European and municipal scale drillability maps: A tool to identify the most suitable techniques to install borehole heat exchangers (BHE) probes

The most suitable drilling technology for shallow geothermal applications in a given geological context plays a key role in the techno-economic evaluation of shallow geothermal solutions. The installation costs are one of the main constraints to the wider application of shallow geothermal heat exchangers and are due mainly to the drilling time and fees. This paper defines drillability as a tool to select the most suitable drilling technique for borehole heat exchanger installations in a given geological and hydrogeological setting. Drillability maps at European and municipal scale are proposed as a guideline for drillers and ground source heat pump designers. The former summarizes the most suitable drilling technology based on lithology and borehole heat exchangers type, whilst the latter provides an insight in the ground thermal properties and the installation timing/costs. In the GEO4CIVHIC Project, a database correlating drilling technique to rock types, time needed to drill a borehole at 100 m depth and drilling costs was created. Local drillability features are compared to in situ real data in four test sites. Two out of four test sites, completed at the time of writing, shows a good agreement between local maps and real site conditions, validating the innovative methodology proposed.