Influence of Well Yield Changes on Qualitative Parameters of Geothermal Water

Abstract

The change in geothermal water exploitation (replacing of the free outflow by increased pumping) could cause unwished changes in geothermal water properties and consequently also some technological problems during utilization and liquidation of the used geothermal water. In the end, the long-term overexploitation of geothermal water could influence not only a single geothermal well, but consequently, the whole geothermal structure.
Influence of well yield changes was studied on the example of geothermal well VZO-13 Zlatna na Ostrove (Slovakia) located in the central depression of the Danube Basin. Qualitative properties of geothermal water exploited by free outflow of 7.5 l/s were compared with those, exploited by pumping with the rate of 18.33 l/s. The value of TDS has increased from 2.5 g/l to 13 g/l. The change was accompanied by changes in gas composition and content. The total gas content increased from 274 l/m3 to 1117 l/m3, nitrogen content decreased from 20 l/m3 to 1.8 l/m3, CO2 content increased from 15 l/m3 to 57 l/m3 and the methane content increased from 239 l/m3 to 1058 l/m3. The change in gas content resulted in the change of bubble point depth which decreased from 150 m (by the free outflow of 7.5 l/s) to the depth of 250 m below the surface (by the pumping rate of 18.33 l/s). Hydrodynamic calculations also documented that by the long-term abstraction of the 18 l/s, the free outflow will disappear and the piezometric head of geothermal water will decrease to the depth of 85 m below the surface.