Study On Crack Evolution And Permeability Change Mechanmism Of Medium And High Rank Coal In The Process Of Bearing Stress

The study on the variation characteristics of mechanical deformation,crack evolution and gas permeability is the basic work of gas extraction and disaster prevention in coal containing gas under stress.The laboratory tests,theoretical analysis and field tests methods were applied on coal containing gas.The micro-structure characteristics of coal was analyzed.The change characteristics of mechanical deformation,crack evolution and gas permeability of the coal samples was discussed under stress.The two models were established to evaluate effective porosity evolution and crack damage evaluation in coal samples.The law of gas migration under the condition of mining was summarized.The main conclusions are as follows:(1)Based on the RMT-301 rock and concrete mechanical test system,the up-grade reformation of the equipment was carried out,which enables it to carry out the tests of coal strain,acoustic emission and gas seepage under different stress paths.(2)By mercury injection,low temperature liquid nitrogen and electron microscope scanning test method,the microstructure characteristics of coal samples with different coal rank were researched:? Micropore and keypore is bigger contribution than mesopore and macropore for pore specific surface area,while mesopore and macropore for pore volume;?The total pore volume of bituminous coal is relatively larger,and the total pore specific surface area of the anthracite is relatively larger;?The tectonic stress on the primary structure coal has certain modification effect on coal structure,the pore volume of which is smaller than the tectonic deformation structure coal,but the effect of the tectonic stress on each pore section of different coal samples is not the same.(3)The mechanical deformation,crack evolution and permeability change characteristics and mechanism were analyzed in coal samples under the total stress and strain test:?The strain characteristics and acoustic emission characteristics in anthracite and bituminous coal samples were similar at the stage before peak and had the big difference at the stage after peak;?Before peak,there were three stages(the initial decline stage,the intermediate stability stage and the later rise stage)in the overall change trend of the permeability and the number of ringing bells,and the overall change trend was also "J" type;?n the initial decline stage,the permeability and axial stress relationship have negative exponential relationship,which is expressed by the formula "k=ae-bx",and in the later rise stage the positive exponential function is used to express their relationship;? Initial pore structure and mechanical properties of coal is some affect to control the crack evolution and permeability change,and the controlling action on the crack evolution and permeability change were different in different stages before peak.(4)The characteristics and mechanism of mechanical deformation,crack evolution and permeability change were analyzed in coal samples under cyclic loading:?In the process of entire cycle,stress-strain curve of coal samples showed a sparse-dense-sparse change rule,and the peak ringing count and permeability change present a "J" type and the change trend experienced three stages(the initial decline stage,the intermediate stability stage and the later rise stage);?In the initial decline stage,the permeability and axial stress relationship have negative exponential relationship,which is expressed by the formula "k=ae-bx",and in the later rise stage the positive exponential function is used to express their relationship;?The essence relationship is analyzed on deformation,crack evolution and gas permeability in coal samples.(5)The crack and effective porosity evolution in coal were analyzed:? The crack damage model of rock is also suitable for coal;?An assessment model "?=log(kr/k0)(kt/k0)" is proposed to reflect the evolution of the effective porosity in coal.(6)The gas flow was simulated under the condition of coal seam,and a gas extraction experiment was carried out at field,which summary were as follows:?The round hole gas flow model is established to simulate the effect of the adsorption layer thickness,aperture and slippage on gas flow under the geological conditions;?The gas extraction experiments showed that the gas permeability of coal seam will increase in the unloading area,and its gas permeability will decrease in the pressurization area,which is consistent in the results of the laboratory experiments and theoretical analysis.