So today the geothermal guys from UMR started digging their wells - and let me tell you, that was awesome to watch. I'd never really witnessed a well being drilled before and it was cool. It was basically just two big trucks, one water truck pumping the water they needed and another big drill rig, with two guys operating the whole show.
They use carbide drill bits to bore the holes, about six inches wide. The bit is fit on the end of a long pipe, maybe 35' long. The pipe is held vertical, screwed into the drill motor that is at the top of the drill rig's boom. The pipe then is rotated and the whole drill motor pushes it down into the hole. The pipe that holds the drill bit is hollow allowing water, thickened a little with Bentonite to help it do its job, is pumped down the pipe the whole time, pushing out the middle of the drill bit head itself. This serves to lubricate the process, keep the drill bit cool and also flush the drilled earth up and out the hole. 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 operators were a well oiled machine, 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 can figure about 1 drill bit per hole, more if the subsoil is particularly hard or they're drilling through rock.
I found out that about 110-130 feet down the first hole they hit a combination of hard, cement like gravel and black granite. The initial plan had been 4 wells for the geothermal, each about 150-170 feet. Hitting that hard layer meant that it makes more sense now to drill 5 holes instead, each a little shallower.
After they'd drilled their hole the crew plumped it with more bentonite. This means that the whole is sealed and any aquifers will not be contaminated. The pipe going into and coming back out of the whole will carry liquid through it but there will be no mixing with the groundwater. A quick wikipedia search brought this up:
"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 SD and the Big Horn Basin in MT.
Wikipedia also has this to say: "Supposedly the world's largest current reserve of bentonite is Chongzuo in China's Guangxi province"