Page 171 - Geothermal Energy Systems Exploration, Development, and Utilization

3.6 Drilling a Well 147
– Freeing a stuck pipe: Sometimes it is even necessary to part the drillstring in
the hole intentionally in order to be able to recover at least the upper free part.
This is done in several working steps.
– Freepoint measurement: The first step is normally to measure the ‘‘freepoint’’
by running a special measurement tool on an electric cable into the drillstring.
Such a freepoint measuring tool consists mainly of three parts, an upper
anchoring device, a lower anchoring device, and a strain gauge between both.
Operation is as follows: The tool is anchored in a certain depth and then
tension or torque is applied to the drillstring. If the anchoring depth is above
the stuck point, a change in tension or torque will be measured because of the
small elongation or twist of the drillstring between the two anchoring units.
If no movement is measured, the freepoint is above the measuring depth. The
measurement is repeated until the lowest free connection between drillpipe
joints is found.
– String shot: In a second step, a ‘‘string shot’’ (= some strings of explosive cord)
are lowered on wireline into the found lowermost free drillpipe connection.
Then left-hand torque is applied to the drillstring and the shot is fired. If the
amount of explosive was sufficient and the torque was properly applied the
connection will be unscrewed due to the shock and the left-hand torque.

The working principle is similar to hammering onto a tight screw while applying
torque to it.

– Try to free the stuck part of drillstring: After having pulled out the freed drillstring
part normally, the recovery of the stuck part too is attempted. This is done by
running in a heavy fishing jar which acts like a drilling jar described earlier but
offers remarkably higher blow energy. The jar assembly then has to be screwed
on to the top of the fish and made-up properly; the jarring can then start and will
run for several hours.
If the jarring attempt is not successful it can be tried to ‘‘wash-over’’ the stuck
pipe. This is done with a kind of ‘‘coring operation’’ using a wash-over shoe
on several casing joints which offer an outside diameter smaller than the hole
diameter and an inside diameter large enough to pass over the drillstring or DC.
When the string (or a part thereof) has been freed by the wash-over assembly
that is left in the hole, the washpipe is pulled and again a fishing string with jar
is lowered to be connected with the fish and to try to pull the fish out of the hole.
• Sidetracking: Sometimes a well has to be sidetracked, for example, if a fishing
job had to be stopped without success or because the well did not find the
reservoir in expected condition. Sidetracking is to leave the existing hole in a
certain depth and start from there a new hole. This can be done either in open
hole (mostly done from a cement plug) or inside a casing string (mainly done
from a whipstock).
– Sidetrack from a cement plug: In this case, a cement plug of a certain length
(approximately 100 m) is set in the desired depth of the borehole. After a
sufficient waiting time to assure to get hard cement – normally between