I'm building a new ICF home and the heat gain/loss calcs I've done indicate I'll need ~2.5 tons of heating/cooling.  I'm leaning towards either a ClimateMaster or Water Furnace 2-speed 3 ton system with closed loop heat exchanger.  I live in a heavily wooded area and plan to go with a vertical loop.  Estimates that I have seen show that I will need around 175'-200' of well per ton for my area/soil conditions, so I'm assuming at this point that I will have 3 wells dug 200' deep and spaced ~20' apart in my soon to be front yard.

I currently have a well for home use which is located ~100' away from the proposed geothermal wells.  It is located on the same elevation as the geo wells.  This well is 6" diameter, 100' deep and has ~90' of water with an almost unlimited supply of water.  I initially considered using this water for the geothermal system in an open loop system, but this water has a very high mineral content, so I have ruled that out.  My existing well went through ~30' of hard red clay before hitting shale at which point the water was located.

Assuming I get similar results with my geothermal wells where I go through ~30' of clay then hit shale for the remainder of the bore and the wells fill almost full with water, would there be any advantage to backfilling these wells with grout after the heat exchanger pipes are installed?  My thoughts are that it is hard to beat a full well of water for heat transfer, so why not case the wells through the clay as required, install the heat exchanger pipes, then apply a cap/seal over the casing to keep surface water out of the wells?

Another option I've considered is backfilling the wells with sand and then grout the last 20' or so.  The problem is that well drilling regulations around here only permit backfilling with sand if the well goes through sand/gravel which I doubt will be the case here.  The other options are to fill the well with grout (bentonite or concrete) or case and cap the well.  I really don't want to go the trouble of drilling these wells and then back filling with bentonite which provides such poor heat transfer characteristics.  I've read up on thermally enhanced grout, but the well drilling regulations around here are geared towards drinking water, not geothermal, and the regulations as I read them only allow bentonite or concrete grout.

If you can keep the wells open, your geo system will operate better. The issue is what the regulations will let you do.

If the regulations say.....
Really though, grouting the loops with a beonite grout will provide an engineered thermal link, regardless of the lithology or water content in the bore. If you do not grout, the issue is protecting a source of groundwater from contamination. The aquifer you save could be your own.

I have a driller friend in Ohio who has done some testing with the state there on this.

He backfilled the holes with pea gravel to the top of the aquifer, and sealed the top of the hole (at least the top 30') with thermally enhanced grout. In side by side tests, the pea gravel holes have proven to be better than any high solids bentonite thermally enhanced grouts. It has shown what I've thought all along. He meets the requirement of sealing the borehole from the possibility of surface contamination, and gets to have the benefits of the aquifer as the grout/connection.

One question I would have is will the 200' holes go through more than one aquifer? The grout in a vert. hole is to protect from surface water contamination AND cross contamination of multiple aquifers the drilled hole encounters.

I agree that protection of the aquifers trumps all other considerations.
One thing we can't have both ways is calling this earth friendly and then putting drinking water at risk.
I have a question arkie- can they get a loop down the existing well? If you are using a 2.5 ton unit and you drill 2- 200' wells for each full ton, 100' would be fine for the last 1/2 ton and save you a couple bucks. Obviously the other loops would have to be nearby for this to work....
J

Thanks for the input guys. Based on the research that I have done online, a 200' well would not pass through more than one aquifer. We have a state water well commission and I think I will contact them directly to get more detailed info on my proposal. But another consideration I had was if somehow I had a dry geothermal hole that actually drained my 100' well used for my home water supply. I guess in that case I wouldn't have any choice but to fully grout the geothermal wells.

My 100' deep water well is 6" diameter and I could possibly get a geothermal loop down there (I think it would be a tight fit getting past the pitless adapter for my submersible pump), but I would have to cut through the roots on some very large red oak trees to get from my mechanical room to the water well. At a cost of only ~$5/ft to drill wells around here I don't think I would save enough trying to use my water well to justify the risk to my large oak shade trees.