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116 3 Drilling into Geothermal Reservoirs
Figure 3.1 Rotary table with kelly bushing und kelly (hexag-
onal). (From Schlumberger Oilfield Glossary.)
Cross section
End-on view
Outside view
Figure 3.2 Illustration of a kelly (hexagonal).
Themodern alternative is thetop drive(Figures 3.3and3.4). This device hasa
built-in engine (either with an electric or hydraulic motor) to rotate the drillstring
and allows traveling and rotating the string without the need of a kelly. However,
very often a rotary table is installed as a backup for the top drive.
3.2.1.3 Mud Pumps
The mud pumps are used to circulate the drill mud in a kind of a ‘‘closed loop’’
through standpipe, swivel, drillstring, and the bit down to the borehole and in the
annulus between the drillstring and borehole wall back upward to the surface.
