Page 142 - Geothermal Energy Systems Exploration, Development, and Utilization

118 3 Drilling into Geothermal Reservoirs
Normally, the mud pumps are equipped with strong engines and are the main
energy consuming machines of a drill rig because they have to circulate the mud
with high pump rates (up to 4000–5000 l min −1 ) and at high pressure (up to 350
bar). Usually two to three mud pumps are available to be run parallel in order to
give sufficient flow rate.

3.2.1.4 Solids Control Equipment
When the mud comes back to the surface it has to be cleaned of drilled solids in
order to prevent an increase in density due to accumulation of solids content and
to prevent excessive abrasive wear in pumps and lines.
This cleaning is done with a chain of solids control equipment consisting of
• shale shakers (single or double deck) with screens and
• hydrocyclones (desander, desilter).
Often, centrifuges are also used to remove ultrafine solids, which otherwise will
increase the viscosity of the mud excessively.

3.2.1.5 Blowout Preventer (BOP)
This is a safety device which allows to shut-in the borehole in case of an emergency.
The preventer stack normally consists of two to three ram-type preventers and one
annular-type preventer. The annular-type blowout preventer (BOP) is capable of
closing around different diameter drillstring parts and also to shut completely. One
of the ram types is dedicated to close the borehole totally even if any pipe is in the
hole (and is capable to cut drillpipe and even drill collars! (DCs)), the others close
around the drillpipe or DCs of specific diameters.
A special high pressure accumulator closing unit is a substantial part of each
BOP stack.

3.2.2
Drillstring

The drillstring consists of the bottomhole assembly and the drillpipe (DP). All
elements are hollow to allow circulation of drill mud through them (normally
inside down, outside up = direct circulation).

3.2.2.1 Bottomhole Assembly
The bottomhole assembly consists of different items like drill bits, stabilizers and
reamers, drill collars (DCs), a jar, a shock sub, and heavy wall drillpipe.
• Drill bits: Drill bits are the tools to destroy the rock; the selection is made
according to rock types and properties and the applied drilling technique. Some
tools are shown in Figure 3.5.
In general, the following types are used for different applications:

• Rollercone bits: Rollercone bits have (mostly) three conical shaped rollers, which
are equipped either with steel teeth or with special inserts made of tungsten