In the oil and gas industries, coiled tubing refers to a very long metal pipe, normally 1 to 3.25 in (25 to 83 mm) in diameter which is supplied spooled on a large reel. It is used for interventions in oil and gas wells and sometimes as production tubing in depleted gas wells. Coiled tubing is often used to carry out operations similar to wirelining. The main benefits over wireline are the ability to pump chemicals through the coil and the ability to push it into the hole rather than relying on gravity. Pumping can be fairly self-contained, almost a closed system, since the tube is continuous instead of jointed pipe. For offshore operations, the ‘footprint’ for a coiled tubing operation is generally larger than a wireline spread, which can limit the number of installations where coiled tubing can be performed and make the operation more costly. A coiled tubing operation is normally performed through the drilling derrick on the oil platform, which is used to support the surface equipment, although on platforms with no drilling facilities a self-supporting tower can be used instead. For coiled tubing operations on sub-sea wells a Mobile Offshore Drilling Unit (MODU) e.g. semi-submersible, Drillship etc. has to be utilized to support all the surface equipment and personnel, whereas wireline can be carried out from a smaller and cheaper intervention vessel. Onshore, they can be run using smaller service rigs, and for light operations a mobile self-contained coiled tubing rig can be used.
The tool string at the bottom of the coil is often called the bottom hole assembly (BHA). It can range from something as simple as a jetting nozzle, for jobs involving pumping chemicals or cement through the coil, to a larger string of logging tools, depending on the operations.
Coil tubing has also been used as a cheaper version of work-over operations. It is used to perform open hole drilling and milling operations. It can also be used to fracture the reservoir, a process where fluid is pressurised to thousands of psi on a specific point in a well to break the rock apart and allow the flow of product. Coil tubing can perform almost any operation for oil well operations if used correctly.
Coiled tubing is usually employed to conduct operations such as drilling, sidetracking, production tubing and low-bottomhole pressure drilling etc. This long pipe of metal is curled up on a big roll and it is straightened out before it is pushed inside a wellbore. The tool string attached at the bottom of this coil is referred to as BHA, i.e., bottom hole assembly. Coiled tubing operations are carried out on the surface as well as sub-sea wells, where drillship, or semi-submersible equipment are brought into use. Through coil tubing, all kinds of oil well operations including milling, drilling and fracturing can be performed.