Hydrogeological parameterisation of the Daruvar thermal aquifer: integration of fracture network analysis and well testing

Abstract

Highly fractured Mesozoic carbonate rocks are the main reservoir of many geothermal resources in northern  Croatia, being of environmental, cultural, and economic value for the local and regional communities. The  Daruvar thermal springs (temperatures < 50°C) represent the outflow area of an intermediate scale, tectonically  controlled, hydrothermal system hosted in Triassic carbonate rocks. Several investigations have been conducted  in the Daruvar area detailing the architecture of regional and local fracture networks and  quantifying the hydrogeological parameters of the thermal aquifer. In this work, an integrated approach based  on structural and hydrogeological investigations was employed to model the network of fractures in the  reservoir and quantify its impact on the hydraulic properties. Structural investigations were conducted in the  Batinjska Rijeka quarry, considered as an outcrop analogue of the thermal aquifer, employing both a classical  field approach and the virtual quantitative analysis of a 3D digital outcrop model. Structural analysis of the  digital outcrop model allowed identification of two sub-vertical systems of discontinuities, dipping to the NW  and the WSW respectively, in accordance with the data collected through direct field measurements. The main  geometric features of the discontinuity network and their statistical distributions were employed to construct  discrete fracture network models at both the outcrop scale (approximately 100 m) and the aquifer scale in  Daruvar (approximately 700 m). Calibration of the input parameters allowed modelling of porosity and  permeability values that reproduce the field values assessed through pumping tests, well tests, and well logging. This work highlights the importance of integrating geological and hydrogeological investigations to  obtain a more reliable reconstruction and quantification of the processes driving the fluid flow in fractured  aquifers and affecting the spatial distribution of their hydraulic properties.