Assessment Of Water Inrush Risk Of Coalfloor After CBM Development

Coal seam is important enriched horizons for coal bed methane(CBM)formed in the four of eight coal-forming period.Among them,Ordovician limestone aquifer is the under lying horizon of Carboniferous-Permian coal seams in North China.If the distance between the layers of Ordovician limestone aquifer is small,CBM exploitation for groundwater impermeable layer may lead to disturbances damage easily,so that coal mining has the water inrush risk of coal floor after CBM development.At present,Chinese scholars have more abundant research results on water inrush risk of coal floor,but the research is relatively weak about water invasion regularity after the CBM.This paper mainly studies the relationship between the groundwater and extraction after CBM development,reveals the influencing mechanism of coal mining water inrush risk in the future,and appropriate measures are taken,which can have important theoretical and practical significance for the future coal mining safety.Chengzhuang coal in Shanxi Province is selected as the study area.For the 15 coal floor after CBM development,water inrush risk is assessed by vulnerability index method based on AHP.The main controlling factors of water inrush risk of coal floor after CBM development are determined,based on the analysis and research about mining topography,meteorology and hydrology,formation,structure,the coal quality and so on,which are Ordovician limestone aquifer pressure,aquifer water abundance,the effective thickness of bottom impermeable layer,fault fractal dimension,the minimum horizontal stress and rock burst pressure,drawing the corresponding thematic maps.The judgment matrix is constructed by using AHP to calculate the weight of different impact factor.Thematic maps are assigned,weighted and stacked to produce water bursting hazard zoning map.Finally,the comparison is made between it and the map plotted on the basic of common vulnerability index,which doesn't consider the main factors related with CBM development.The results show that: the vulnerable areas and less vulnerable areas are mainly distributed in the western region of the study area,which has larger aquifer water pressure and smaller effective thickness playing the important role on water-blocking.Floor damage of some places has even developed to the aquifer,and protection should be strengthened in the process of coal mining.Transition zone and safer area still have the possibility of water inrush,which should be observed signs of water inrush during mining.In addition,compared with ordinary vulnerability index,this new method has more accurate determination for the more vulnerable areas and vulnerable zones,and the evaluation results are more realistic.