Page 90 - Hydrocarbon Exploration and Production Second Edition

Drilling Engineering 77

hole
OD
pipe
tool join

Figure 4.26 Development of key seating (plan view of hole).

An initial estimate of where the string is stuck can be calculated by applying a pull
on the drill string in excess of the drill string weight and measuring the observed
stretch in the pipe. This information may be used to decide where to ‘back off’ the
string if the deeper part cannot be recovered.
If the string indeed cannot be retrieved by overpull, an explosive or chemical
charge is lowered inside the pipe to the top of the stuck interval and the pipe above
the stuck point is recovered after severing the string. Since drilling assemblies and
redrilling of the borehole in a sidetrack are expensive, a further attempt to retrieve
the tubulars (often called a ‘fish’) left in the hole will then be made. This is one
application of fishing operations as described below.

4.7.2. Fishing

Fishing refers to the retrieval of a foreign object from the borehole. Fishing
operations will be required if the object is expected to hamper further drilling
progress either by jamming the string or damaging the drill bit. This ‘junk’ often
consists of small non-drillable objects, for example bit nozzles, rock bit cones or
broken off parts of equipment. Other common causes for fishing are
drill pipe left in the hole (either as a result of twist off, string back off or
cementing operations)
items that have been dropped into the hole which can cause major drilling
problems (e.g. rig floor tools, parts of the drill string).
Bottom hole assemblies and certain types of downhole equipment (e.g. logging
tools, MWD tools) cost several hundred thousand US$. Some logging tools will
have radioactive sources which may need to be recovered or isolated for safety and
legal reasons. However, prior to commencing fishing operations, a cost–benefit
assessment will have to be made to establish that the time and equipment
attributable to the fishing job is justified by the value of the fish or the cost of
sidetracking the hole.
Due to the different nature of ‘junk’, a wide variety of fishing tools are
employed.