## 1 Introduction
In studies [15; 16], methods of statistical analysis were used to demonstrate that the waterjet radial channel technology is more effective in comparison with the technology of hydrochloric-acid treatment of bottomhole formation zone and the technology of \"reperforation/quick-gain perforation\". Today, multiple equipment configurations are used for waterjet stimulation both in the oil and gas industry and in the construction and public utility sector, including waterjet nozzles of various designs. For example, study [17] investigates energy converting efficiency and proposes the most optimal design of the waterjet nozzle determined by numerical modeling. Figure 1: Schematic diagram displaying the working principle of the jet drilling. As a rule, waterjet nozzles have separate holes for front and rear jets. The waterjet nozzle is self-propelled, i.e., its movement inside the rock along the radial channel it creates is propelled by the reactive forces of jets from the rear holes of the nozzle. Literature [4] also describes important aspects of influence of distance between the waterjet nozzle and the rock under stimulation on the efficiency of radial drilling, but these phenomena are not fully studied. The main tool for radial drilling was borrowed from the municipal economy. Hydraulic monitoring nozzles are widely used for cleaning the inner surface of various types of pipelines. Pipe blockages are removed using through-type nozzles, rotary type nozzles or punch type. Figure 2 schematically shows the specified types of nozzles. Most often, through-type nozzles are used to intensify oil or gas production. The bodies of such nozzles are monolithic and have separate holes for front and rear jets. This specific nozzle type is reliable from a constructive point of view, however, the area of impact of the frontal jets will depend on the number of front separate holes. The rotary nozzle type has a housing consisting of two different parts, one of which rotates due to the pumped flow through the nozzles. In the literature [18,19], there are nozzle designs with both a front rotating part and a rear one. The use of this type of nozzle makes it possible to cover the treated area more evenly. In the case of oil and gas production, this nozzle type will enable the creation of highly permeable radial channels of the correct geometry with a circular cross section. However, this type of nozzle is prone to a large number of breakdowns. The third type of nozzle does not have front separate holes and is not used in oil and gas production. This nozzle type is used only in the communal sector. Further in this article, only through-type nozzles are considered as the most common in the oil and gas industry at the moment. The process of radial drilling of highly permeable channels comprises several key stages: cutting off the side channel (cutting off the metal casing), hydraulic jet destruction of the rock, and washing out the resulting sludge to the surface. Each of the listed stages must be considered separately in detail. For example, the issue of raising cuttings by liquid flows during the radial drilling process is discussed in detail in [20]. However, this article is devoted to a detailed study of only the stage of rock destruction in order to create a highly permeable channel. In [20], the authors made a significant contribution to the study of waterjet nozzle movement and the forces propelling it. The study presents the results of mathematical modeling of the thrust force of the waterjet nozzle motion and shows an example of determining the optimal design parameters of the waterjet nozzle. The authors made very important conclusions that underlie the most effective approach to practical application of radial drilling technology for the purpose of enhanced oil recovery, specifically:  
1. Self-propulsion capability of the waterjet nozzle decreases significantly as the number of front holes increases. 2. The angle of the front holes has a negligible effect on the self-propulsion capability of a waterjet nozzle. 3.