Ground motions induced by pore pressure changes at the Szentes geothermal area, SE Hungary

Abstract

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 1970s. Regional-scale exploitation of geothermal reservoirs without re-injection resulted in basin-scale pressure drop in the Upper Pannonian (Upper Miocene) sediments, leading to compaction. This compaction resulted in ground subsidence primarily through poro-elastic coupling. We investigated surface deformation at the Szentes geothermal filed, SE Hungary, where the largest pressure decline occurred. Subsequently, hydraulic head recovery in the western part of the geothermal reservoir was initiated in the mid-1990s. We obtained data from the European Space Agency’s Envisat satellites to estimate the ground motions for the period of November 2002–December 2006. We applied inverse geomechanical modeling to estimate reservoir properties and processes. 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. Ground movements together with the modeling results show that uplift of the Szentes geothermal field occurred during the observation period. Since no injection wells were operated at Szentes before 2018, and production temperatures remained relatively constant through the entire production period, we explain ground uplift with pore pressure increase due to natural recharge. The estimated decompaction coefficients of the reservoir system characterizing the elastic behavior of the Szentes geothermal reservoir varies between ~ 0.2 × 10–9 and 2 × 10–9 Pa−1. Compaction coefficients of the reservoir system corresponding to the earlier depressurization period, from ~ 1970 to the mid-1990s, may be significantly larger due to the potential inelastic behavior and permanent compaction of clay-rich aquitards. The improved parametrization enables better forecasting of the reservoir behavior and facilitates the assessment of future subsidence scenarios that are helpful for the establishment of a sustainable production scheme.