Study on gas flow characteristics and pulse blockage removal regulation in unstable collapsed boreholes

Gas drainage boreholes easily suffer hole wall collapse and coal particle blockage under in-situ stress and mining disturbance, drastically lowering gas extraction efficiency. To avoid the drawbacks of traditional dredging technologies, including drainage interruption and borehole wall damage, a gas pulse blockage removal method is proposed to achieve non-contact dredging using transient high-pressure gas. By establishing a gas pulse dredging test system for collapsed and blocked boreholes, the flow resistance characteristics under the coupling effect of collapse position and coal particle size are revealed. The regulation mechanisms of pulse pressure and frequency on blockage removal performance are clarified, and prediction models for pore response and structural penetration degree are developed, considering energy attenuation and coal structure evolution. The results indicate that collapse position and particle size jointly determine borehole flow resistance. Fine coal blockage at the borehole opening creates a global flow bottleneck, bringing the lowest extraction peak concentration and serious gas retention in surrounding coal seams. Pulse energy decays exponentially with transmission distance: bottom-blocked boreholes show weak unblocking effect with response lag over 10 s, while orifice blockage responds rapidly and achieves prominent concentration growth. Pulse pressure presents a marginal diminishing promotion effect on dredging efficiency. Under the conditions of this experiment, 0.4 MPa is the optimal pressure for most conditions, with gas concentration increment ΔC reaching 110–186%; further increasing pressure to 0.6 MPa limits ΔC growth below 20.3%, and fine coal at the opening undergoes turbulent secondary deposition to weaken dredging effect. Pulse frequency dominates cumulative energy input, and unblocking performance evolves following a three-stage rule of slow growth, accelerated breakthrough and decelerated growth. The optimal frequency is 4 cycles per 10 min for middle-borehole collapse (ΔC increment of 94.7%) and 3 cycles per 10 min for orifice collapse (ΔC increment up to 168.0%). The model developed quantifies key parameters, showing that the comprehensive reaction coefficient A decreases as particle size increases, whilst the energy accumulation index B rises with increasing propagation distance. Based on this, a differentiated pulse strategy of ‘high-frequency, short-duration pulses near the source and low-frequency, cumulative pulses far from the source’ is proposed, which provides a theoretical basis and guidance on parameter design for the efficient and precise unblocking of gas extraction boreholes.