Research On Anisotropic Characteristics And Evaluation Model Of Coal Seam Permeability

Gas flow in a coal seam deals with the multi-physical fields that coupling of mechanical deformation,gas-phase flow,adsorption effect,etc.The initial anisotropic permeability of the coal seam is caused by the difference of internal material composition and pore-fracture structure.During the development of coalbed methane,the effective stress changes and desorption shrinkage deformation proportionally to pore pressure.Anisotropy of stress deformation and gas adsorption and desorption deformation in coal seam causes the dynamic change of coal seam permeability anisotropy.Study on the anisotropic characteristics and evaluation of coal seam permeability is a matter of great importance to the optimal design of a coalbed methane development scheme.In this paper,the raw coal samples with different metamorphic degree in Permian typical mining areas in North China are used as the research object.The deformation of the coal seam and the evolution law of the anisotropic permeability were tested by using the independently developed coal seam physical property characteristics testing system,and coal adsorption and desorption deformation testing device.Based on the characteristics of coal seam fracture and fracture deformation,by combining theoretical analysis with experimental research,the anisotropy of the initial permeability of coal seam and the evolution law of anisotropic permeability of coal seam were studied.The main conclusions are as follows:(1)It is revealed that the dynamic elastic modulus of coal seam is anisotropic and has a quadratic polynomial function relationship with the loading pressure.The two directions of the parallel bedding are obviously larger than that of the vertical bedding.The measured values of three directions increase with the pressure raising.The amplification is more obvious at the initial stage of loading.However,with the further increase of the pressure,the compressibility of the fracture in coal seam decreases,which makes the range of change decrease.(2)It is revealed that the swelling deformation of coal is anisotropic and has a hyperbolic function with adsorption pressure.The swelling deformation of coal in different directions when adsorbing gas is different.The swelling deformation of coal in vertical bedding direction is obviously larger than one in the parallel bedding direction.The deformations in each direction are increasing with the gas pressure increase,but the growth rate is decreasing.(3)It is revealed that the permeability of coal seam is anisotropic,and the degree of anisotropy changes dynamically with the change of effective stress and gas pressure.The permeability of coal is the largest along the face cleat and the minimum in the vertical bedding.The permeability of the coal samples decreases with the increase of effective stress.The results show that the there is a negative exponential function relationship between the permeability and effective stress.With the increase of effective stress,the attenuation rate of permeability in different directions of coal is different,vertical bedding direction is the largest.The coefficient of the anisotropic permeability increases with the increase of effective stress.The permeability in different directions decreases with the increase of gas pressure.The relationship between permeability and gas pressure in different directions of coal seam is negative exponential function.Based on the anisotropy of elastic parameters and the anisotropy of adsorption swelling deformation of coal seam,the evolution law of anisotropic permeability of coal seam is analyzed.The mechanism of dynamic variation of anisotropic permeability caused by anisotropy of stress deformation and adsorption swelling deformation is revealed.(4)The main value and dominant orientation of permeability of coal seam are determined by using permeability tensor model.According to the characteristics of the coal fracture,the permeability tensor model of the coal seam considering fracture penetration coefficient is established.Based on the quantitative observation of the coal seam fractures,the main value and dominant orientation of permeability of coal seam is evaluated by using modified permeability tensor model.(5)The dynamic evolution model of anisotropic permeability of coal seam is established.Considering the fracture structural anisotropy under the initial state of coal,anisotropy of physical and mechanical parameters and anisotropic characteristics of adsorption swelling deformation.Moreover,by means of rock mechanics,and parallel plate seepage theory,the dynamic evolution model of anisotropic permeability of coal seam is established.The model is verified by the laboratory test.It provides a theoretical basis for the prediction of coalbed methane productivity and the rational selection of CBM production plan.