A potential impact of geothermal energy exploitation on the underground of the Upper Phanerozoic following various schemes of thermal water extraction and injection to the productive aquifer and other aquifers was modelled on an example of Klaipėda demonstration geothermal plant (KDGP). The obtained results showed that: 1 - thermal energy resources in the productive Viešvilė aquifer are sufficient for operation of KDGP at project geothermal output 20.8 MW for 50 years; 2 - at a distance 0.2-0.5 km around the injection wells, the water level in the productive layer rises above the ground; 3 - the temperature in the productive complex falls down in an area of approximately 15 km 2 ; 4 - the spent mineralized (95 g/l) water of productive aquifer returned into the Upper Permian-Žagarė aquifer may reach the freshwater waterworks in 10 years; 5 - yet its short-lasting emergency injections represent no real hazard for waterworks; 6 - the interaction of groundwater of different temperature and chemical composition may slightly elevate the saturation of mixture with the ferrous minerals (hematite and magnetite) and allow their precipitation in the layer or well filters; 7 - the cooling of Viešvilė aquifer from 37-39 to 11 °C in the impact zone of injection wells during 50 years is an irreversible process; complete regeneration of temperature due to geothermal flux lasts for about 6000 years.