A novel permeable wellbore completion method for gas drainage in soft coal seams: grouting material development and coal fines control mechanisms

Borehole gas extraction in soft coal seams frequently suffers from borehole collapse, coal fines clogging, and rapid permeability decline, severely limiting long-term gas recovery efficiency. To address these challenges, this study proposes a novel permeable wellbore completion method based on in-situ grouting of a pore-tunable porous polyurethane (PU) material. A series of PU slurries were formulated and optimized through 12 iterations by systematically varying the surfactant, flame retardant, and auxiliary foaming agent contents. Three representative formulations (Groups X, Z, and D) with distinct pore architectures were selected for comprehensive evaluation. The permeability, compressive strength, pore structure, and coal fines control performance were investigated under simulated reservoir conditions, including confining pressures of 0.5–3.0 MPa and high-concentration coal fines flow. Mercury intrusion porosimetry, micro-CT three-dimensional reconstruction, and SEM-based particle size analysis were employed to characterize pore-throat alterations, coal fines deposition sites, and effluent particle size distributions. The results show that all three PU groups maintained permeabilities above 1 Darcy under confining pressure, with compressive strengths ranging from 3.80 to 4.86 MPa. Group X exhibited the highest porosity (70.37%), smallest median pore diameter (190.1 μm), and greatest interception efficiency, while Group D, with the largest dominant pore diameter (810.47 μm), showed the lowest permeability damage rate (66%). Coal fines transport and retention followed a composite mechanism characterized by selective deposition at the seepage front, agglomerate/bridge formation at pore throats, and sieve-controlled penetration of finer particles, consistent with the “1/3–1/7” particle-to-pore size criterion. Particles > 100 μm tended to be intercepted, particles < 30 μm passed through more readily, and medium-sized particles (30–100 μm) formed the main internal clogging framework. Despite 66–71% permeability impairment, post-test permeabilities remained above 1 Darcy, satisfying the high-permeability requirements for gas drainage applications. This study provides a validated material design framework for achieving balanced permeability, mechanical stability, and coal fines control in soft coal seam gas extraction, offering a scalable technical pathway for enhancing long-term borehole completion reliability.