COILED TUBING (CT) LOGGING BASICS
Coiled tubing (CT) refers to metal piping, normally 1" to 3.25" in diameter that comes spooled on a large reel. It is used to carry out operations similar to those run on wireline, as well as drilling, fracturing, and acidizing operations. The main benefits over wireline are the ability to pump chemicals through the tubing and the ability to push the tubing  into the hole rather than relying on gravity. This is especially useful in highly deviated or horizontal wells. However, for offshore operations, the footprint for a coiled tubing operation is generally larger than for wireline.

A CT operation can be performed through the drilling derrick or from a self contained unit and a derrick truck. On offshore platforms with no drilling facilities, a self supporting tower can be used instead. On some offshore wells, a semi-submersible has to be utilized to support all the surface equipment and personnel, whereas wireline can be carried out from a smaller and cheaper vessel.

The tool string at the bottom of the tubing is called the bottom hole assembly (BHA). It can range from something as simple as a jetting nozzle, for jobs involving pumping chemicals or cement, to a larger string of logging and steering tools. CT conveyed logging can use wireline inside the tubing or memory tools that record the measurements that are then played back when the tool is retrieved.

A coiled tubing operation in progress

Before the invention of coiled tubing rigs, logging tools were sometimes delivered to the bottom of the hole on the end of drill pipe. The logging tool has to be fitted to the bottom of the drill pipe, and the logging cable run through the drill pipe, a very tedious and time consuming process. Logging tools were often damaged on the way in, as running pipe is a relatively rough process. Logs were recorded while running into the hole, just in case, then also logged on the way out. This process is still done in some parts of the world where coiled tubing rigs are not yet common.

Schematic diagram of a coiled tubing job into a horizontal well. The Bottom Hole Assembly can be a logging tool suite, a drilling and steering assembly, or a hydraulic fracture stimulation setup.