Study on the gas flow resistance state in unstable collapse boreholes and the classification of borehole types

One of the key issues affecting the efficient extraction of coal seam gas is the instability and collapse of extraction boreholes. To investigate the impact of collapsed coal particles on gas flow, a simulation system for gas extraction in collapsed boreholes was established. Based on the characteristics of gas flow resistance distribution within the borehole, the effects of coal particle size and collapse location on gas extraction were investigated. Furthermore, borehole types were categorized according to extraction efficiency. The study revealed that smaller sizes of the collapsed coal particles in the borehole result in higher resistance and complexity in gas flow. The closer the position of the borehole collapse is to the borehole opening, the more obvious the influence of the particle size of the collapsed coal particles on the gas extraction efficiency. The total extraction flow rate and peak concentration of the collapsed borehole decrease as the particle size of the collapsed coal decreases and the position of the borehole collapse gradually approaches the borehole opening, while the residual gas concentration in the coal seam shows the opposite trend. The gas extraction time is jointly influenced by the particle size of the collapsed coal particles and the collapse position. When the collapse occurs at the bottom of the borehole, the gas extraction time prolongs as the particle size of the collapsed coal particles decreases. When the collapse occurs in the middle of the borehole or at the borehole opening, the gas extraction time first increases and then decreases as the particle size decreases. In the evaluation of gas extraction efficiency across different types of unstable boreholes, a score lower than 1.0 indicates a Type I collapsed borehole, while a score higher than 1.0 indicates a Type II collapsed borehole. The research results are helpful to optimize the gas extraction strategy and are of important reference value for the smooth extraction of coal seam gas.