Study On The Structural Characteristics Of "Dual Porosity-Dual Permeability" And The Mechanism Of Zoning-Permeability Of Coal Around Gas Extraction Boreholes

Borehole gas pre-pumping is an important tool for gas management in high gas mines.The stress concentration in the borehole can easily lead to the destruction of the coal structure around the borehole,with the borehole showing a circular "three-zone" damage pattern(fracture zone,crack zone,original rock zone).The structure of the fracture and crack zone of the extraction borehole is the main channel for the transport of gas from the perimeter of the borehole,and there is currently a lack of understanding of the structure of such pore and fracture combinations of coal bodies.The pore and crack network at this fine level is dense and the gas transport process in the peripore fracture and crack zone coal bodies is a mixture of Darcy seepage and Fick diffusion,and the permeation mechanism of the combined media structure coal bodies is unknown.Subsequently,the determination of the effective extraction radius of pre-sumping boreholes is mainly dependent on downhole testing and field experience,which cannot ensure the universality of the design of coal seam gas pre-sumping boreholes,and the design of gas pre-sumping boreholes lacks effective theoretical guidance.Therefore,a combination of theoretical derivation,indoor tests and field tests is used to analyse the physical equivalent structural characteristics of the fracture and crack zones around the perimeter of the borehole.The parameters of the permeability characteristics of the fracture unit and crack network coal bodies are determined,and a theoretical mathematical model of the dual-media permeability of the perimeter coal body of the extraction borehole is established.The zonal permeability mechanism of the coal body around the extraction borehole is investigated.The main research works are as follows.(1)Aiming at the convergence and flow of gas in the coal body around the perimeter of the gas extraction borehole,the damage pattern of the coal body around the borehole is analyzed,and the equivalent physical model of the coal body structure in the perimeter fracture zone and the fracture zone is given.Further,using the dual medium theory,the structure of coal body in the fracture zone is equated to the fracture unit in the dual medium,and the structure of crack coal body is equated to the crack network.Then,the gas flow law in each of the two permeable systems of perforated fracture zone and cracked zone is analyzed.(2)Combining the characteristics of pore and cracked structure of the coal body in the perimeter fracture zone and cracked zone of the extraction borehole.We designed the "lateral limit compression" permeability test system for the pore body,and analyzed the mechanism of the effective stress on the permeability of the pore structure coal body.At the same time,the permeability parameters of different cracked structure specimens were measured by using the triaxial permeability test platform of cracked coal body and the calculation principle of KC equation.Finally,combined with the characteristics of permeability parameters of pore and crack structure coal bodies,the basic characteristics of permeability of peripore fracture zone and cracked zone structures of gas extraction boreholes were mastered.(3)A systematic analysis of the permeability parameters of the peripore fracture and crack structured coal bodies in the extraction borehole is carried out.Based on the basic principles of dual porosity and dual permeability media percolation in the dual media system,expressions for calculating the permeability and porosity of the perforated coal body fracture unit and crack network are given.The new method of calculating permeability of perforated coal body based on the dual media model is proposed,and then the "dual porosity-dual permeability" mathematical model of perforated coal body in gas extraction boreholes is established.It is concluded that there are two porosities and two permeabilities in the peripore fracture zone and crack zone of the coal body.(4)The dual media theory was used to derive the parameters of permeability characteristics for the perimeter fracture unit and the crack network,showing that the form of gas transport in the perimeter fracture unit of the extraction borehole follows Fick’s law of diffusion and the form of gas transport in the crack network follows Darcy’s law.Further field tests on the gas transport pattern of coal bodies with dual media structure around the perimeter of the borehole were carried out,and a new method for calculating the permeability and permeability coefficients of coal bodies was developed using the " dual porosity-dual permeability" model,which can more accurately determine the effective extraction radius of gas from pre-pumped coal seams.Through the above research,the dual media characteristics of the coal body around the perimeter of the gas extraction borehole are structurally revealed.The permeability characteristics of the coal body in the fracture zone and the crack zone around the extraction borehole were analysed,and a theoretical mathematical model of " dual porosity-dual permeability " of the coal body in the fracture zone and the crack zone around the gas extraction borehole was constructed.The model was also used to give a method for optimising the reasonable layout parameters of the coal seam gas extraction boreholes at 40103 working face of Dafosi Coal Mine.This provides an important theoretical basis for the design of coal seam gas pre-sumping boreholes.It has important practical significance.