Abstract
<p>The long-term sustainable exploitation of geothermal resources requires cautious planning and regulation. Exploitation in excess of natural recharge can result in reservoir pressure decline, causing a decrease in production rates. Furthermore, such “overexploitation” of geothermal reservoirs may lead to compaction and land subsidence. Understanding of such phenomena is critical for the assessment of societal-environmental risks, but can also be used for optimization by constraining reservoir processes and properties.</p><p>Excessive thermal water volumes have been extracted from porous sedimentary rocks in the Hungarian part of the Pannonian Basin. Thermal water production in Hungary increased significantly from the early 70’s. Regional-scale overexploitation of geothermal reservoirs resulted in basin-scale pressure drop in the Upper Pannonian sediments, leading to compaction and ground subsidence.</p><p>We investigated surface deformation at the Szentes geothermal filed, SE Hungary, where the largest pressure decline occurred. We obtained data from the European Space Agency’s ERS and Envisat satellites to estimate the ground motions for the periods of 1992-2000 and 2002-2010. We applied inverse geomechanical modelling to understand the compaction behaviour of the reservoir system and to estimate the subsurface properties. We constrained the model parameters using the Ensemble Smoother with Multiple Data Assimilation, which allowed us to incorporate large amounts of surface movement observations in a computationally efficient way. The model requires pressure time series as input parameters, therefore, the lack of regular pressure measurements in geothermal wells of Szentes resulted in significant uncertainties. Still, we managed to identify a potential delay in pressure drop and subsidence, implying a time-decay compaction behaviour of the reservoir system,  and we arrived at realistic estimates for the compaction coefficient of the reservoir. The improved parametrization enables better forecasting of the reservoir behaviour and facilitates the assessment of future subsidence scenarios. This study thus demonstrates the effectiveness of InSAR-based ground motion data and inverse geomechanical modelling for the monitoring of geothermal reservoirs and the establishment of a sustainable production scheme.</p>
Highlights
Hungary is among the most favorable countries for geothermal development within Europe (e.g. Békési et al., 2018; Cloetingh et al, 2010; Horváth et al, 2015; Lenkey et al, 2021; Limberger et al, 2014; Limberger et al., 2018), with an average geothermal gradient of ~45 °C/km, and a mean surface heat flow of 100 mW/m 2 (Lenkey et al, 2002)
PS-InSAR time series of ground deformation together with the modelling results suggest that uplift of the Szentes geothermal field due to pore pressure recovery occurred between 2002-2010
The north-eastern part of the area experienced a moderate increase in hydraulic heads (Bálint and Szanyi, 2015), whereas the InSAR data and modelling results suggest pressure decline indicated by the second source
Summary
Hungary is among the most favorable countries for geothermal development within Europe (e.g. Békési et al., 2018; Cloetingh et al, 2010; Horváth et al, 2015; Lenkey et al, 2021; Limberger et al, 2014; Limberger et al., 2018), with an average geothermal gradient of ~45 °C/km, and a mean surface heat flow of 100 mW/m 2 (Lenkey et al, 2002). Exploitation in excess of natural recharge can result in reservoir pressure decline, causing a decrease in production rates Such “overexploitation” of geothermal reservoirs may lead to compaction, land subsidence, or even induced seismicity (e.g. Allis, 2000; Békési et al, 2019a; Keiding et al., 2010; Maghsoudi et al, 2018; Trugman et al, 2014; van der Meer et al, 2014). The advantages of probabilistic ensemble-based approaches for inversion of surface subsidence have already been shown in several studies outside the geothermal energy arena (Baù et al, 2015; Candela et al, 2017; Fokker et al, 2016). Such ensemble-based approaches have limitations in terms of model parameters, including only a limited number of reservoir properties, and cannot directly distinguish between different subsurface processes. This study is the first application of mapping and modelling ground motions due to geothermal activities in Hungary, that aims to demonstrate the usefulness, requirements, and limitations of inverse geomechanical models for geothermal sites in Hungary and worldwide
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