## 1 Introduction  
Coalbed methane (CBM) is a high-quality source of clean energy but also the main factor responsible for coal mine gas disasters. Hence, the efficient development of CBM resources is an important component of both the readjustment of China's energy structure and continued safe coal mining. However, the CBM reservoirs in China are complex in that they are characterized by low permeability, low saturation, low reservoir pressures, and relatively high metamorphic grades [1, 2]. It is therefore necessary to adopt methods for increasing the permeability of coal seams.  
Some methods have been employed to improve the permeability of coal seams and some results have been achieved, with techniques such as hydraulic fracturing [3, 4], deep hole pre-splitting blasting [5], water jet drilling [6, 7], and other techniques used to increase CBM production. Water jet drilling is widely used for CBM development. It has the advantages of temperature reduction and dust removal, and this drilling technique does not produce sparks and is very effective at breaking rock [8]. Existing water jet bits include rotary jet bits, combined jet bits, and straight-swirling integratedjet bits. Rotary jet bits break a large area but because of relatively low jet velocities near the bit axis, their rock-breaking efficiency is low. A boss is formed at the center of the borehole and this boss hinders drill bit advance and reduces drilling efficiency [9]. A combined jet bit can integrate different jet characteristics, but designing an effective bit of this type is difficult. The two kinds of water jet are easy to interact, and these bits are large [10; 11]. The straight-swirling integrated jet bit is very good at breaking rock and is reliable, but the large bit is not conducive to field application [12]. Hence, some investigators have proposed using self-rotating bits: bits that have high rock-breaking efficiency, low specific consumption, and produce large diameter holes [13; 14; 15; 16]. [PERSON] et al. introduced a self-rotating bit for ultra-short radius self-propelled horizontal drilling of CBM wells [17]. They analyzed the bit's drilling speed and hole diameter but did not investigate the influence of any hydraulic parameters or the jet operating parameters on hole diameter. [PERSON] et al. developed a model for calculating the rotation speed of a self-rotating bit and analyzed the influence of structural parameters on the bit's rotation speed [14]. [PERSON] et al. simulated the effects of jet angle and bit rotation speed on the jet flow field [18]. However, these previous self-rotating drill bit studies have focused on optimizing the bit structure and analyzing the fluid flow around the bit. Moreover, there are few studies which focus on the effect of different water jet parameters.  
Rock-breaking efficiency is determined by water jet parameters. Hence, this paper has carried out rock breaking experiments under different rotating speeds, jet pressures, and incident angles. The influences of jet pressure, rotation speed, and impact angle on the rock-breaking efficiency of a self-rotating jet were obtained. Self-rotating bit design considerations were analyzed. The conclusions from this study can provide a reference and guidance for self-rotating bit design optimization and the determination of performance parameters.