A novel dynamic filling material for plugging fractures around underground gas extraction boreholes: Experimental and engineering performances

Underground gas extraction is an important technical means to solve coal mine gas disaster and utilize coalbed methane resource. Air leakage around extraction boreholes is a key factor for restricting the efficient gas extraction. Cement-based materials and polyurethane are currently the most widely used for plugging fractures to prevent air leakage. However, regenerated fractures can still develop under stress disturbances around boreholes after materials solidify. Regenerated fractures create new leakage pathways for external air to enter a borehole, significantly impacting the efficient extraction of gas. To overcome shortcomings of solid-phase materials in plugging fractures, based on the concept of "solid plugging liquid and liquid plugging gas", a novel dynamic filling material with superior stability, strong permeability, and adaptable performance is developed for plugging fractures in this paper firstly. Then the rheological properties, anti-interference properties, hydrophobic properties and water retention properties of the dynamic filling material are tested and the experimental results show that the addition of coupling agent can improve the rheological properties of the material; the intrusion of coal powder has little effect on destroying the stability of the material; the dense protective film formed by the material can effectively reduce the intrusion of external water and its own water loss. The properties of the materials modified with different additives had good mobility and stability, and can be used for plugging fractures around boreholes dynamically. Finally, the dynamic filling material is applied in underground gas extraction engineering in Tangshan Coal Mine. The field test shows that the average methane concentration of boreholes with the dynamic filling material plugging increases from 8 % to 24 %. The material is suitable for dynamic and long-term filling and leakage prevention in regenerated fractures in coal.