A thorough understanding of the chemistry involved in reinjecting heat-depleted geothermal water into poorly consolidated sandstone is vital for the effective design of treatments targeting subsurface rock formations. The intricate chemical interactions occurring within sandstone systems can result in the dissolution of certain minerals and the subsequent precipitation of others, which may significantly contribute to damage within the formation. This process can alter the physical properties of the rock, potentially leading to reduced permeability and other challenges in resource extraction. Thus, it is imperative to monitor not only the concentration of various chemical species present in the geothermal water and sandstone, but also the spatial distribution of these geochemical reactions. By doing so, we can better predict and mitigate their potential adverse effects on rock formations, ensuring the long-term success and efficiency of geothermal energy extraction and other subsurface activities. In this study, we conducted laboratory experiments using both model and natural formation waters, as well as rock samples, to investigate water–rock interactions in a sandstone reservoir in the Szentes area of Hungary. Geochemical models were run with two different thermodynamic databases to simulate laboratory experiments, predict the effects of heat-depleted geothermal water reinjection into the reservoir, and assess predictions of different geochemical databases. Our study shows that calcite dissolves while quartz, kaolinite, and dolomite form. Other mineral reactions, however, remain less certain. The PHREEQC database indicates chlorite dissolution along with the formation of small amounts of feldspars and hematite, whereas the Thermoddem database predicts montmorillonite dissolution and chlorite precipitation. The reservoir porosity and permeability are expected to change over time as a result of mineral reactions. Modeling results, however, indicate negligible porosity changes as the reservoir reaches equilibrium state. The general concept proposed here, which focuses on the geochemical properties of the reinjected water and reservoir, provides a framework for detailed analysis of the geothermal system—a critical step for ensuring sustainable geothermal operations.

This study establishes a facies-based framework for characterizing reservoir quality in the Upper Pannonian geothermal reservoirs of the Szentes field (Hungary). To evaluate vertical heterogeneity and optimize the selection of geothermal reinjection zones, an integrated core–log–statistical workflow was applied to data from boreholes SZT-1 and SZSZT-IX. The methodology combined petrophysical measurements, petrographic observations, and multivariate statistical analyses, including Hierarchical Cluster Analysis (HCA) and Linear Discriminant Analysis (LDA). The siliciclastic succession was classified into four distinct facies clusters representing a continuum of depositional energy regimes: Rolling, Graded Suspension with Rolling, fine-grained Suspension, and Uniform Suspension. The results demonstrate a dual control on reservoir quality: the primary pore framework is determined by depositional grain-size architecture and sediment transport processes, while mechanical compaction and diagenetic alteration subsequently modify pore connectivity and flow efficiency. Among the identified facies, deposits formed from Graded Suspension with Rolling represent the most favorable reservoir units, combining high porosity (up to 33%) with exceptionally high permeability (>1500 mD). In contrast, suspension-dominated facies deposited from Graded and Uniform Suspension exhibit significantly reduced permeability due to higher matrix content, cementation, and compaction. The results demonstrate that reservoir performance in the Szentes geothermal system is primarily controlled by facies-scale heterogeneity rather than by depth-based stratigraphic divisions alone. This integrated facies-based approach provides a predictive framework for extrapolating reservoir properties to uncored intervals and offers practical guidance for optimizing reinjection strategies and sustainable geothermal reservoir management.