During the drilling of a horizontal well, various technological problems may occur, and most often they are the result of increased values of torsion moment and drag force. The contact between the drill string and the casing/wellbore causes an increase in the frictional force, which results in an increased drag force and torsional moment while drilling the borehole especially during the drilling of curved and horizontal sections of the borehole. In addition to the mentioned friction, there is also wear and tear of the tool joint of drill pipe, considering that the tool joint is the widest part of a drill string and therefore in constant contact with the borehole of the well. The best way to reduce torsional moment and drag force during drilling is to use oil-based or synthetic muds instead of water-based muds. However, the reduction of the torsional moment and drag force can also be effectively achieved by adding commercial lubricants to the water-based mud, which have the task of reducing the friction coefficient. Since during the drilling of extended reach wells, most of the drilling takes place through the reservoir section, a polymer mud was chosen for this test. One of the possible solutions to this problem is the use of protective sliding rings that would be placed on the tool joint of the drill pipe in combination with additives (lubricants) in the polymer mud to reduce the torsional moment and drag force. Therefore, three materials were selected such as polyamide 6 (PA6), polytetrafluorethylene (PTFE – in the following text - Teflon) and polyoxymethylene (POM), from which it was intended to create a protective sliding ring that would be placed on the drill joint of a drill pipe. Also, three commercial lubricants Z, X and Y from two different manufacturers were used, which have the role of reducing the friction factor and thus the torsional moment and drag force. As part of this doctoral thesis, laboratory tests were carried out in two phases. The first phase of testing was carried out in the Laboratory for Drilling Fluids at the Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, where the basic physical properties of the polymer mud were tested, and a test was carried out on a device for determining the lubricity of the mud. The first phase is divided into 3 parts where in the first part the properties of the polymer mud to which lubricant Z has been added are tested, in the second part lubricant X, while in the third part lubricant Y is tested. In the first part basic physical properties testing was conducted on polymer mud with weighing agents in which lubricant Z was added in different concentrations from 1% to 6% and the lubricity of the mud using metal block and blocks made from Teflon, PA6 and POM. After analysing the data, it was concluded that increasing the concentration of lubricants to 5% and 6% it did not provide additional decrease IV in friction force, so it was decided to continue with concentrations from 1 to 4%. By this conclusion, same tests were conducted using polymer mud without weighing agents in which lubricant Z was added in different concentrations from 1% to 4%. From the obtained data, it has been decided for the second and third part of testing to use polymer mud with and without weighing agents in which lubricant X or Y was added in a concentration of 0,5% as a minimum value, 2% as a mean value and 4% as a maximum value. In the second and third part of testing same tests were conducted as in the first part. In the second part polymer mud with and without weighing agents in which lubricant X was added in concentrations of 0,5%, 2% and 4%. In the third part of first phase polymer mud with and without weighing agents in which lubricant Y was added in concentrations of 0,5%, 2% and 4%. After analysing all the obtained data on the friction force and mud lubricity, it was observed that the value of the friction force created by using test blocks made of Teflon is the lowest compared to all other test materials such as metal, POM and PA6. Based on this, it was decided to continue the laboratory tests in second phase using test blocks made of Teflon and polymer mud with and without weighing agents in which one of three lubricants Z, X and Y was added in concentrations of 2% and 4%. Second phase of testing is divided in two parts, and it was conducted at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, on a machine that measures mechanical wear of mechanical elements that are in direct contact (Timken) and provide the value of friction factor. The first part was testing mechanical wear of blocks made from Teflon in a polymer mud without weighing agents in which one of three lubricants Z, X and Y was added in concentration of 2% and 4%. To simulate real parameters, the test was conducted at room temperature and at higher temperature (50 °C). The second part was testing mechanical wear of blocks made from Teflon in a polymer mud with weighing agents in which one of three lubricants Z, X and Y was added in concentration of 2% and 4%. After completing all the tests as part of this doctoral thesis, it can be concluded that the use of a test block made of Teflon significantly reduces the friction factor compared to a metal block. Also, after analysing all the data obtained on the device for measuring the mechanical wear of mechanical elements that are in direct contact, it can be concluded that the mechanical wear of the test block made of Teflon is minimal (0.08-2.79%) and considering the imagined idea of placing sliding protective rings on the tool joint of drill pipe should reduce their mechanical wear, i.e. in this specific case, the protective sliding ring will wear out and not the tool join of the drill pipe.