Page 170 - Geothermal Energy Systems Exploration, Development, and Utilization

146 3 Drilling into Geothermal Reservoirs

Drillpipe

Keyseat
cross section

Keyseat
Drill
collars

Soft formation
Hard formation

Figure 3.19 Keyseat. (From Schlumberger Oilfield Glossary, 2009.)

of structural geology and geomechanics to understand borehole breakout initia-
tion and to specify a certain mud window (Moeck, Backers, and Schandelmeier,
2007; Backers, Stephansson, and Moeck, 2008).
• Stuck pipes or tools: Trouble shooting by down pumping a high viscose pill (for
cleaning the borehole/remove filtercake), decrease of mud weight, and reducing
friction coefficient of the mud by adding friction-reducing additives if differential
sticking is assumed; increase of mud weight if cavings and borehole breakouts
are assumed; eventually cleaning circulation run.
• High dog legs (depending on diameter of casing): Hole sections with high angle
changes (inclination and azimuth) over a short distance (= ‘‘dog legs’’) may cause
problems for both drilling and run casing. During drilling the rotation of the
drillpipe body under a certain sideforce may cause to form a ‘‘keyseat’’ in the
hanging side of the borehole (Figure 3.19). When pulling the string the next time
the drillpipe may jam in the keyseat with the tool joint due to its larger diameter.
This may result in a fishing operation and – in worst case – even in the loss of
the drillstring part (and of course the hole section) below the keyseat.
• Fishing: The activity of recovering parts (e.g., from a drill bit or a broken
drillstring) which had been left in the borehole is called fishing;consequently,
thelostpartiscalled fish. Depending on the specific situation and shape and size
of the ‘‘fish’’ special tools are available to increase the chance to catch the fish.
There are tools to catch a drillstring part internally or externally, to catch small
pieces of junk and a lot of other tools. However – like with real fishing – success
is never guaranteed.