Geothermal energy is becoming increasingly popular not only in the scientific community but also in the oil&gas industry. Companies are building strategies for acquiring energy from the Earth's heat, investing in R&D departments, and taking steps to reduce their carbon footprint. This proves the worldwide positive direction aimed at replacing some of the energy obtained so far from fossil fuels, with energy from renewable sources. In recent years, the use of deep geothermal energy has been investigated more frequently, especially regarding areas with less favourable geological conditions for the construction of shallow geothermal installations. Such a solution requires greater investment capital, but brings an individual set of benefits. Due to the high cost of drilling, and a large number of existing wells, a frequently debated topic is the possibility of adapting old wells for geothermal purposes. In this context, it is worth considering preparing a well for exploitation of both oil&gas, and later geothermal energy, already at the planning stage of a given well. It is known that heat losses negatively affect the efficiency of a geothermal installation. In the case of deep borehole heat exchangers, these losses occur due to the interaction of the heated working fluid with the colder fluid in the annular space. They also occur when the working fluid in the annular space has a higher temperature than the surrounding rocks. The second of the presented cases also takes place in geothermal wells, where hot water is transported to the surface, often from a depth of several kilometres. Reducing the heat loss will allow a higher water temperature at the outlet of the borehole. This may be assisted by the appropriate selection of the parameters of the cement slurry located between the casing and the rock mass, in particular the thermal conductivity coefficient. Adjusting it to suit the geological conditions of the future geothermal well will contribute to achieving optimal operating conditions. The work describes the methodology and laboratory tests results of cement slurry formulas with the end goal of reduced thermal conductivity. These tests were carried out in the laboratories of the Faculty of Drilling, Oil and Gas, AGH in Krakow. The influence of individual additives on the thermal conductivity coefficient, measured with a heat flow meter, are presented.

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