The large borehole annulus should be tightly and permanently backfilled to protect the pipes, to ensure good heat transfer from the rock to the pipes and finally to the circulating water-glycol fluid („brine“) and to stabilize the borehole wall.

For reasons of groundwater protection, the incursion of surface water should be prevented and a seal should be made between two different groundwater levels (Note: This case does not apply to Bavaria, where only drilling in the 1st groundwater level are permitted).