A joint methodical case study for understanding severe formation clogging processes within a highly utilized geothermal reservoir in the Pannonian Basin of Hungary

Geothermal energy is a promising renewable energy source, considering the costs of fossil energy and the need to reduce greenhouse gases drastically. The Dunántúli Group (DG, a sandstone formation), located in the Hungarian part of the Pannonian Basin, stands out as being most promising for Hungary´s national geothermal energy production. Currently, over 600 operational wells targeting the DG exist in Hungary. Although a 1:2 ratio of production to reinjection wells is targeted for this reservoir, only 5% of all wells serve reinjection purposes. Limited knowledge exists about microscale pore structures and flow properties of this formation, crucial for both, successful water reinjection and for geothermal energy extraction. Severe clogging processes afflict geothermal wells in this region, shutting down injection wells within a very short amount of time, causing significant extra costs and delays in progressing Hungary´s energy transition.

This study investigated selected reservoir samples from the DG, utilizing classical mineralogical and petrophysical methods, an extensive core-flooding program for controlled clogging of artificial and natural core samples, as well as X-ray µ-CT imaging, qualitative and quantitative 3-D image analysis and pore scale simulations with special focus upon the physical pore clogging. Due to the influence of (micro-) tectonic activities, many micro-fractured and hence potentially movable grains can be found (e.g., movable by re-injection of formation water). Although the storage properties are favourable, pore throats, which control fluid flow, are nearly of the same size as the movable particles within the pore-system. This fact, coupled with smaller particles present in the re-injection fluid, leads to significant and deep-seated clogging of flow pathways. Furthermore, the absence of cement in the permeable part of the DG will cause formation compaction during reinjection, if pressures are higher compared to the formation pressure. This leads to a reduction of the average pore throat size, causing progressive clogging of the entire injection zone. For the first time, these processes have been validated successfully through pore-scale clogging simulations using GeoDict software.

These results significantly enhance the understanding of transport in the DG as they provide insights into the geology and hydrogeology of the Pannonian Basin, encouraging industry and scientists to develop and drill more and sustainable reinjection wells.