The most complicated problems in deep-well drilling technology derive from the damaged borehole walls due to the action of different mechanical and physio-chemical factors. Insufficient borehole stability occurs during drilling through different rocks including shales whose pores are of nano-dimensions and average in size from 10 to 30 nm. Classical additives to such rocks cannot produce a high-quality mud cake, but this can be achieved if nanoparticles are added to the drilling mud. They can enter shale pores, physically plug them and reduce the penetration of the filtrate, thereby reducing the pressure transfer from the wellbore to shale rocks. This results in strengthening of the well walls and increases the stability of the wellbore. In this doctoral thesis, laboratory tests regarding the influence of the size and concentration of SiO2 nanoparticles (sizes 8, 20, 60 and 80 nm), Al2O3 (sizes 20, 30 to 60 and 100 nm), TiO2 (size 30 to 50 nm) and Fe2O3 (sizes 20 do 30 nm and 50 nm) were performed. The following drilling mud properties are determined: 1) density, 2) rheological properties at room temperature and temperature 50 °C, 3) API filtration, 4) mud cake thickness, 5) PPT filtration, the ability of mud with nanoparticles to plug pores of the ceramic disc, at differential pressures of 55 bar and 34.5 bar and a temperature of 88 °C, 6) lubricating properties of the mud and 7) influence of mud without and with nanoparticles on the swelling of laboratory-prepared pellets. The tests were performed in three phases, and the nanoparticles were added in 4 concentrations of 0.5, 1, 3 and 5 wt%. In the first phase of laboratory testing, the influence of nanoparticle size and concentration on the properties of 7 wt% bentonite suspension was determined. In total, the properties of 41 muds, which all have different compositions, were examined. According to these measurement, particular types, sizes, and concentrations of nanoparticles were selected due to their ability to increase the stability of the wellbore. This was determined by measurements. Thus, SiO2 nanoparticles (sizes 8 and 20 nm), TiO2 (sizes 30 to 50 nm) and Fe2O3 (sizes 20 to 30 nm) in concentrations of SiO2 (1, 3 and 5 wt%), TiO2 (1 and 3 wt%) and Fe2O3 (1 wt%) were selected for conducting the second phase laboratory tests. In the second phase of laboratory tests with selected nanoparticles, the influence of nanoparticles on the properties of mud A, which has a more complex composition was determined. Mud A was prepared by adding an additive to reduce filtration PAC R at a concentration of 1 g/l in the bentonite suspension used in the first phase. In total, the properties of 10 muds of different compositions were examined. According to these measurement, specific types, sizes, and concentrations of nanoparticles were selected, based on the measurements pointing to their ability to increase the wellbore stability. Based on the results of the second phase, only SiO2 nanoparticles (sizes 8 and 20 nm) in concentrations of 1, 3 and 5 wt% were selected for further tests in the final third phase of the influence of nanoparticles on the wellbore stability. In the third phase, the test was performed using a mud prepared so that optimal rheological properties for the muds from the second phase were adjusted by increasing the concentration of PAC R additive to 2 g/l and additive barite was added to prepare a higher density mud. This is important for the safe drilling of the wellbore to prevent unwanted inflow into the wellbore. According to laboratory testing, SiO2 nanoparticles were shown to be the only ones that can be added to water-based muds with the aim of increasing the wellbore stability. Other nanoparticles tested in this doctoral thesis were found to be able to partially increase the wellbore stability but can be used to adjust other mud properties. For example, Al2O3 and Fe2O3 nanoparticles could be used to adjust rheological properties. Several discoveries were made during the testing performed for the purpose of the creation of this doctoral thesis: a procedure to examine the effect of concentration and size of SiO2, Al2O3, TiO2 and Fe2O3 nanoparticles on water-based mud properties under atmospheric and simulated well conditions was designed; it was found that improving filtration and lubricating properties of water-based muds depends on the type, concentration and size of added nanoparticles; the addition of TiO2 and Fe2O3 nanoparticles partially increases the stability of the wellbore but has a significant impact on increasing the rheological parameters of water-based mud; adding SiO2 nanoparticles up to 20 nm in the water-based mud increases the stability of the wellbore, but the best results were achieved by adding SiO2 nanoparticles (size 8 nm) at a concentration of 5 wt%. Finally, the possibilities of applying drilling mud enriched by nanoparticles, primarily SiO2, to increase the stability of the wellbore and drilling through rocks prone to swelling and collapse were determined.