Equivalent Characteristics Of Dual-Porosity Coal Structure And Their Effects On Coal Mechanical Properties And Permeability

Coal is generally regarded as a complex dual-porosity porous medium which consists of coal matrix containing pores and fracture.The abundant pore and fracture volume and tremendous specific surface area in coal make it can store a mountain of gas and provide the space for the gas migration.The gas seepage characteristics and permeability evolution law in coal is closely related to the coal matrix pore and fracture system.However,because of the complexity of coal structure itself,the elements that make up the dual-porosity coal structure(i.e.the matrix and the fracture)are often simplified to geometries with regular shapes when the relevant theoretical research is conduted.Therefore,the scale characteristics of the simplified geometries are often regarded as the equivalent scale characteristics of coal matrix and fracture.Equivalent scale characteristics of the simplified coal matrix and fracture will have an important impact on the gas seepage characteristics and permeability evolution law in coal.The paper is established in the study of equivalent characteristic of dual-porosity coal structure and its influence mechanism on coal mechanical properties and permeability evolution law,and discusses the quantitative characterization method of equivalent matrix scale and fracture width of coal and studies gas ad-desorption and diffusion properties in coal by the method of theoretical analysis,laboratory experiments,numerical simulation and engineering research comprehensively.Meanwhile,the effect of dual-porosity coal structure on coal mechanics properties and permeability evolution mechanism by combining experiment and theoretical analysis.the coal permeability evolution model based on equivalent characteristic of dual-porosity coal structure was established and the model is used to predict the coal permeability.The main conclusion of the paper are as follows:1)Based on the fractal theory,the interface between pore and fracture is 33.06 nm-35.42 nm.According to the initial structure permeability and the initial fissure rate,it is calculated that the equivalent matrix scale of the natural coal samples is 0.2696 mm,which is 3.51-76.23 times over the briquette coal,and the equivalent fracture width of natural coal is 0.7279 ?m,which is 0.23-1.68 times over the briquette coal.The equivalent matrix scale and fracture width of briquette coal increases with increasing coal particle size.Meanwhile,it is found that coal permeability is the joint effect of matrix scale and fracture width and both are indispensable.2)It is found that there is some difference in the pore and fracture of standard coal sample and coal powder,however,they have same change law.Therefore,the adsorption,desorption and diffusion properties of standard coal sample can be described qualitatively based on those of coal powder to a certain degree.The Langmuir volume VL and ultimate desorption volume decreases with the increasing coal particle size,and adsorption isotherms can be divided into four obvious stage as the coal particle size changes.The Langmuir pressure Lp has no obvious change with the increasing coal particle size when the coal particle size less than 1 mm,and Lp increases to 0.9318 MPa only when the coal particle size is 1-3 mm.A non-constant diffusion coefficient(non-CDC)model could accurately describe the gas diffusion properties in coal.3)It is found that the effect of dual-porosity coal structure on the coal volumetric strain is very obvious.Under the same conditions,the peak stress of coal sample increases with the increasing equivalent matrix scale,and the volume strain corresponding to the peak stress at the uniaxial loading increases first and then decreases with the increasing equivalent matrix scale,however,the volume strain corresponding to the peak stress at the triaxial loading is not showing a clear regularity.The complete stress-strain curve of natural coal is mainly made of linear elastic stage,strain softening stage and residual strength stage,however,the nonlinear elastic-plastic stage and ideal plastic stage of briquette coal is also obvious.The macroscopic damage of natural coal is mainly brittle failure,and that of briquette coal is mainly composed of shear expansion and has significant ductility.Coal cohesion and elasticity modulus both have a tendency to decrease first and then increase and Poisson's ratio has a tendency to increase first and then decrease with the increasing coal matrix scale.Gas has a significant weakening effect on coal strength,which is controlled by the equivalent characteristics of dual-porosity coal structure.The maximum adsorption expansion strain of coal increases with the increasing equivalent matrix scale of coal.4)Coal permeability evolution law was controlled by multiple factors,such as effective stress,swelling deformation of coal matrix and the malconformation of coal adsorption characteristics,and the eventual evolution law is the result that three factors complete together.However,when the particle size of coal powder used to suppress the briquette coal is too big,the inhomogeneity of coal particles will result in the gas migration space more complex and every factor is not sufficient to have an absolute effect on the attenuation of coal permeability alone when the effective stress is equal and the gas adsorption equilibrium pressure increases,so the change of attenuation degree of coal permeability has been unable to show a clear regularity.5)Under the same conditions,the permeability of the coal with different dual-porosity structure is in the elastic phase of the whole stress-strain process,and its permeability decreases slowly with the increase of axial strain.However,in the nonlinear elastic-plastic stage,the permeability of coal began to increase with the increase of axial strain.In the ideal plasticity and strain softening stage,the permeability increases rapidly with increasing axial strain,and natural coal permeability has an increase of 300 to 500 times and briquette coal permeability has also an increase of more than 100 times.In the residual strength stage,coal permeability is basically stable.It is found that coal permeability in the residual strength stage decreases first and then increases with the increasing equivalent matrix scale of coal in the same conditions.Based on the reasonable assumption of the dual-porosity coal structure,the coal permeability model in the elastic phase and the nonlinear elastic-plastic stage based on the equivalent characteristic of dual-porosity coal structure(ECDP model)is constructed,and the effect of equivalent matrix scale and fracture width of coal on the coal permeability is discussed.6)Through comparative analysis,it is found that ECDP model not only cover the effect of effective stress and coal matrix swelling deformation caused by adsorption on the coal permeability evolution law in the process of gas migration,but also bring the dual-porosity structure of coal in,meanwhile,it not only describes the distribution of coal permeability but also give an actual meaning for the specific coal permeability.Therefore,the ECDP model has guiding significance to the coal mining.Moreover,the dual-porosity structure of coal is introduced in the ECDP model,so it has a bright prospect in terms of describing the coal permeability evolution law in the nonlinear elastic-plastic stage.After the engineering test is conducted,it is found that the predicted permeability by the ECDP model has the same order of magnitude with the field measurement and the error is very little,which indicates that ECDP model is reasonable to guide the field engineering practice.