Numerical Simulation Study On Seepage Characteristics Of Methane In Compacted Crushed Rock Masses In Coal Mine Gobs

Methane resources of gobs in China is rich,but the recovery factor is low.The classic “three-zone” theory holds that: the bottom of the borehole is arranged in the “fracture zone” to efficiently extract methane.However,the pressure of methane in the gob is low,and the extraction range is small.Methane extraction of gob faces difficulties such as "no gas production","less gas production",and "large gas production difference".However,there are few studies on the extraction of methane in the caved zone,and only the caved zone is assumed to be a "black box",and the empirical setting of void ratio in different layers of the gob is to study the seepage of methane in the "black box".In this paper,the numerical simulation,experiment and on-site measurement methods were used to study the distribution characteristics of voids in different horizons in the crushed rock mass of the gob and the seepage characteristics of low-pressure methane in the voids.The main research results were as follows:Under the Lagrangian framework,the Hertz-Mindlin non-sliding contact model in the soft ball model was used for force analysis,describing the collision forces between rock blocks and rock blocks,and between the rock blocks and the wall.Comprehensively considering the gravity of the rock block,a mathematical model of the compression of the crushed rock mass was establish;For the methane containing air in the gas phase,the gas flow equation was established under the Euler framework using the discrete element method;By combining the equations constructed above,a mathematical model of methane seepage in a crushed rock mass was established.Based on the model of methane seepage in crushed rock mass,the seepage process of low-pressure methane in the void of crushed rock mass was simulated.The void of crushed rock mass provided space for the flow of methane from bottom to top.The larger the void between the rock blocks,the greater the flow rate of methane.The denser the particle arrangement,the stronger the ability to hinder the flow of methane.The void between the rock blocks at the bottom of the crushed rock mass was large,the arrangement of the rock particles was relatively loose,and a large amount of methane was stored,resulting in large methane pressure and flow velocity at the bottom of the crushed rock mass;As the distance from the bottom of the crushed rock mass increased,the void between the rock blocks gradually decreased,the pressure of the methane gradually decreased,and the flow velocity of the methane basically stabilized.At the same level of the crushed rock mass,with the increase of methane concentration,the flow velocity of methane also gradually increased.In this paper,three closed mine gobs with similar geological conditions were selected.Among them,# 1 borehole,#2 borehole and # 3 borehole were located in the fracture zone,the upper of the caved zone and the bottom of the caved zone,respectively.The methane extraction volume of # 1 borehole,# 2 borehole and # 3 borehole increased in turn,indicating that the # 3 borehole was the best extraction borehole for low-pressure gob.This was due to the low pressure of methane in the gob and the small effective extraction range.When the bottom of the borehole was arranged in the fracture zone,it was difficult for the low-pressure methane in the gob to flow into the borehole.The void in the bottom of the caved zone was large,and a large amount of methane was stored.The borehole was located in the bottom of the gob,which could quickly and effectively extract the methane desorbed by the left coal.