When identifying the conditions required for the sustainable and long-term exploitation of geothermal resources it is very important to assess the dynamics of processes linked to the formation, migration and deposition of particles in geothermal systems. Such particles often cause clogging and damage to the boreholes and source reservoirs. Solid particles: products of corrosion processes, secondary precipitation from geothermal water or particles from the rock formations holding the source reservoir, may settle in the surface installations and lead to clogging of the injection wells. The paper proposes a mathematical model for changes in the absorbance index and the water injection pressure required over time. This was determined from the operating conditions for a model system consisting of a doublet of geothermal wells (extraction and injection well) and using the water occurring in Liassic sandstone structures in the Polish Lowland. Calculations were based on real data and conditions found in the Skierniewice GT-2 source reservoir intake. The main product of secondary mineral precipitation is calcium carbonate in the form of aragonite and calcite. It has been demonstrated that clogging of the active zone causes a particularly high surge in injection pressure during the fi rst 24 hours of pumping. In subsequent hours, pressure increases are close to linear and gradually grow to a level of ~2.2 MPa after 120 hours. The absorbance index decreases at a particularly fast rate during the fi rst six hours (Figure 4). Over the period of time analysed, its value decreases from over 42 to approximately 18 m3/h/MPa after 120 hours from initiation of the injection. These estimated results have been confi rmed in practice by real-life investigation of an injection well. The absorbance index recorded during the hydrodynamic tests decreased to approximately 20 m3/h/MPa after 120 hours.

M embrane-based water desalination processes and hybrid technologies are often considered as a technologically and economically viable alternative for desalination of geothermal waters. This has been confirmed by the results of pilot studies concerning the UF-RO desalination of geothermal waters extracted from various geological structures in Poland. The assessment of the feasibility of implementing the water desalination process analysed on an industrial scale is largely dependent on the method and possibility of disposing or utilising the concentrate. The analyses conducted in this respect have demonstrated that it is possible to use the solution obtained as a balneological product owing to its elevated metasilicic acid, fluorides and iodides ions content. Due to environmental considerations, injecting the concentrate back into the formation is the preferable solution. The energy efficiency and economic analysis conducted demonstrated that the cost effectiveness of implementing the UF-RO process in a geothermal system on an industrial scale largely depends on the factors related to its operation, including without limitation the amount of geothermal water extracted, water salinity, the absorption parameters of the wells used to inject water back into the formation, the scale of problems related to the disposal of cooled water, local demand for drinking and household water, etc. The decrease in the pressure required to inject water into the formation as well as the reduction in the stream of the water injected are among the key cost-effectiveness factors. Ensuring favourable desalinated water sale terms (price/quantity) is also a very important consideration owing to the electrical power required to conduct the UF-RO process.

Geothermal waters are a source of clean energy. They should be used in a rational manner especially in energyand economic terms.

Key factors that determine the conditions in which geothermal waters are used, the amount of energy obtainedand the manner in which cooled water is utilised include water salinity. Elevated salinity levels and the presence oftoxic microelements may often lead to difficulties related to the utilisation of spent waters. Only a few Polishgeothermal facilities operate in a closed system, where the water is injected back into the formation after havingbeen used. Open (with water dumped into surface waterways or sewerage systems) or mixed (only part of the wateris re-injected into the formation via absorption wells while the rest is dumped into rivers) arrangements are morefrequently used. In certain circumstances, the use of desalinated geothermal water may constitute an alternativeenabling local needs for fresh water to be met (e.g. drinking water).

The assessment of the feasibility of implementing the water desalination process on an industrial scale islargely dependent on the method and possibility of disposing of, or utilising, the concentrate. Due to environmentalconsiderations, injecting the concentrate back into the formation is the preferable solution. The energy efficiency and economic analysis conducted demonstrated that the cost effectiveness of implementing the desalinationprocess in a geothermal system on an industrial scale largely depends on the factors related to its operation,including without limitation the amount of geothermal water extracted, water salinity, the absorption parameters ofthe wells used to inject water back into the formation, the scale of problems related to the disposal of cooled water,local demand for drinking and household water, etc. The decrease in the pressure required to inject water into theformation as well as the reduction in the stream of the water injected are among the key cost-effectiveness factors.Ensuring favourable desalinated water sale terms (price/quantity) is also a very important consideration owing tothe electrical power required to conduct the desalination process

The aim of the presented work was an attempt to verify the geothermal conditions in the Polish Lowlands (Lower Jurassic and Lower Cretaceous reservoir) based on new geological information. The paper presents geothermal conditions in the Polish Lowlands according to the state of recognition at the end of 2022 in order to update the hydrogeothermal conditions in selected regions. Based on the scientific and research works published so far as well as numerous geothermal investments, and geological information from twenty-three new exploratory drilling events performed in the years 2000–2022 (nineteen of which were performed/documented after 2006), the authors undertook to update forecasts of the top surface of Lower Jurassic and Lower Cretaceous formations, the total thickness of these formations and the potential discharge of wells. The analysis was performed using the QGIS Desktop 3.24.1 software, a cross-platform and free open-source geoinformation software application (GIS ) that enables the viewing, editing and analyzing of spatial data and the creation of maps. The correction covered the course of the isolines on all six analyzed maps. The presented analysis made it possible to make a spot correction of the forecasted course of the isoline in relation to the maps published earlier in the Atlas of geothermal resources in the Polish Lowlands. Mesozoic formations developed in 2006, edited by Wojciech Górecki. Information obtained from newly drilled geothermal boreholes enabled the local correction of the forecasted values of individual parameters while maintaining the general trend.