## 4 Cavitation Jet Drilling  
Drilling into rock using a continuous fluid stream becomes feasible when surpassing a threshold pressure, which, especially for hard rocks like granite and basalt, can often exceed 200 MPa, representing the rock's compressive strength limit. However, the high  
Figure 1: The main components of a drilling nozzle taken into account during the literature review. threshold pressure poses challenges due to issues with the seals employed in high-pressure equipment, limiting the applicability of such methods. To circumvent these challenges, solutions involving a jet with pulsating pressure are often utilized, employing devices like jet cannons to momentarily increase the pressure to high values. An alternative approach involves drilling using a continuous cavitation jet, which incorporates gas bubbles. These bubbles contribute to an increase in the backpressure acting on the rock. The pressure induced by a continuous fluid stream can be mathematically expressed by the following equation of the pressure of stagnation P\({}_{0}\) in a fluid stream, which can be written using [PERSON]'s equation, describing the relationship between the fluid's pressure, density, and velocity:  
\[P_{0}=P+\frac{1}{2}\rho v^{2} \tag{1}\]  
where  
_P\({}_{0}\)_--pressure of stagnation;  
_P_--static pressure;  
_P_--liquid density;  
_v_--jet velocity. This equation demonstrates how the pressure of stagnation (_P\({}_{0}\)_) is composed of the sum of the static pressure (_P_) and the dynamic pressure term, \(\frac{1}{2}\rho v^{2}\), which is dependent on the density (_\(\rho\)_) and the velocity (_v_) of the fluid jet. [PERSON] [23] showed that for isothermal compression, the build-up pressure _p\({}_{i}\)_ produced by the cavitation flux is expressed by Equation (2):  
\[p_{i}=\frac{p_{s}}{6.35}exp\left(\frac{2}{3\alpha}\right) \tag{2}\]  
where  
_p\({}_{i}\)_--build-up pressure produced by the cavitation flux;  
_p\({}_{s}\)_--saturation vapor pressure;  
_\(\alpha\)_--parameter influencing the cavitation process (the gas content of the liquid, expressed as the ratio of the partial pressure of the gas to the stagnation pressure at the beginning of bubble destruction). [PERSON] and [PERSON] [23] assumed that the angle \(\alpha\) varies from 1/6 to 1/10, as shown in Figure 2. In this case, backpressure and stagnation pressure are related by Equation (3). [PERSON] [24] concluded that stagnation pressures as high as 1380-2700 MPa are produced by cavitating fluxes. \[p_{i}=(8\div 124)p_{s} \tag{3}\]  
Tests were conducted in hydronautics [25] on coal samples subjected to liquid jet drilling. The tests were carried out at a pressure of 13.2 MPa with a jet diameter of 6.4 mm and a 15 cm/s rate of penetration, as illustrated in Figure 3. Under these conditions, a cutting gap of 6-20 cm in diameter was achieved. The results presented in Table 1 reveal variations in specific energy based on different parameters. Specifically, the specific energy decreased as the rate of penetration (ROP) increased and increased with a larger nozzle diameter. The collected data indicate that the lowest specific energy value was observed when using the smallest nozzle, measuring 1.8 mm, coupled with a maximum ROP of 91.4 cm/s. These findings suggest that there exists an optimal combination of nozzle diameter and rate of penetration speed, resulting in minimized specific energy requirements for the hydraulic drilling of coal samples using a cavitation jet. This information is valuable for optimizing the process and enhancing the efficiency of radial drilling in coal utilizing this technology. As mentioned earlier, a cavitation jet allows for drilling through rock at lower pressures generated by pumps, even in hard rock, compared to a continuous jet. However, it is important to note that the specific energy required to cut a fracture using a cavitation jet, as discussed by [18], is higher than that needed with a continuous jet.