Applications Manual Bosch Geothermal Heat Pumps
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Bosch Thermotechnology Corp.
Data subject to change
10.6 Typical Methods Used for Vertical
Borehole Construction
10.6.1 Air Rotary Drilling
Air Rotary Drilling is used to construct vertical boreholes
in rock formations (Fig. 63). High pressure, high volume
compressed air is forced down the drill string to ush
cuttings out of the borehole. A Down Hole Hammer (DHH)
is attached to the bottom of the drill string and used to
break rock in a process similar to a jack hammer breaking
concrete. No uids are reclaimed or re-circulated when
using this method.
Fig. 63
Courtesy of Durbin Geothermal
10.6.2 Mud Rotary Drilling
Mud Rotary Drilling is the most common method used in
vertical borehole construction (Fig. 64). It is often viewed
as the most ef cient method of drilling unconsolidated or
soft overburden formations as well. Fluid, usually clean
water with engineered bentonite additives is circulated
from a mud tub through a suction hose by a mud pump
and forced down the drill string through portals in the drill
bit to ush cuttings out of the borehole while the bit is fed
down the hole. The uid and cuttings are then run through
a series of separation chambers so that the solids can be
removed and the uid re-circulated.
Fig. 64
Courtesy of Durbin Geothermal
10.6.3 Combination Air and Mud Rotary Drilling
Combination Air and Mud Rotary Drilling is typically used
when unstable formations are found on top of hard rock
formations (Fig. 65). The process is relatively simple but
more time consuming than either separate method. The
mud rotary method is used to drill down to the bedrock
zone. When it is necessary to stabilize the formation steel
casing is inserted into the hole. The air rotary method is
then used to complete drilling to the desired depth. Once
the loop has been inserted and the borehole has been
properly grouted the steel casing can be removed from the
hole.
Fig. 65
Courtesy of Durbin Geothermal
