| The Ordos energy source base is rich in coal resources which are in a stable occurrence condition, there is a great potential for development and utilization. But Ordos energy base is located in the west part of China with a fragile ecological environment. Taigemiao coal field is located in the Ejin Horo Banner, the coal seams are deep buried and the roof strata is thin and unstable and even lost in some regions. The coal seam roof aquifer in big thickness threatens coal mine safety mining. So there is a need to evaluate the 2# and 6# roof aquifer water inrush risk and to predict the coal mine inflow, as a result, flow field of and hydraulic connection between each aquifer after mining are analyzed.Hydrogeological parameters are much important in aquifer water abundance evaluation and mine water inflow prediction. In order to calculate the hydrogeological parameters more accurately, pumping tests which have three water level drawdown were selected to calculate the well losses using empirical formula which shows that the well losses is proportional to the cubic of pumping flow. Theoretical pumping well groundwater drawdown are obtained by subtracting well losses from the measured groundwater drawdown. Then analyzing the hydrogeological conditions of the study area and demonstrating that it is not possible to develop a steady flow when pumping in a closed aquifer. So Theis formula can be used to calculate hydraulic conductivity and specific storage of each aquifer using multi-levels drawdown superposition principle. The process can be accomplished in MATLAB software by programming. This method overcomes the defect that single-hole pumping tests can’t solve the specific storage and improves the accuracy of hydraulic conductivity calculated by Dupuit formula and empirical influence radius formula. The accurate hydrogeological parameters are data foundation to the aquifer water abundance evaluation and coal mine water inflow prediction.The classical “three maps” method to evaluate roof aquifer water bursting risk is not suitable for the specific conditions of thin or missed water-resisting layer overlying coal seam and poor water yield capacity of coal seam roof aquifer in large thickness. So on the basis of “three maps” method, the roof aquifer water bursting risk evaluation method is further researched from two aspects, one is the roof caving and fractured degree, the other is the roof aquifer water abundance. The roof caving and fractured degree is represented by water-conducting zone height. While the aquifer water abundance evaluation method is largely promoted on the theoretical basis of water-richness index method. On the one hand, making sufficient use of the geology and hydrogeology investigation information, for example permeability coefficient, sandstone thickness, flushing fluid consumption, core recovery percentage and thickness ratio of brittle rocks to ductile rocks, and taking them as the main controlling factors which controls aquifer water abundance. On the other hand, the limited measured specific capacity are used to calibrate the evaluation result of the water-richness index method. The new method works out the problem of roof aquifer water abundance evaluation on condition of low hydrogeology investigation. For the weights of each main controlling factors, on the one hand, the subjective weights were determined on the basis of expert scoring using analytic hierarchy process(AHP); on the other hand, the grey correlation analysis method was adopted to calculate the objective weights. The weighted average value of subjective weight and objective weight are adopted as the final weight of each controlling factor. The thematic maps of each main controlling factor were overlaid according to the final weights. Then the scheme by which to overlay the aquifer water abundance zoning map and the roof caving and fractured zoning map is established and the roof aquifer water inrush risk zoning map can be get. According to the evaluation results, the relatively risk area of 2# coal seam roof aquifer inrush are mainly at the middle and northern part of the second well field, there also is a small part relatively risk area in the fourth well field. It is relative danger because of the better aquifer water abundance and the relatively high water-conducting zone height. The relatively lower danger area are at the border of the second and fourth well field, the southern part of the first well field. It is because of the better aquifer water abundance. The other relatively lower danger area is at the middle of the third well field because of the relatively high water-conducting zone height. The other area of 2# coal seam roof aquifer is in a low water inrush condition. The 6# coal seam roof aquifer water inrush risk is generally low because of the low aquifer water abundance. Goaf seeper after mining the 2# coal seam is the main threaten to the safe mining of 6# coal seam. Prevention and controlling measures of mine water inrush can be developed according to the water inrush risk zoning map of 2# and 6# coal seam roof aquifer.When making clear the hydrogeological conditions of the hydrogeological units which the study area belongs, the conceptual hydrogeological model and mathematical model can be established. The three-dimensional groundwater flow numerical model of the study area can be established by Visual modflow 2011 software. Firstly, discrete the simulation time and space, then input the hydrogeological parameters, boundary condition,initial water level and source sink term to the numerical model. To recognize the aquifer hydrogeological parameters by the pumping tests data and verify it by longterm borehole water level observation data. The aquifer boundary conditions, hydraulic conductivity of the aquitard is recognized by the long time simulation. By analyzing the sensitivity of the hydraulic conductivity in horizontal and vertical direction and specific storage of the aquifer, we found that the aquifer water level is most sensitive to specific storage, the hydraulic conductivity in the horizontal direction is the second influence factor while the hydraulic conductivity in vertical direction is the most insensitive one. By analyzing the sensitivity of the hydraulic conductivity in horizontal and vertical direction of the aquitard, we found that the hydraulic conductivity in vertical direction is the most important factor to the aquifer water level while the hydraulic conductivity in horizontal direction is less important. So it is very important to make clear the specific storage of aquifers and the hydraulic conductivity in vertical direction of aquitards.The low aquifer water abundance and the large aquifer thickness determined it is not suitable to use pumping well to simulate the mine water inrush. For the pumping well can only be set in one cell, the weak aquifer water abundance it will lead to a sharp groundwater table decline in the grid cell setting the pumping well, while groundwater table around the cell decline slightly. So this time we use the Drain module to forecast mine water inflow, this method can avoid errors in flow calculating caused by dry cell in the process of drainage.Based on the model which has been recognized and verified, the 2# and 6# coal seam roof aquifer water inflow are predicted in natural and mining condition respectively, and also calculate the mine water inflow using “big well” method to cross validate the numerical results. The mine water inflow in natural condition is predicted using the natural hydrogeological parameters. While the aquifer overlying the coal seam is separated into two layer by fractured zone height of which the upper layer’s parameters is not changed while the under layer’s hydraulic conductivity is largely increased by the water-conductivity zone. The hydraulic conductivity in the under layer is determined by the study of former researcher. Mine water inflow in the mining condition is predicted using the changed parameter model. Hydraulic conductivity in the water-conducting zone is set with two scheme. The first scheme is to increase the hydraulic conductivity in horizontal and vertical direction by 4 times and 9 times respectively. The second one is to increase the hydraulic conductivity in horizontal and vertical direction both by 9 times. Prediction results shows that mine water inflow in 2# coal seam roof aquifer is 10 times of that in 6# coal seam roof aquifer, mine water inflow in mining condition is 1.27~1.82 times of that in natural condition, mine water inflow in scheme two is 1.26~1.41 times of that in scheme one, mine water inflow calculated by “big well” method is the most small one as the influence radius, reference radius and the reference influence radius in the big well formula is empirical formula and the flux doesn’t include the static reserve, its precision is low. The measured tunnel water inflow in Hulusu mine at the southwest of Taigemiao coal field is very close to the numerical simulation result, so the numerical simulation result is recommended as the reference for mine water prevention measures and drainage system design.After drainage, the water level in 2# roof aquifer declined at about 760 m, while the water level in 6# roof aquifer declined at about 600 m. Based on this simulation, drainage in coal seams roof aquifer didn’t influence the water inflow in cretaceous aquifer and quaternary aquifer. The author thinks there are two reasons, the first one is the waterconducting fracture zone haven’t reach to the cretaceous aquifer, so there is not hydraulic connection between the cretaceous aquifer and the coal seams roof aquifer. The other one is that this time drained the first mining area of the first, second and third well field in the southern part of coal mine and the first working face of the other well fields, however when the coal mine is put into production the whole area will be drained. In addition, the aquitard in the southern part is much thicker than that in the northern part. So based on this simulation and prediction we can’t ensure that the cretaceous aquifer water level will not decline when the northern part of the mine is put into production in large scale. It is very necessary to get a more detail production scheme and a more accuracy hydraulic conductivity in mining condition to make the model more accuracy in future, by which we can get a more accuracy mine water inflow prediction.In the second part of this paper, we evaluated the ecological environment condition of the study area using NDVI factor and groundwater depth. Evaluation results show that ecological environment condition in the southern part is better than that in the northern part of study area. In addition, the aquitard in the northern part is much thinner than that in the southern part, when coal mine is put into production, there is a large risk that the water-conducting zone will reach to the cretaceous aquifer in the northern part, and caused groundwater level declining which will do harm to the ecological environment. So before mining, we must take measures to prevent the water-conducting zone to break the aquitard underlay cretaceous aquifer, at the same time to prevent the large groundwater level decline in cretaceous aquifer. |
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