Mitigating depletion of a porous geothermal aquifer in the Pannonian sedimentary basin

This paper investigates depletion rates and available thermal water resources of the transboundary Upper Pannonian loose sandstone geothermal aquifer of the Mura Formation in the Mura-Zala sedimentary basin in north-eastern Slovenia, and outlines a regional reinjection strategy to mitigate depletion. The research monitoring network of 12 geothermal wells, which is being constantly upgraded since 2009, has highlighted that the current abstraction rate of 2.4 million m3 in 2014 is not sustainable because hydraulic state has been continuously deteriorating regionally while the chemical state is affected only locally. The average regional drawdown rate in observation wells is 0.67 m annually, while very rough average value for abstraction wells is 3.0 m per year. The cumulative historical regional drawdown is above 16 m. The available thermal water resources are addressed as the cumulative abstraction rate which reverses the observed declining trend of groundwater levels, and were numerically assessed to be ~1.3 million m3 of thermal water per year. These numerical models set up in the AUTOUGH2 code also provided the quantification tool of the regional groundwater balance and confirmed the gravity-driven regional groundwater flow with prevailing conductive heat transfer mechanism. Simulations of the regional reinjection strategy outline that the cumulative reinjection rate of ~1.48 million m3 of thermal water per year should be sufficient to reach good status of the aquifer if the cumulative regional abstraction rate does not change. If it does, the rate should be adjusted accordingly. The water is provided by eight users of geothermal heat and should be returned by the existing reinjection wells in Lendava and Murska Sobota and two new wells situated in the central part of the basin by 2021 the latest. These findings should enhance optimization of exploitation practice and implementation of geothermal doublets in the region as the successful example of a doublet in Lendava exemplifies that no major technological issues should occur if the system is properly designed.