Aquifer characterisation of the karstified Massenkalk carbonates as potential geothermal reservoir: implications from borehole logging and hydraulic testing (Hagen, Germany)

The hydrogeological characterisation of karst aquifers poses challenges due to their highly heterogeneous nature. To investigate the hydrogeological properties of karstic aquifers as potential geothermal reservoirs, a 224-metre-deep borehole was drilled into the Devonian Massenkalk carbonate in the Steltenberg quarry in Hagen, North Rhine-Westphalia, Germany. Borehole geophysical investigations and hydraulic tests revealed highly heterogeneous and permeable karstified structures. The transmissivity averages 6.75 × 10–4 m2/s. The storage coefficient, averaging 3.95 × 10–4, points to a confined aquifer. The step-drawdown test in the new well results in 141.4 m3/d × m as the well yield. A notable characteristic of the investigated karst aquifer is its vuggy porosity, which contributes to its bulk porosity and hydraulic storage capacity. Major fluid pathways in the aquifer are predominantly controlled by a series of highly conductive karst systems, such as dissolution- widened fractures and pre-existing vugs. Other distinct fracture sets discovered with borehole logging are less hydraulically relevant.