Drilling engineering experience gained from MD-BTES construction phase of SKEWS demo-site

As a result of the current energy crisis triggered by war and increasing shortage of resources, renewable energy sources are becoming increasingly important. The storage of heat from fluctuating energy sources is an essential component for independent and base-load capable energy supply. A promising technology are medium-deep geothermal storage systems, which store excess heat in the crystalline subsurface and offer significant advantages over near-surface geothermal storage systems. At the Lichtwiese campus of the Technical University of Darmstadt, the world's first medium-deep research geothermal storage system was constructed in the crystalline bedrock with three 750 m deep boreholes with a distance of approx. 8.6 m (research project SKEWS, project administrator Jülich, funding code 03EE4030A). The outer casing of the coaxial system has a diameter of 7", on which an attached glass fiber cable records temperature and strain measurements. Research operations began in the spring of 2023, which consists of an initial enhanced Geothermal Response Test (eGRT) followed by five heating and cooling phases.

The experience and knowledge acquired are intended to demonstrate the basic construction and operational feasibility of such storage systems, as well as to be used as a basis for the planning, dimensioning, construction and costing of future projects.

With the current project status, it has already been possible to evaluate the processes of the drilling phase and their effects on the drilling operation. The encountering of deviating geological and hydrogeological conditions to the prognosis from the planning phase required, among other things, the change of the drilling technique from water hammer to rotary drilling with a clay-fresh-water fluid and accordingly also affected the verticality of the drillings. Based on the detailed drilling data recorded and the geological conditions explored, the drilling phase of the storage system could be evaluated in terms of its material usage, drilling accuracy, costs and energy consumption.