# Physical Simulation Experiment for Visualizing Pulverized Coal Transport in Propped Fractures
## 1 Introduction  
Pulverized coal output is more common during coalbed methane (CBM) production, leading to the fragility of propped fractures, stuck pumps, and buried pumps, which ultimately reduce the production capacity of CBM wells [1; 2; 3; 4]. This problem considerably limits the productivity of coal seam gas wells [5]. Consequently, understanding the transportation dynamics of pulverized coal and implementing effective prevention and control measures are pivotal for the economic and efficient development of CBM. Variations in pulverized coal production characteristics occur across different geological and engineering conditions [6; 7]. Notably, significant fluctuations in pulverized coal output occur during various stages of discharge mining [8]. Among these stages, the single-phase water flow phase is particularly prone to pulverized coal occurrences, wherein the coalparticles primarily originate from the erosion of cracked walls by proppants, resulting in small particle sizes [9]. Therefore, studies on pulverized coal transportation dynamics as well as prevention and control countermeasures should pay special attention to the single-phase water flow stage. The transportation and deposition patterns of coal dust within propped fractures during the single-phase water flow stage have been extensively investigated. The transportation of pulverized coal requires a specific critical start-up flow rate, which varies depending on the characteristics of the coal particle size [10; 11]. Some researchers have considered the adhesive properties of coal particles and utilized hydrodynamic theory to establish a mechanical model for the initiation of pulverized coal deposition within coal seam fractures during the single-phase water flow stage. Their findings indicate that the initiation velocity exhibits an initial decline followed by an increase with increasing particle sizes. Moreover, through an analysis of the forces acting on individual static pulverized coal particles within a cutting system, three primary modes of pulverized coal transport under the influence of single-phase water flow have been identified as rolling, lifting, and sliding [12; 13; 14]. Numerous factors influence the transportation and settling of pulverized coal, including the internal structure of the porous medium, transport pathways, ambient pressure, permeability, seepage velocity, flow field, particle size, particle size distribution, and fluid viscosity [15; 16; 17]. To investigate the laws governing the transport and settling of pulverized coal, certain scholars have utilized flow conductivity testers and self-designed experimental apparatus (including a visualization parallel plate and high-transparency tempered glass) to conduct physical simulation experiments on coal transport within cracks [18; 19; 20; 21; 22]. These experiments explain why suspended pulverized coal tends to deposit more readily at the edges of porous media. The key findings are as follows: as the particle size of pulverized coal increases, its trajectory tends to lower, increasing the likelihood of deposition; higher fluid flow rates and viscosity enhance the suspension capacity of pulverized coal, making it less prone to deposition; pulverized coal can travel greater distances within cracks with smaller curvature and better connectivity, resulting in higher coal output; however, pulverized coal can also obstruct transportation channels. However, these studies mainly focus on the fluid flushing of pulverized coal or pulverized coal mixed solutions driving within cracks, neglecting visualized physical simulation experiments of pulverized coal mixed solutions entering proppant-filled zones. Furthermore, there is a lack of experimental studies on the impacts of sedimentation, aggregation, and dispersed transport of pulverized coal on the permeability of coal reservoirs and the inflow capacity of proppant fractures.