Coiled tubing

Coiled tubing

Coiled tubing refers to metal piping normally 1 to 2 centimeters in diameter and is used for various interventions in oil and gas wells, which comes spooled on a large drum. The main advantage of a wireline is the ability to pump chemicals through the coil and then push it into a hole rather than relying on gravity. However, it consumes more space and offshore requires larger and more robust rig, which makes it more expensive on small platforms. Onshore they can be run using smaller service rigs and for light operations, the mobile self-contained coiled rig.

The tool string which lies at the bottom is called the bottom hole assembly. It can range from a simple jetting nozzle and is used for jobs such as pumping of cement or chemicals through the hole or it can be a larger string of logging tools depending on the type of operations.

Uses

Coiled tubing is used for the following purposes:

Circulation

The most popular use of coiled tubing is circulation. A hydrostatic head maybe initiating flow of formation fluids due to its weight, the safest but not the cheapest solution would be to circulate out the fluid using a gas, most often nitrogen is used. By running in coiled tubing from top to the bottom of the hole and pumping in gas the kill fluid can be forced out to production. Circulating can also be used to clean out light debris.

Pumping

Pumping through coiled tubing can also be used or dispersing fluids to a specific location in the well such as from cementing perforations or for chemical washes of downhole components such as sandscreens. In former case coiled tubing is comparatively more efficient as compared to simply pumping the cement from surface as allowing it to flow through the entire completion could potentionaly damage important components, such as the down hole safety valve.

Drilling

The modern drilling technique involves coiled tubing instead of a conventional drill pipe. This has the advantage of less effort to trip in and out of the well. Instead of rotating the drill, bit by using a rotary table or top to drive at the surface, it is turned by a downhole motor and powered by the motion of drilling fluid pumped from surface.

Lodging and perforating

These tasks form the realm of wireline. Since coiled tubing is rigid, it can be pushed into the well from the surface. This is an advantage over wireline which depends solely on the weight of the tool string to be lowered into the well.

Coil tubing rigup

The main engine of a coiled tubing invention is the injector head. This component contains the mechanism to push and pull the coil in and out of the hole. An injector head has a curved guide beam on the top called gooseneck which threads the coil into the injector body. Below the injector is the stripper, which contains rubber pack of elements providing a seal around the tubing to isolate the wells pressure.