_Article_  
**Investigation on Coal Fragmentation by High-Velocity Water Jet in Drilling:**  
**Size Distributions and Fractal Characteristics**  
**[PERSON] 1,2,3, [PERSON] 1,2,3,*, [PERSON] 1,2,3, [PERSON] 1,2,3,4, [PERSON] 1,2,3 and [PERSON] 1,2,3**  
1 State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China; [EMAIL_ADDRESS] (S.X. ); [EMAIL_ADDRESS] (Y.L. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (Q.L. ); [EMAIL_ADDRESS] (L.W). 2 College of Resources and Environmental Science, Chongqing University, Chongqing 400044, China
3 National & Local Joint Engineering Laboratory of Gas Drainage in Complex Coal Seam, Chongqing University, Chongqing 400044, China
* Correspondence: [EMAIL_ADDRESS]; Tel. : +86-23-6510-6640 2018  
for meeting the global energy demand for sustainable development, but also necessary for improving coal mine safety and reducing greenhouse gas emissions [2; 4; 5]. Unfortunately, CBM reservoirs in China are characterized by low saturation, low permeability, low reservoir pressure, and relatively high metamorphic grade, resulting in a low CBM production rate [6; 7; 8]. Thus, some measures must be taken to improve the permeability of coal seams by increasing the number of cracks and amount of gas desorption in coal. Water jet drilling (WJD) technology is a highly efficient new method to develop oil and gas for reservoirs with low permeability [9; 10; 11]. For the ground and underground CBM development, WJD is designed to create multiple lateral micro-holes or gas drainage boreholes in the CBM reservoir, respectively, utilizing high-pressure water jets to break the coal formation. This enables the gas formation to be exposed as much as possible, and it has been effective in developing unconventional resources such as CBM and shale gas in many countries [12]. It is necessary to remove the coal fragments effectively from the micro-holes in order to reduce the resistance to the bit and working pipes and form a long hole in the formation. However, the coal fragments generated by WJD are larger, more irregular, and are shaped like lumps compared with the cuttings generated by the traditional mechanical drilling method. This makes it more difficult for WJD holes to discharge coal fragments. For the cuttings generated by a mechanical drill bit, there have been many studies to investigate the transportation of cuttings in the well and the effects of drilling parameters on cutting transportation, including pump rate, well dimensions, fluid properties, solid sizes, solid loading and well inclination [13; 14; 15; 16; 17; 18]. However, the coal fragments generated from WJD are the results of coal fragmentation under multiple water jet impact. There are few studies that analyze the coal fragment transportation in WJD holes, and there is a lack of understanding regarding the fragmentation features of coal fragments, including the particle size distribution and morphology. Moreover, the fragment sizes and shapes have a dramatic influence on fragments transportation in WJD holes. The large coal fragments are difficult to discharge from the hole with water flow, resulting in sticking and serious friction resistance [19; 20]. Therefore, it is very necessary to study the fragmentation size distribution characteristics of coal generated from WJD. Research on the fragmentation size distribution characteristics for brittle materials has become a popular field of study [21; 22]. The fragments can be described by some statistical regularity. Rosin-Rammler and Weibull distribution functions are usually used to represent the size distribution of rock fragments [23]. Some models have been proposed to predict the mean fragment size for impact fragmentation and rock blast fragmentation [24; 25]. The three-parameter generalized extreme value distribution (GEV) can suitably describe the frequency distribution of fragment sizes generated from impact test using an SHPB [21; 26].