Cements, Waters, and Scales: An Integrated Study of the Szeged Geothermal Systems (SE Hungary) to Characterize Natural Environmental Conditions of the Thermal Aquifer

Abstract

The study area, Pannonian Basin (Central Europe), is characterized by high heat flow and presence of low-enthalpy geothermal waters. In the Szeged Geothermal Systems (Hungary), having Miocene to Pliocene sandstone aquifers with dominantly Na–HCO3-type thermal water, unwanted carbonate scaling was observed. An integrated approach consisting of host rock and scale mineralogical and petrographic analyses as well as water chemistry led to a better understanding of the characteristic natural (geogenic) environmental conditions of the geothermal aquifers and to highlight their technical importance. Analyses of the reservoir sandstones showed that they are mineralogically immature mixed carbonate-siliciclastic rocks with significant macroporosity. Detrital carbonate grains such as dolomite and limestone fragments appear as important framework components (up to ~20–25%). During water–rock interactions, they could serve as a potential source of the calcium and bicarbonate ions, contributing to the elevated scaling potential. Therefore, this sandstone aquifer cannot be considered as a conventional siliciclastic reservoir. In mudrocks, a significant amount of organic matter also occurs, triggering CO2 producing reactions. Correspondingly, framboidal pyrite and ferroan calcite are the main cement minerals in all of the studied sandstone samples which can suggest that calcite saturation state of the thermal fluid is close to equilibrium in oxygen-depleted pore water. Analysis of the dominant carbonate crystals in the scale can suggest that growth of the feather dendrites of low-Mg calcite was probably driven by rapid CO2 degassing of CO2-rich thermal water under far-from-equilibrium conditions. Based on hydrogeochemical data and related indices for scaling and corrosion ability, the produced bicarbonate-rich (up to 3180 mg/l) thermal water has a significant potential for carbonate scaling which supports the aforementioned statement. Taking into consideration our present knowledge of geological setting of the studied geothermal systems, temporal changes in chemical composition and temperature of the thermal water during the heating period can indicate upwelling fluids from a deep aquifer. Regarding the pre-Neogene basement, hydrologic contact with a Triassic carbonate aquifer might be reflected in the observed chemical features such as decreased total dissolved solids and increased bicarbonate content with high scale-forming ability. The proposed upflow of basin-derived water could be channeled by Neogene to Quaternary fault zones, including compaction effects creating fault systems above the elevated basement high. The results may help to understand the cause of the high carbonate scale precipitation rates in geothermal systems tapping sandstone aquifers.

Guided Transformation of Abandoned Hydrocarbon Fields into Heat Storage Solutions Using Enhanced MCDA-AHP Geostatical Based Method, Hungarian Case Study

“…For the Data Collection segment of our study, we utilized data from 67 wells in the study area which is located in the Southern Part of the Bekes Basin, Hungary, a site previously explored for hydrocarbon but now considered for its geothermal potential [29]. This field encompasses two potential formations, the shallow Szolnok Formation as an aquifer [30] and the deeper Bekes Formation [29], offering a unique lens to examine heat storage capabilities. The selection of 3.88 km as the side length for our analysis squares derives from the optimal area typically utilized for injection and production processes within such formations.…”

Permeability and Mineralogy of the Újfalu Formation, Hungary, from Production Tests and Experimental Rock Characterization: Implications for Geothermal Heat Projects

“…However, this does not address the issue of site-specific data, relevant to the present study, being unavailable. One of few recently published studies, by Varga et al [22], on reservoir properties of Pannonian sandstone, provides a qualitative evaluation of scaling and cementation for a geothermal site at Szeged (Figure 1). In contrast, one of the most recent publications to quantify porosity and permeability of Pannonian sandstones dates from 1994 [11].…”