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.
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
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.