Temporal And Spatial Evolution Of Overburden Ultra Thick Aquifer Permeability During Coal Seam Extraction

The Cretaceous ultra-thick aquifer overlies on the Jurassic coal seam,of which the permeability changes within the area influenced by working face mining,bringing varieties to water recharging intensity of the aquifer and the water inflow to the mining working face.Therefore,it is of tremendous urgency to deepen the research on permeability spatial and temporal evolution of the Cretaceous ultra thick aquifer and to enhance prevention systems on proof water hazards.Taking the Cretaceous ultra thick aquifer of the No.1 mining area in Zhaoxian coal mine,Shaanxi province as the research target,this thesis combines various methods together such as field test,theoretic analysis and numerical simulation for the final conclusions.Regional sedimentary environment and micro-facies are studied in this thesis.Temporary and spatial permeability evolution regulation of overburden aquifer during mining is studied based on the data from pumping test and former drainage processes A three-dimensional groundwater model is also established based on the pumping test and hydrogeological conditions for parameter inversion of the aquifer after working face mining.Finally,an optimized scheme for pumping well arrangement for aquifer drainage during working face mining is put forward.Conclusions and results of the thesis are as follows:(1)Meandering fluvial,braided fluvial and shoal lake sedimentary were formed respectively in Yan’ an Formation,Zhiluo Formation and An’ ding Formation.Yijun Formation is mainly constructed by huge thick conglomerates,developing a typical alluvial fan system including braided channel deposit and sheet flow deposit.(2)According to the evolution of hydrogeological parameters,the temporal and spatial evolution regulation of overburden aquifer permeability during mining is summarized.Temporally,at the early period,the aquifer subsides affected by working face mining,fissures develop within the aquifer and the permeability increases gradually to the climax.Along with the mining process,strata overlying the aquifer subside in sequence,compacting the aquifer downward and causing the fissures to close and bringing a decrease to the aquifer permeability.When the working face finishes the mining process gradually,the movements of the overburden strata become stable,bringing a stable state to the permeability.Spatially,permeability on the direction of mining improves more than that of other directions.(3)Based on the geological and hydrogeological conditions of Yijun Formation,the boundary and layer conditions are generalized.Combining pumping test data,micro-facies layout of Yijun Formation,post-mining unit-flow of Yijun Formation and permeability coefficients in pre-mining area together,a three-dimensional groundwater numerical model is established.The groundwater seepage process is simulated during the pumping test for numerical values of permeability during the pumping test.(4)Theoretical method and numerical method adopted,the law of pumping well distribution for overburden aquifer drainage is summarized.Based on the law,an optimized scheme for pumping well distribution for aquifer drainage during working face mining is put forward.The numerical simulation results show that the water levels of different observation well on Yijun Formation decrease to the level near the floor of Yijun Formation after drainage.The designed pumping wells in the optimized scheme can work well during the working face extraction.The target is reached of reducing water-recharging intensity of the overburden aquifer and ensuring safety of working face extraction.This thesis contains 65 figures,20 tables and 94 references.