Today the geothermal guys started
digging the wells that will transfer warm air between the subsoil and
the house. The rig for drilling is basically two big trucks, one drill
rig with a large drill bit, and a water truck to pump water alongside
the drill into the borehole. Two guys are able to run the entire
outfit.
Carbide drill bits are used
to bore holes about six inches wide in diameter. The bit is fit on the
end of a long pipe, maybe 35 feet long and held vertical by being screwed
into the drill motor that sits at the top of the drill rig's boom. The
pipe is then rotated and the whole drill motor pushes it down into the
hole. The pipe that holds the drill bit is hollow, allowing water thickened
with a clay called bentonite to be pumped down the pipe, pushing out
the middle of the drill bit head itself. The water mixture lubricates
the process, keeps the drill bit cool, and also flushes the drilled
earth up and out the hole.
The operators have a well-organized
regimen, removing the drill motor from the pipe, raising the motor to
attach another section of pipe, and then drilling down through that.
They just repeated that process all day, occasionally flushing out the
hole with more water, changing drill bits as needed. Apparently they
use one drill bit per hole, more if the subsoil is particularly hard
or they're drilling through rock.
It was amazing to see how fast
they dug through the first pipe's length, not even 5 minutes and the
drill had pushed all the way down. The operator told me the first 30
or 50 feet is all sand, loose gravel and some clay at our site. Easy
drilling.
The initial plan had been 4
wells for the geothermal, each about 150-170 feet. But with the very
first hole, at about 110-130 feet they hit a combination of hard, cement-like
gravel and black granite. Hitting that hard layer meant that it made
more sense to change their plan, and drill 5 shallower holes in place
of the four deep ones originally planned.
After drilling the wells the
crew pumped them with more bentonite. This means that the hole is sealed
to prevent adjacent aquifers being contaminated. The pipe going into
and coming back out of the hole will carry liquid through it as a medium
for transferring heat, but there will be no mixing with the groundwater.
A Wikipedia search brought
up this information:
"Bentonite is an
absorbent aluminium phyllosilicate generally impure clay
consisting mostly of montmorillonite, (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2ยท(H2O)n.
Two types exist: swelling bentonite which is also called sodium
bentonite and non-swelling bentonite or calcium
bentonite. It forms from weathering of volcanic ash, most often in the presence of water."
Not sure if they were using
the swelling or non-swelling versions. I'll ask tomorrow.
Factoids of the day: In 2005
USA was the leading producer of bentonite, with roughly 1/3 the world's
share. The majority of the high-grade commercial bentonite in the US
comes from the area between the Black Hills in South Dakota and the
Big Horn Basin in Montana.
Wikipedia also has this to
say: "Supposedly the world's largest current reserve of bentonite
is Chongzuo in China's Guangxi
province"
Geoff