High-Permeability Anti-Clogging Porous Polyurethane for Coal-Fine Control in Gas Drainage Borehole Completions

Efficient gas drainage in soft coal seams is commonly impeded by two coupled issues: coal-fines-induced clogging of screens and boreholes, and instability of the borehole wall. To overcome these limitations, an in situ grouted porous polyurethane system was developed for borehole completion. The polyurethane slurry, consisting of isocyanate, polyether polyol, catalyst, foam stabilizer, cell-opening agent, cross-linker, and water as a blowing agent, was formulated to coordinate foaming and gelation kinetics. By adjusting the type and dosage of catalyst, the gel time could be precisely controlled within 10–1500 s to suit different construction requirements. After curing, the material exhibited an interconnected open-cell structure with a porosity of approximately 83%, permeability greater than 4 D, and a uniaxial compressive strength of about 1.72 MPa. Mercury intrusion porosimetry revealed a highly connected, multiscale pore network, with an accessible porosity of 78.9%, a median pore size of 125 μm, and a dominant pore-size range of 1–301 μm, indicating favorable conditions for gas flow. Flow-through experiments under simulated methane drainage showed that coal-fine production is strongly dependent on flow rate: fines generation was negligible at flow rates ≤20 L/min and became noticeable at around 30 L/min. Scanning electron microscopy confirmed that coal fines were confined to the upper ~5 mm of the consolidation layer, where bridging and straining within small near-surface pores limited deeper penetration. Although near-surface fines deposition reduced permeability from the intrinsic polyurethane value (~4.0 D) to ~2.0 D, the permeability stabilized above ~1.5 D under dynamic conditions. Overall, these laboratory-scale results demonstrate that the porous polyurethane can effectively intercept coal fines within a shallow surface zone, provide sufficient mechanical support to stabilize the borehole, and maintain high permeability under the tested conditions, suggesting its potential as a candidate material for enhancing methane drainage performance in soft coal seams. Further field validation and comparative studies against conventional completion systems are needed to assess its true engineering viability.