve an object, called a “fish” or “junk” from the well. This fish, or junk, maybe in the well as a result of a broken down-hole tool, drill pipe, drill bit or an object that has fallen down the well from the surface. These items cause an obstruction for drilling to progress, as the drill bit is not designed to drill out steel objects. Following fishing, services are required to remove these obstructions.
Fishing services are used at the following stages of the oil and gas field lifecycle:
There are various tools available that are run into the hole to fish an object. Each tool is designed to perform a certain task for a certain type of object lost in the well. Broad categories are 1) Catch Tools, 2) Junk Tools, 3) Milling Tools, 4) Accessory Tools.
- Catching tool (spear) - When a pipe must be fished, an internal catching tool (spear) is used. The tool fits inside the pipe (fish) and grips it for further retrieval, in case a fishing operation failed. The tool should be retrievable mechanically or hydraulically.
- Overshot - When a down-hole tool or pipe must be gripped outside, a tool called overshot is used in case of fishing operation failed. The tool should be retrievable mechanically or hydraulically.
- Washpipe - is a large diameter thin-wall pipe, with cutting teeth at the bottom, that mill the fish to a more manageable shape, after which drilling fluid is used to remove debris and another fishing tool used to remove the remaining fish. This is also called “wash-over operation” and used as a last choice, due to the high risk of stucking in the well.
- Junk Basket is lowered to the well after the fish has been grinded. Drilling fluid would force the debris to travel towards the junk basket, after which the basket is retrieved to the surface
- Fishing Magnets are used for retrieval of any steel debris and small metal particles in the well.
- Junk Magnet is a tool that removes metal particles from the drilling fluid
- Impression Block is a tool that identifies shape, condition and dimensions of the upper end of the fish and position in the well, in order to determine next course of actions and select the right fishing tool.
- Wire-line Spear recovers damaged wireline or wire rope from the well.
- Junk Mill is a tool that grinds the fish into smaller pieces
- Mills - there is various type of milling tools used for specific applications. The list of the tools is diverse and includes tapper mill reamer, cement mills, string taper mills and conebuster mills. The major difference between them is the purpose of the milling job, i.e. light vs. heavy milling, the size and shape of the milling object.
- Whipstock is a tool that allows to deviate the well directionally. It serves as a deflection tool to change the course or during a sidetrack. This tool is used less and less and replaced by bent-housing motors.
- Jars - a jar, hydraulic or mechanical, is a percussion tool that provides jarring power by releasing a stroke/piston inside the tool. The tool installed on top of the fishing assembly and provides rapid pulling force when the fishing string cannot be pulled
- Jar Impact Amplifier - a tool that amplifies jarring power
- Drilling Safety Joint is a deliberately made weakest link in the drill string, to release the fishing string from the drill pipe, if required. It is vital to have it when the fish is caught and secured, but the assembly cannot be removed from the well, due to high tension.
- Bumper subs - serves as a dampener and allows high torque and fluid circulation, when bumping up and down is required. It could be used during pipe and tubing recovery, packer retrieval and heavy milling.
The size and handling capabilities of the equipment vary and primarily driven by the well depth and the trajectory. Offshore deep-water wells need very heavy equipment to handle tubulars. Selection criteria are based on: what was lost, where it is lost, its size and shape, and, how damaged it is.
Overall costs of fishing services are very low and represent less than 3% of a well cost. However, in higher-risk operations, fishing costs may contribute a significant proportion of well cost, in particular when rig days are incurred and a fish cannot be removed for a long period of time.