## 2 The Mechanism of Hole Cleaning for Different Well Profiles  
It is possible to improve the cleaning quality of a vertical borehole by reducing the final settling velocity of the cuttings. However, as the zenith angle increases, the reduction in the final settling velocity contributes less and less to the cleaning quality (Figure 1a). The sludge transport efficiency has to be corrected, because only the axial component of the final settling velocity is taken into account. It is therefore more difficult to predict the cleaning quality in such cases [22]. When predicting and controlling well cleaning, the formation of cutting layers has to be taken into account. Because of this, the volumetric concentration of the cuttings increases [23; 24]. Moreover, in the process of deviated well drilling, problems often arise with bottom hole cleaning and the removal of broken rock to the surface due to the deposition of rock particles on the bottom hole or the borehole walls under ineffective cleaning modes. The efficiency of borehole cleaning from cuttings depends not only on the main hydrodynamic indicators and technological parameters of the drilling regime but also the geometry of the annular space and the borehole profile. In turn, the geometry of the annular space is often determined by the eccentric position of a drill string in a wellbore, and the borehole profile is determined by the zenith angle [24]. More importantly, the transport of cuttings has a significant influence on extended reach drilling. Cuttings created in horizontal wells often settle on the bottom side of the annular section. A stagnant cuttings bed will develop if the cuttings are not sufficiently cleared. This may result in a number of operating problems, including high torque and drag, low penetration rates, clogged pipes, lost circulation, and well control problems. Drilling activities must thus be halted sometimes for specific hole cleaning procedures, adding to the amount of unproductive time (Figure 1a) [25; 26; 27]. Figure 1b shows that holes with inclinations between 35 and 60 degrees are more difficult to clean compared to near-vertical sections, which are the easiest to clean.  
Moreover, various classifications of borehole zenith angles are known, affecting the degree of clearance of the drilled rock particles. In mechanical cuttings' removal, the mud behaves differently depending on the zenith angle (Figure 2). In intervals with zenith angles up to 30\({}^{\circ}\), at significantly higher velocities (zone 1), the cuttings are effectively carried away by the mud flow and no sediment is formed on the walls. At lower velocities (zone 3), cuttings are slowly carried away. At zenith angles greater than 30\({}^{\circ}\), the cuttings are deposited on the bottom wall of the borehole. At lower velocities (zone 4), the deposited  
Figure 1: Schematic of transport cutting: (**a**) shows the transport of cuttings in a horizontal well, and (**b**) shows the difficulty of hole cleaning in relation to inclination.  
cuttings are mixed upwards as a single mass, while in turbulent mode (zone 2), they rise on the contact surface of the cutting jwith the drilling fluid in the form of dunes. Conditions corresponding to zones 1 and 2 are ideal for cleaning the cuttings from the borehole. The conditions corresponding to zone 5 guarantee the occurrence of tool puffs, as there is no cuttings' outflow at low rates of upward flow. As can be seen, cutting outflow is more difficult with an increasing zenith angle and lower flow velocity in the annulus. The greatest difficulties arise at zenith angles from 30 to 60\({}^{\circ}\), as under these conditions, the deposited cuttings tend to slide down the borehole and form plug [24,28].