Anisotropic Characteristics Of Gas Diffusion And Seepage In Coal Seamunder Stress Load

Considered anisotropic characteristics caused by pore-fractures of coal bed,anisotropic characteristics of coal bed methane seepage diffusion under load were studied to drainage gas efficiently,eliminate blank zone of mine and decrease hazard of coal and gas outburst.Some investigations were cayyied out to study anisotropic characteristics of coal bed methane seepage under load.The Theoretical aspects include that A.Mohr's circle was used to represent permeability in any direction;B.the relationship between anisotropic permeability and strain was derived from combining Kozeny-Carman equation and flat plate equation;C.gas anisotropic seepage equation was derived from combining continuity equation with the relationship between anisotropic permeability and stress by defining directional compressibility and directional sensitivity coeff-icient.Under stress load,anisotropic seepage experiments were carried out by using of coal samples at 0°,45°,and 900 directions.The results show that,under the definition of permeability in the direction of the gradient,the permeability anisotropic ratio is greater than 5.62 less than 9.89 when the range of gas pressure is from 0.5MPa to 2MPa and greater than 2.84 less than 3.23 when the range of gas pressure is from 2.5MPa to 3MPa.Under the definition of permeability in the direction of the flow,the anisotropic ratio is greater than 8.67 less than 15.7 when the range of gas pressure is from 0.5MPa to 2MPa and greater than 3.10 less than 4.17 when the range of gas pressure is from 2.5MPa to 3MPa.The permeability decreases,in the form of negative exponential function,as the confining pressure increase at elastic stage.The wider the width of equivalent fracture in Coal simple is,the larger the initial directional compressibility is,higher the directional sensitivity coefficient is.With the increase of compression in the form of negative exponential function,permeability decrease at elastic stage.The wider the width of equivalent fracture in Coal simple is,the less the initial directional compressibility is,higher the directional sensitivity coefficient is.Some investigations were cayyied out to study anisotropic characteristics of coal bed methane diffusion under load.The Theoretical aspects include that A.Mohr's circle was used to represent diffusion coefficient in any direction;B.considering multi-scale pores in the coal,the cylindrical unidirectional multi-scale diffusion new model of dynamic diffusion coefficient was established on the basis of anisotropic diffusion continuity equation.Under stress load,anisotropic diffusion experiments were carried out by using of coal samples at 0°,45°,and 90° directions.The results show that the initial diffusion coefficient anisotropic ratio(?)is greater than 2.07 less than 3.62 when the range of gas pressure is from 0.5MPa to 3.0MPa.The initial diffusion coefficient(D0)increases as gas pressure increases.Decay coefficient(?)has a tendency to increase as gas pressure increases.At the same adsorption saturation gas pressure,DA10>DA20>DA30.Diffusion coefficient(D0)increases as confining pressure decreases,when the range of confining pressure is from 2MPa to 4MPa.At the same confining pressure,DA10>DA20>DA30.Decay coefficient(?)has a tendency to decrease as confining pressure increases.The function of pressure didtribution and the relationship between effective gas drainage radius and time,under the condition of single borehole,were derived to calculate the effect of gas drainage.Considering anisotropic permeability,pressure didtributions were simulated in the ways of uniform or non-uniform borehole distribution.The simulation results show that gas drainage cannot reach the standard in the way of uniform borehole distribution after 120 days,but it can reach the standard in the way of optimal non-uniform borehole distribution within 90 days.At the same time 12 boreholes were saved per hundred meters by using of the non-uniform borehole distibution.