| Dynamic coal reservoir permeability impacts the production of coalbed methane(CBM)and has been extensively studied.In this paper,a total of ac.110 coal samples from 41 underground coal mines lacated in the different coal-bearing basins/areas of China were carried out for a series of laboratory experiments,including gas permeability measurement and real-time strain monitoring,acoustic emission monitoring,X-ray CT analysis,porosity and permeability measurement under overburden pressure,mercury porosimentry and coal petrology analyses.According to the experiments,the characteristics and controlling mechanism of dymanic change in coal permeability to gas(CO2),the coupling relationships between coal stress-strain,material composition,pore-fracture system and the permeability change,and the identification patterns for the change types of the coal permeability to gas were investigated and researched.Results are as follows.(1)Through physical simulation experiments,the change trendencis of different rank coals permeability to gas were summarized as "decline type","rebound type","increase type" and "transition type".Under a constant confining stress,the permeability change is the superimposed result of effective stress increase,coal matrix shrinkage and gas slippage that occur during the gas pressure depletion process.Based on the controlling mechanism of the permeability change,a prediction model for the gas permeability change in coals with different metamorphic grade was estabilished.(2)According to the real-time strain data monitored as the gas seepages within coals,the coupling relationship between coal stress-strain and the gas permeability were revealed.Results show that the permeability is positively related to the total strain of coal.Moreover,the generation mechanism of coal strain as the gas seepages within coal were proposed,that is the coal strain is the comprehensive result of matrix shrinkage and gas slippage during the gas pressure decline under a constant confining stress.According to the analyses on geological fracors,an empirical model was developed predicting the Young's modulus(elastic characteristic)of coals.(3)The controls of coal material composition on the gas permeability were clarified.Results show that coal metamorphic grade,macerals and coal quality can be used to indentify the types of the permeability change.According to the cross-plots including maximum vitrinite reflectance-initial permeability,"fixed carbon content/ash yield"-initial permeability,"inertinite content/vitrinite content"-initial permeability,"inertinite content/vitrinite content"-"fixed carbon content/ash yield" and "inertinite content/vitrinite content"-"fixed carbon content/ash yield"-initial permeability,the identification patterns for the change types of coal permeability to gas were estabilished.(4)The change characteristics of the coal porosity and permeability and the coupling relationship between them were revealed.Coal porosity and permeability change resulting from the change in effective stress,and the permeability increases with the increasing porosity in the form of exponent.The effect from the pore-fracture system within coals on gas-water two phase flows and single phase gas flow can be described as:the more developed open seepage pores are,the slower the changes of gas-water relative permeabilities occur;during the gas pressure decline,open fracture content primarily impact the shape of gas permeability change curve.(5)The vertical anisotropy of physical properties within high rank coal reservoir in Zhengzhuang block was quantitatively analyzed by using variable coefficient.CBM adsorption time comprehensively refleclts the process of CBM desorption-difussion-seepage,and can be used for predicting the favorable area within coal reservoir.Based on the dynamic change of the coal permeability to gas,a new method predicting CBM favorable area with high and stable gas production was established considering the CBM exploration and development. |
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