Study On Distribution And Variation Of The Negative Pressure Of Gas Drainage Bore In The Coal Seam

Gas pre-drainage in the coal seam is an important measure to prevent and control gas accident in coal mines,and it is also an effective measure for the gas outburst prevention.Study on the influence of the distribution variation law of the negative pressure in the borehole on the gas pressure distribution around the borehole,and the gas flow of borehole with drainage time changing will be of significant importance in improving the efficiency of gas drainage.In this paper,the seam is considered as a porous medium.And the adsorbed gas move as diffusion in the coal matrix,while the free gas move as seepage flow in the fractures.Using the finite element method analyze and propose the calculation method of the distribution of the gas flow and negative pressure within the borehole.And then based on the above theory,the equation takes coal seam deformation into account.The process of the gas drainage of the borehole has been calculated using the COMSOL software.And it analyzes the influence of the negative pressure in the borehole on the distribution of the gas pressure around the borehole and the the distribution of the gas flow along the along the length in the borehole.The results show that:(1)The coal seam gas pressure of the same location from the wall of the borehole is equal to each other.And the distribution of gas pressure within the borehole has not much influence on the gas pressure around the borehole at the same location.(2)When the borehole is complete,the value of the negative pressure loss is very small.Therefore,the negative pressure loss of the borehole have no effect on the gas flow at the the gas flow which comes from the coal seam at various locations within the borehole of borehole.So,Along the hole length,the distribution of the gas flow from the opening to the bottom of the hole linearly decrease.(3)When the borehole is not complete,the coal of the top of borehole stack in the lower of borehole,and it results in the reduce of the cross-section of the borehole and will be smaller with the levels of hole collapse increasing.At the same time,with the increased levels of hole collapse,the value of the negative pressure loss will be bigger and bigger.But the value of loss is still small compared to the drainage negative pressure.Therefore,the negative pressure loss of the borehole have no effect on the gasflow which comes from the coal seam at various locations within the borehole of borehole.So,the distribution of the gas flow from the opening to the bottom of the hole linearly decrease,still.(4)When the borehole is plugging,the value of the negative pressure loss is small in the section of the no-plugging borehole,and the pressure in the the plugging borehole is increasing and close to coal seam gas pressure.This permeability of the plugging section of the borehole is bigger than the permeability of coal which is next to the plugging section.But the negative pressure has little effect on the plugging section.