Damage Mechanism of Solid Particles in Deep Coalbed Methane Reservoirs: Experimental and Numerical Simulation Studies

With the depletion of shallow coalbed methane, the development of deep coalbed methane has become crucial. Deep coalbed reservoir conditions are complex and highly susceptible to permeability damage caused by solid particles during drilling. This study systematically investigates the permeability damage mechanisms of solid particles in fractured coal reservoirs via combined experimental and numerical approaches. Scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and X-ray diffraction (XRD) analyses characterized pore structure, clay mineral composition, and surface hydrophobicity. Hydration and velocity sensitivity tests elucidated how internally generated particles reduce permeability under varying conditions. Permeability damage experiments quantified drilling fluid particle size effects, while computed tomography (CT) and particle migration-deposition modeling revealed microscale dynamics and concentration effects. Results demonstrate that clay mineral types govern internal particle generation, while seepage velocity controls migration. Permeability damage follows a multiscale synergistic mechanism: “size matching - concentration - pore structure response”. Fine particles penetrate macropore throats through microfractures; coarse particles form bridge plugs at fracture entrances. Elevated particle concentrations induce stepwise porosity reduction and exponential permeability decline. The coal’s hydrophobicity restricts filtrate invasion but enhances particle retention. Dispersible clays release fines under shear stress, aggravating internal blockages. Microscopically, highly connected fractures provide low-resistance pathways, whereas pore-dense zones become deposition hotspots due to high flow tortuosity. Macroscopically, permeability exhibits nonlinear decay patterns correlating with particle spatial distribution. A predictive framework integrating particle size distribution, concentration thresholds, and pore heterogeneity was established, providing theoretical guidance for reservoir protection in deep coalbed methane development.