Hydrogeological parametrization of a carbonate thermal aquifer through integrated pumping test and virtual outcrop reconstruction (Daruvar, Croatia)

Geothermal resources are renewable sources of energy and raw materials. A sustainable utilization for the long-term preservation of the resource requires a site-specific plan that needs to be based on geological and hydrogeological reconstructions. Northern Croatia is rich in geothermal resources that are generally hosted in carbonate rocks. The occurrence of thermal waters (temperatures of 38-50°C) in the town of Daruvar has been documented since the Roman age. In this research, the characterization of the Daruvar carbonate thermal aquifer was detailed using an integrated approach combining hydrogeological and structural investigations and discrete fracture network (DFN) modeling. Hydrogeological investigations consisted in the well logging and pumping tests of a 190 m deep well in Daruvar. Structural investigations were conducted NE of Daruvar where the carbonate rock complex of the aquifer is exposed at the surface. They included the measurement of the discontinuity sets and the photogrammetric reconstruction of the outcrop. The results were used to calibrate a DFN model at the scale of the aquifer explored by hydrogeological investigations (700x700x150 m).

The porosity distribution of the aquifer was obtained from the neutron log of the well ranging from 0.03 to 9.1% (average = 2.7%). The permeability was calculated using transmissivity values from the analysis of pumping tests and literature data resulting in a range from 7.4 to 122.8 D (average = 46 D). Structural analyses in the outcrop analog of the aquifer depicted two dominant systems of discontinuities (241/65 and 296/75). A highly fractured section of the outcrop was selected for the statistical analysis of the geometrical features of the discontinuity network to derive the input parameters for the DFN modeling. Discontinuity aperture was estimated based on the calibration of the DFN model. The results show a linear and power correlation of the aperture with porosity and permeability, respectively. Considering the average porosity of the aquifer, the calibrated aperture value was 3 mm obtaining a permeability of 1.5×10⁵ D. Such high value was interpreted as connected to the porosity value used for the calibration, which was measured through the neutron log depicting the total porosity. On the other hand, the fluid flow and the aquifer permeability are influenced by the effective porosity, which is at least an order of magnitude lower than the total porosity in carbonate aquifers. This difference was accounted for by testing a “dual aperture” approach. Considering the experimental dataset, a porosity of 0.2% (10th percentile of the distribution) was tested. It resulted in a calibrated fracture aperture of 0.22 mm obtaining a permeability of 60.5 D, comparable with the experimental dataset.

The obtained results highlight the importance of integrating structural and hydrogeological approaches to investigate fractured aquifers. Structural data can be used to determine the architecture of the fracture network in the rock mass, while hydrogeological investigations supported by numerical modeling and structural results can provide a solid hydrogeological parametrization of the aquifer.

Acknowledgment: This research was funded by the HyTheC project of the Croatian Science Foundation, grant number UIP-2019-04-1218.