Page 25 - Well Logging and Formation Evaluation

Basics 15

conditions during drilling, and the logging program may be adjusted to
take this into consideration. In general it is found that the longer a hole is
left open, the greater the likelihood of hole problems occurring. Three
sorts of sticking are found to occur:

• differential sticking,
• key seating, and
• holding up.

Differential sticking occurs when either the cable or the toolstring
becomes embedded in the borehole wall and gets held in place by the dif-
ferential pressure between the mud and the formation. In such a situation,
it is impossible to move the toolstring either up or down. The usual pro-
cedure is to alternately pull and slack off on the tool, pulling up to 90%
of the weakpoint of the cable (that point at which the cablehead will shear
off the top of the toolstring). Strangely enough, this procedure is often
successful, and a tool may become free after 30 minutes or so of cycling.
Key seating occurs when a groove is cut into one side of the borehole,
which allows the cable, but not the toolstring, to pass upward. The tool-
string is effectively locked in place at a certain depth. Unfortunately this
often means that when the weakpoint is broken, the toolstring will drop
to the bottom of the hole and may be hard to recover or may be damaged.
Holding up occurs when a constriction, blockage, dogleg, or shelf
occurs in the borehole such that the toolstring may not pass a certain depth,
although it can be retrieved. The usual practice in such a situation is to
pull out of the hole and reconfigure the tool in some way, making the tool-
string either shorter or, in some cases, longer in an attempt to work past
the holdup depth.
Once it is no longer possible to recover a tool on wireline, there are two
options: cutting-and-threading or breaking the weakpoint. In the cut-and-
thread technique, the cable is cut at the surface. The drillpipe (with a
special fishing head called an overshot) is run into the hole with the cable
being laboriously threaded through each stand of pipe. At a certain depth
it may be possible to install an SES, which means that when the toolstring
is being recovered, log data may be acquired in a similar fashion to pipe-
conveyed logging. If the weakpoint has been broken (either accidentally
or on purpose), cutting-and-threading is not possible. Then the pipe is run
into the hole until the toolstring is tagged, although often the tool will
drop to the bottom of the hole before it can be engaged by the overshot.
Most oil companies will specify that they do not wish to break the
weakpoint on purpose, even if the cut-and-thread technique is much