The objective of this project has been to employ chemical leaching to achieve sufficient hydraulic contact through fissures in the rock to enable them to serve as part of the circulation system of a borehole in the heat store.^The trials were carried out both in the laboratory and in the field.^The results obtained in the field were investigated by means of hydraulic tests that allowed the hydraulic conductivity to be determined.^The results were not as expected, rather they were the opposite.^Fissures in the rock were sealed instead of being opened up, and the hydraulic conductivity decreased instead of increasing.^The explanation for this lies in the fact that the leaching liquid, a solution of NaOH, became saturated by dissolved minerals which were then precipitated elsewhere in the fracture system.^However, this undesired result may turn out to have a number of geological engineering applications, as there is normally a greater need to seal fissures in rock than to open them.