Bed Separation Dynamic Development And Response Mechanism Of Extra-thick Cretaceous Aquifer Induced By Coal Mining

The Ordos Basin has been the largest coal-producing area in China.Statistically,the available coal reserves reach 658.99 billion tons which accounts for about 38.8% of total coal reserves,and the predicted coal reserves account for more than 65.5% of that in China.Currently,there are four nationally large-scale coal bases in the Ordos Basin,which are the Shaanxi Coal Base,the Shendong Coal Base,the Huanglong Coal Base,and the Ningdong Coal Base.With abundant reserve resources and great development potential,the Ordos Basin is extremely important for the socioeconomic development and energy sustainability of the country,which guarantees China’s coal energy supply in the next 50-100 years.However,in recent years,water inrush hazards from the bed separation spaces frequently occurred during coal mining.Due to the lack of obvious signs of water inrush,destructive damage,and the difficulties in early warning,the water inrush hazards induced by bed separation caused the flooded panels,stop mining and casualties,which posed a great threat to mining safety and resulted in the huge financial loss to the coal mines.The formation and water storage of the bed separation spaces are the fundamental causes of the water inrush hazard.Thus,the investigation of dynamic development characteristics of mining-induced bed separation and response mechanism of the Cretaceous aquifer play an important part in the formation of bed separation water,the mechanism of water inrush and the prevention of water hazard.This thesis takes Yingpanhao coal mine in the Ordos Basin,China,as the study area,with extra-thick Cretaceous sandstone overlying the coal seams as the main research object.Based on the in-situ test,lab experiment,numerical simulation and theoretical analysis,the dynamic development of bed separation and response mechanism of the hydraulic head,permeability and seepage field of the Cretaceous aquifer were determined.The main results are as follows:(1)The method using magnetic loops to monitor the subsidence of deep strata with the large displacement induced by coal mining in a drilled hole was proposed.The insitu test of dynamic development of multi-separation spaces in the extra-thick overburden was implemented.The monitoring results show that the strata overlying coal seams has obvious differential subsidence characteristics of engineering geology rock group,and the monitoring strata were divided into the upper group of Cretaceous strata,the lower group of Cretaceous strata,and the Anding Formation group according to the subsidence trend.The subsidence differences between rock groups resulted in the formation of bed separation spaces.There were three bed separation spaces with depth about 285 m,365-402 m,434 m in the monitoring strata.The development of bed separation was divided into four stages,which are slow development stage,rapid development stage,stable development stage and closed stage,respectively.The break of the strata induced the close and transfer of bed separation spaces in the overburden,showing the jumping occurrence law from the low-location strata to the high-location strata.During the stage from the rapid development to the stable development,the spatial development of the bed separation space changes from the vertical expansion to the horizontal expansion.(2)The collaborative monitoring technology of hydraulic pressure variation influenced by the bed separation development in the extra thick overburden was proposed to investigate formation of mining-induced bed separation water.The fiber Bragg grating osmometer sensors were adopted to in-situ monitor the variation of the hydraulic pressure of the Cretaceous sandstone aquifer during coal mining.The monitoring results show that the pressure variation of the monitored aquifer with different depths affected by the development of the lower bed separation spaces.As the separation spaces develops,the monitored hydraulic pressure decreases.While it increases if the water filling of separation spaces completes.With the advance of the coal mining,the bed separation spaces gradually expand to the monitoring hole zone which make the hydraulic pressure rapid decrease due to the changing from lateral supply to vertical supply of monitoring aquifers.The bending deformation of the strata expanded the bed separation space which made the hydraulic head of the monitoring aquifer rapid decrease,while the break of strata induced the close and transfer of the bed separation which caused the abrupt drop of hydraulic head,during which the water inrush hazard of bed separation was prone to happen.Influenced by the hydraulic pressure and negative pressure,the water storage of bed separation spaces was rapid which almost was synchronous with the development of bed separation.The influences of hydraulic head caused by bed separation development and water-conducting fractured zone was analyzed which revealed the decrease mechanism of Cretaceous aquifer but there was no bed separation water inrush in Ordos Basin,and the classification of bed separation water was proposed.(3)Influenced by the coal mining,the increase of the aquifer permeability accelerated the accumulation of water resources into the bed separation spaces.In order to analyze the response mechanism of the permeability of the Cretaceous aquifer for the coal mining,the axial compression servo permeability test was carried out on the Cretaceous sandstone sample,and the stress-permeability coupling relationship was established.According to the coupling formula,three-dimensional numerical simulation of permeability change of Cretaceous sandstone was performed,and the response characteristics of permeability for the coal mining were obtained.The results of lab experiment show that influenced by the mining-induced stress,the permeability variation of weakly cemented Cretaceous sandstone was divided into three stages,which were slow increase stage,rapid increase stage and jumping increase stage due to the damage.According to the characteristics of sandstone permeability in the different stages,the three-stage coupling formula of the stress-permeability was established.The results of the numerical simulation show that the height of water-conducting fracture zone reached the maximum when the mining footage was about 400 m,and the permeability of the lower Cretaceous strata significantly increased induced by the coal mining which increased to 1.5-5 times of the initial permeability.(4)The double-scale numerical simulation models of Yingpanhao coal mine and Hailiutu River Basin were established to analyze the response mechanism of the seepage field of Cretaceous aquifer for the coal mining.The seepage boundary condition was adopted to simulate the impact of bed separation development on the seepage field of aquifer,whose opening and closing were dominated by the maximum outflow.The simulation results show that the vertical seepage recharge area was formed above the bed separation space,in which hydraulic head of aquifer rapidly decreased,while the decrease of hydraulic head was slow in the lateral seepage recharge area.With the advance of coal mining,the superposed-decreasing influence on the hydraulic head in mined area was caused by the bed separation.Based on the established numerical model,the influencing factors including the mining height of coal seam,permeability of Cretaceous aquifer and mining speed were selected to analyze the impact of bed separation on the seepage field.According to the changing characteristics of the seepage field,the hydraulic zoning model induced by the bed separation was proposed.(5)Considering the spatial characteristics of the bed separation development,the elastic thin plate mechanical model was established.The space volume calculation formula of the vertical expansion and the horizontal expansion developing stage were proposed.The main water accumulating location of bed separation in the overburden was determined and the breaking distance of Cretaceous strata was calculated.The results of research show that the bed separation volume slow increased in the vertical expansion developing stage while rapidly increased in the horizontal expansion developing stage.Taking the water inrush accident of 21301 panel in Cuimu coal mine as an example,the predicted formula of bed separation volume was applied and tested.(6)The recovery of the hydraulic head of Cretaceous aquifer induced by bed separation was evaluated from the perspective of time.Based on the Theis well function model,the recovery of the hydraulic head when there was no bed separation water inrush was analyzed.The results of research show that the hydraulic head rapidly increased in the initial stage,but with the increase of time,the recovery gradually slowed down because the recharge intensity of surrounding aquifers to the vertical recharge area decreased.The calculation results show that recovering initial hydraulic head of 98% degree needed 377 days,and the depth of the hydraulic head can increase to 3.7 m in a hydrological year.With the increase of the groundwater depth,aquatic vegetation and mesophytes vegetation gradually degenerated to xerophytes vegetation in arid and semiarid area.Thus,the coal mining may have a negative impact on the vegetation growth.The thesis contains 98 figures,18 tables,and 200 references.