Influences of negative pressure on air-leakage of coalseam gas extraction: Laboratory and CFD-DEM simulations

Comprehensive understandings of the influences of negative pressure on air-leakage and its driving mechanism are critical for controlling methane concentration during coal seam gas extraction. Laboratory experiments were conducted using an integrated apparatus to study the air-leakage process of coal seam gas extraction under different negative pressures. In addition, a coupled computational fluid dynamics-discrete element approach was used to simulate the process of air flow and particle transport in a natural coal fracture to understand the influential mechanism of negative pressure on air-leakage at the mesoscale. Experimental results show that with an increase in the absolute value of negative pressure, the air-leakage experiences three stages of increase, decrease and increase at Stage I, Stage II and Stage III, respectively. It is reported for the first time and in good agreement with field test data. From experimental and numerical results, at Stage I the air-leakage increases with the absolute value of negative pressure because there is no blockage in fractures around borehole. At Stage II, the further increased absolute value of negative pressure contributes to particles motion in fractures. It gradually leads to the formation of blockage structure composed of aggregated particles in fractures, and the decrease of fracture conductivity and air velocity. With the absolute value of negative pressure rising continuously, the contact forces between the aggregated particles increase. Therefore, the balance of the blockage structure tends to be broken, which helps to understand the air-leakage increase with the absolute value of negative pressure at Stage III. Between Stage II and Stage III, there is a valley point corresponding to the minimum air-leakage, which is important to the improvement of gas extraction performance. The contact forces on the aggregated soft particles are larger than those on the aggregated hard particles under the same negative pressure. Therefore, the blockage structure consisting of soft particles is much easier to be broken than that formed by hard particles. This explains why the absolute value of negative pressure corresponding to the valley point for hard coal model is larger than that for the soft coal model.