We have been very happy for four years with an open loop residential geothermal system.   I remember  our well to be 350 feet deep.   I am now involved the building of a new civic structure in town and I pursuaded the town to do a feasibility study for a geothermal system.   The people who did that said they would need two wells 1400 feet deep.   I can understand that, since it is a larger building, it would need a bigger system, but why would the wells be so much deeper?...

It might also be because their design is based on bentonite-packed wells, with a fluid circulating in a closed tube, rather than direct contact between water and rock. That makes the depth needed on the order of maybe 200 feet per ton of heat load, rather than, say, 80-100 for open loop or standing column well.

Google on Haverhill Library ground source heat pump, and you can see some very detailed info on what they did and how well it performed. First hole was 1500 feet; worked well. When they built the addition, they drilled another well, same size.