# Structural Optimization of Slotting Nozzle Used to Improve Coal-Seam Permeability
## 1 Introduction  
Coalbed methane (CBM) is a green resource with high energy efficiency. Extraction of CBM not only increases energy supply, but can also reduce greenhouse gas emissions and the occurrence of gas disasters. China is rich in CBM resources, ranking third in the world [1], but is highly dependent on natural gas imports. In 2018, China became the world's largest importer of natural gas, with an import volume of 125.4 billion m\({}^{3}\)[2, 3, 4]. CBM occurrence conditions in China are complex, mainly due to three factors: (i) low gas saturation, with an average gas content of 45%; (ii) low reservoir pressure, with a pressure coefficient of generally less than 0.8; and (iii) high degree of coal-seam metamorphism [5]. Therefore, CBM is mainly exploited by underground extraction in China. CBM reservoir permeability in China is extremely low (10\({}^{-4}\)-10\({}^{-3}\) mD), which is three to four orders of magnitude lower than that in Australia and the United States [6]. Therefore, improving coal-seam permeability and increasing underground extraction of CBM has become a focus of research. Methods that have been employed to increase the permeability of coal seams include dense boreholes, deep-hole blasting, hydraulic flushing, hydraulic slotting, and hydraulic fracturing [7; 8; 9; 10]; however, these technologies have drawbacks. In particular, the range of coal-seam permeability enhancement is small and uneven, and the construction cost is high. To solve these problems, some scholars have proposed the use of slotting directional hydraulic fracturing (SDHF), which combines hydraulic slotting with hydraulic fracturing [11; 12; 13]. The principle of this method is connecting a coal-seam slotting device with a high-pressure sealing drill rod. Water is forced into the slotting device through the drill rod by a pump and the water forms a high-speed jet after acceleration through the slotting nozzle; the slotting device simultaneously moves slowly into the borehole. In this way, coal around the borehole will be slotted and a regular slotted hole will be formed [14]. After hydraulic slotting, the slotted hole is sealed and then connected to a high-pressure pump for hydraulic fracturing. Hence, the exposed area, coal-seam structure ratio, and permeability are increased. The gas flow conditions are improved and the gas extraction rate will increase. Use of this technology significantly decreases the number of drill rods required and shortens the time required for gas pre-drainage. [PERSON] found that the size of the slotted hole is the key parameter affecting slotting-directional hydraulic fracturing (SDHF) [4]. The design of the slotting nozzle can enhance the water jet and its structure will significantly affect its ability to slot the coal seam [15]. To improve the effectiveness of slotting, much research into optimized structures has been undertaken, based on various conditions [16; 17; 18; 19; 20; 21]. [PERSON] first proposed a conical straight slotting nozzle structure [22]. Due to its high energy-transfer efficiency and simple manufacture, this nozzle has been widely used in mining, petroleum, fire, and other industries. Some scholars have found that streamlined inner-surface slotting nozzles designed on the basis of hydrodynamic theory have high energy-transfer efficiency. [PERSON] used a simple conical slotting nozzle to study the cavitation of a water jet [23]. The results showed that the slotting nozzle can intensify the shear between the water jet and the surrounding liquid, resulting in a large amount of cavitation. The Rankin-Shape water jet slotting nozzle manufactured by Aqua-Dyne (USA) can maintain convergence and high efficiency of the water jet for a larger target distance [24]. [PERSON] studied the influence of slotting nozzle shape on the water jet flow coefficient and flow field [25]. [PERSON] analyzed the influence of throat length of a convergent-expansion slotting nozzle on the flow-field parameters, both theoretically and experimentally [26].