Abstract
The gas permeability and mechanical properties of coal, which are seriously influenced by mining-induced stress evolution and gas pressure conditions, are key issues in coal mining and enhanced coalbed methane recovery. To obtain a comprehensive understanding of the effects of mining-induced stress conditions and gas pressures on the mechanical behavior and permeability evolution of coal, a series of mining-induced stress unloading experiments at different gas pressures were conducted. The test results are compared with the results of conventional triaxial compression tests also conducted at different gas pressures, and the different mechanisms between these two methods were theoretically analyzed. The test results show that under the same mining-induced stress conditions, the strength of the coal mass decreases with increasing gas pressure, while the absolute deformation of the coal mass increases. Under real mining-induced stress conditions, the volumetric strain of the coal mass remains negative, which means that the volume of the coal mass continues to increase. The volumetric strain corresponding to the peak stress of the coal mass increases with gas pressure in the same mining layout simulation. However, in conventional triaxial compression tests, the coal mass volume continues to decrease and in a compressional state, and there is no obvious deformation stage that occurs during the mining-induced stress unloading tests. The theoretical and experimental analyses show that mining-induced stress unloading and gas pressure changes greatly impact the deformation, failure mechanism and permeability enhancement of coal.
Highlights
According to the specific report from the Intergovernmental Panel on Climate Change, human activities are estimated to have caused approximately 1.0 ◦ C of global warming above pre-industrial levels, and global warming is likely to reach 1.5 ◦ C between 2030 and 2052 if it continues to increase at the current rate [1,2]
The in situ stress and coalbed methane (CBM) pressure significantly increase, and the coal mass presents obvious nonlinear mechanical behavior resulting in a difficulty in conducting the CO2 -ECBM
Most existing theories are founded by conducting conventional triaxial compression (CTC) tests, and complete stress-strain relationships are used to analyze and describe the basic mechanical behavior and failure damage process [11,12,13,14]
Summary
According to the specific report from the Intergovernmental Panel on Climate Change, human activities are estimated to have caused approximately 1.0 ◦ C of global warming above pre-industrial levels, and global warming is likely to reach 1.5 ◦ C between 2030 and 2052 if it continues to increase at the current rate [1,2]. The emission of carbon dioxide (CO2 ) is considered to be the main cause of the greenhouse effect and global warming [3]. Researches show that the coalbed methane (CBM) is not just a major source of energy [4] and one of the most harmful greenhouse gases [5,6], whose the greenhouse effect is 21–34 times that of carbon dioxide (CO2 ) [7,8]. To prevent the mining-induced CBM excessive emission from causing environmental problems and greenhouse effects, the CBM should be pre-extracted and utilized, and a CO2 -enhanced coalbed methane (CO2 -ECBM). The in situ stress and CBM pressure significantly increase, and the coal mass presents obvious nonlinear mechanical behavior resulting in a difficulty in conducting the CO2 -ECBM. The advantage of the method is that the test results are comparable
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