Numerical Simulation Study On The Impact Of Gas Migration Around The Borehole Dynamic Permeability

Permeability is a important parameter index,which is the key parameter to measure the difficulty degree of the gas drainage.Research on the influence of coal permeability distribution and permeability change around the borehole on gas migration,which can improve the gas drainage rate,guide field gas drainage and promote coal mine safety in production is of great significance.Based on the equation of gas migration in the coal seam and the equation of deformation control,the drilling extraction model under the condition of different permeability is established.The paper compares and analysis the law of coal gas migration when the permeability is constant,the permeability of ending drilling and dynamic permeability.The conclusions were obtained as follows:(1)Under the conditions of permeability change,the coal permeability of drilling decreases with the increase of extraction time;When the extraction time is 120 days,the gas pressure decreases 0.83 MP and 0.67 MPa where the horizontal distance of drilling is1 m and 3m,and the permeability decreases 1.6% and 1.3% which relative to the initial permeability.(2)Research on the influencing factors of the coal permeability around drilling,which can get the drilling diameter and drill drainage under negative pressure have little effect on the permeability,and stress and initial gas pressure has a greater effect on the permeability.(3)The effective gas drainage radius and gas flow quantity distribution of initial permeability slightly less than permeability of ending drilling,and the difference is little,when the extraction time is 120 days,The effective gas drainage radius and gas flow quantity distribution of permeability of ending drilling increase 3.1% and 4.9% relative the permeability is constant.It shows that the condition of two kinds of permeability have little influence on the seam gas migration.(4)The effective gas drainage radius and gas flow quantity distribution of initial permeability larger than the dynamic permeability,and the difference is increasing with the extraction time,when the extraction time is 120 days,The effective gas drainage radius and gas flow quantity distribution of dynamic permeability decrease 10.7% and13.4% relative the permeability is constant.Due to the changes of permeability showed gas flow has some influence in the process of drilling drainage,so in the future we should consider the influence of dynamic permeability on the gas flow.