# Analysis of Application Parameters of Hydraulic Slotting Technology in Jointed Coal Reservoirs
## 1 Introduction  
Coalbed methane (gas) is a new form of clean energy stored in coal seams [1; 2; 3; 4]. According to statistics, the global gas resources buried at a depth of less than 2000 m amount to 240 trillion cubic meters, which is twice that of conventional natural gas resources and has great exploitation potential [5; 6; 7]. Moreover, gas exploitation can effectively prevent and control coal and gas outbursts, and reduce production accidents during coal mining [8; 9]. China has the third largest gas reserves in the world. However, most coal mines in China are characterized by complex geological structures and low-permeability coal seams, which makes the effective extraction of gas difficult using traditional technologies. Currently, hydraulic fracturing technology and water jet slotting technology are typically used for the enhancement of coal-seam permeability [10; 11]. Hydraulic fracturing technology has effective applications; however, various shortcomings, such as high initiation pressure and the existence of non-fractured areas, still exist and prevent the application of this technology to coal seam structures because they may induce outburst accidents [12; 13]. To a certain extent, water jet slotting technologycan significantly improve the effect of gas extraction and is applicable to all coal seams. Moreover, it is currently an indispensable technology for enhancing the permeability of coal seams [11; 14]. To investigate the permeability-enhancing mechanism of hydraulic slotting and optimize the hydraulic slotting effect, many studies produced relevant investigations by experimental tests. [PERSON] et al. [15] implemented hydraulic slotting in the Wangxingzhuang coal mine in China and found that the effective diameter and average gas-extraction concentration of the slotted borehole were approximately 12.8 and 3.7 times those of a conventional borehole. Additionally, their results revealed that the hydraulic slotting technique is effective. [PERSON] et al. [16] proposed different hydraulic slotting techniques and carried out field tests. Their results revealed that the average gas-extraction concentration increased by more than two times. [PERSON] et al. [17] derived the velocity distribution law and the pressure attenuation law of a water jet based on theoretical analysis, and established a maximum slot depth prediction model. Additionally, the error range of the field test results and maximum slot depth prediction model was less than 10%. [PERSON] et al. [18] tested and analyzed the coal parameters before and after hydraulic slotting and coabled methane pre-drainage, and reported that the effective stress of the coal seam sequentially changed during the slotting and pre-drainage processes, and that the pore proportion and coalbed methane adsorption properties of the coal changed significantly. [PERSON] et al. [19] established a fluid-solid coupling test system, which provided real air pressure and stress, and used it to simulate the entire hydraulic slotting extraction process. In this experiment, the law governing the stress variation during the slotting process was analyzed, and the deformation characteristics and gas migration were obtained with different slotting parameters. For numerical simulation, many scholars analyzed fluid-solid coupling by extended and generalized finite element methods [20; 21]. In order to make the numerical simulation results more accurate, [PERSON] et al. [22; 23] used an extended finite element method to study fluid-solid coupling, and optimized the couplings between stress and pressure. [PERSON] et al. [24] and [PERSON] et al. [25] used an extended finite element method optimized in different ways. In the numerical simulation of hydraulic slotting technique, [PERSON] et al. [26] investigated the self-excited oscillation pulsed jet slotting technology by conducting a numerical simulation, and proposed and analyzed a cross slotting arrangement to reduce the slotting construction.