Page 335 - Drilling Technology in Nontechnical Language

326 Drilling Technology in Nontechnical Language Second Edition

Explosives can also be run on wireline or pipe to break up junk, which
can then be dealt with using a washover tool or a drill or mill. The explosive
used is a shaped charge, designed so that the force is projected downwards
onto the junk.

Fishing for wireline and logging tools

Finally, wireline sometimes breaks, resulting in a tricky fishing job.
Wireline is fished with a spear that has barbs on the side. The spear is run

into the top of the wire and rotated. The barbs catch on the wire and cause
the wire to wrap around the spear so that it can be pulled out.
If a logging tool gets stuck, the best way to fish it is by stripping over

the wire. The wire is cut at the surface and is fed through a special tool that
latches onto a profile at the top of the logging tool. This fishing tool is then

run in the hole over the wire so that the wire guides the fishing tool to the
logging tool. It is a slow job because each stand of pipe has to have the wire
fed through it before it can be connected to the string at the rotary table.

Economics of shing

Fishing is an expensive activity because normal operations cannot
continue until the situation is resolved. If the fish cannot be recovered, the

alternatives are as follows:
▫ Place cement on top of the fish and drill a sidetrack around it.

▫ Finish the well early (if enough of the original objectives can
be achieved in the shallower wellbore).
▫ Abandon the well (either drill another well or abandon
the project).
These alternative courses of action have a cost attached to them. There

comes a point when fishing should be discontinued and the best alternative
pursued. This is an economic decision.

Fishing jobs have the best chance of success on the first attempt. If
the first attempt fails, it is only worth trying again if, in the process of

the first attempt, something was learned that would improve the chances
of success on the second attempt. The longer a fish stays in the hole, the

harder it becomes to recover it. This is because hole conditions deteriorate

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