There are 2 kinds of geothermal systems: an open loop system and a closed loop system. In an open loop system, a water well is drilled which will produce the appropriate amount of gallons per minute to run the unit, and then another well is drilled for the return water.
A closed loop system is for situations where you may not get an adequate water supply to run the system, so you drill multiple boreholes with loops installed. These loops go in the hole and out the hole to the next hole in a series, then you put a bentonite grout surrounding the loop, filling the rest of the space in the borehole. This bentonite grout is thermally-inhanced to maximize the proper temperature transfer. Then, water and glycerin is put inside the loop. All air is purged where the fluid will circulate through the system in and out of each loop over and over in that series.