## 2 Laboratory Testing  
Over the years, significant advances have been made in the development of lubricants aimed at reducing friction, and thus the torque that will occur during drilling. Due to the difficult working conditions in the field and the impossibility of the continuous monitoring of the changes occurring in the well, it is necessary to carry out detailed laboratory tests aimed at selecting the appropriate lubricant and material to produce the protective sliding rings. All laboratory tests, whose results are shown in this paper, have been conducted in the laboratory for drilling fluids at the Faculty of Mining Geology and Petroleum Engineering, University of Zagreb, and in the laboratory for machine elements at the Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb. In the laboratory for drilling fluids, according to API standards (API RP 13B-1 [33] and 13B-2 [34]), the physical parameters of the used drilling fluid have been tested. The OFITE lubricity tester shown in Figure 3 was used for the assessment of the lubricating potential of drilling fluids, providing data to evaluate the type and quantity of lubricating additives that may be required in known fluid systems. Materials Used for Measuring  
On the lubricity tester, a test was carried out on different materials such as steel (regularly used), Teflon, PA6 and POM, which could potentially be used for the manufacturing of protective sliding rings. To ensure the uniformity of the laboratory test, the blocks made from PA6, Teflon and POM are produced in the same dimensions as the steel block as it is described in the manual of the OFITE lubricity tester. The blocks made from steel, PA6, Teflon and POM are shown in Figure 4.  
**Polyamide 6 (PA6)**  
Polyamides are a group of mostly crystalline plastomers which, according to their use, belong to the group of structural plastomers and are a common material used to produce polymer gears and other structural elements [35]. **Polyoxymethylene (POM)**  
Polyoxymethylene (POM) is one of the polymers with excellent mechanical properties developed in recent years. Polyoxymethylene (POM) belongs to the type of crystalline poly  
Figure 4: Blocks made from Teflon, PA6, POM and steel used for research on lubricity tester. Figure 3: OFITE lubricity tester used for measuring coefficient of lubricity. mer material with a degree of crystallinity over 70%. Thanks to the strong -C-O- structure in its main chain, POM maintains the characteristics of stable chemical compositions and excellent mechanical properties, such as high strength/stiffness, favorable impact/creep resistance, and prominent long-term durability [36]. _Teflon_  
Teflon is a synthetic fluoropolymer made up of a tetrafluoroethylene monomer. The chemical name of Teflon is poly (1,1,2,2 tetrafluoroethylene). It is a thermoplastic polymer. Teflon's chemical formula is (C\({}_{2}\)F\({}_{4}\))\({}_{n}\). The Teflon formula shows repetitive or n numbers of C\({}_{2}\)F\({}_{4}\) units. It has an ability to maintain high strength, toughness, and self-lubrication at low temperatures (around \(-268.15\)\({}^{\circ}\)C (5 K)), and good flexibility at temperatures above \(-79.15\)\({}^{\circ}\)C (194 K) [37]. When testing the drilling fluid lubricity, two steel surfaces are brought into contact and in between them a friction force occurs, i.e., a rotating ring is placed on a rotating shaft and during the measurement it is brought into contact with a steel block. In this way, the friction force that occurs is simulated between the drill string and the casing string both made from steel material. The lubricity Test should only be performed when the unit has been successfully calibrated with deionized water and gives a coefficient of friction reading of \(34\pm 2\) (that is, the friction force value). After the device is calibrated, mud lubricity testing is carried out for 5 min, after which the torque value is read and recorded. The measurement was performed for 15 min, and the reading was recorded after 5, 10 and 15 min.