Water yield calculation of drainage boreholes based on water exchange between aquifers and boreholes

Depressurization by dewatering is a main engineering method for the prevention and control of roof water disaster in coal mines.The calculation of water yield and parameter optimization of inclined drainage boreholes in the compound water filled aquifers is a key technology to be developed.Based on the theory of coupled model of seepage and pipe flow,the law of energy conservation,taking the water exchange between aquifer and borehole as the coupling point,a coupling model of aquifer borehole system was established for the inclined drainage boreholes of coal seam roof compound water-filled aquifer and the reliability of the model was verified by the data from the dewatering test of the temporary coal bunker in Muduchaideng coal mine.Based on this model,the water yield of different elevation angles,number and the water yield per unit length were calculated,and the optimal elevation angles and number of boreholes were determined.Meanwhile,the characteristics of borehole water yield of different dewatering heads and the drawdown inside the working face in the range of 600 m working face under 150 m drilling field spacing were calculated and analyzed.The results showed that the calculation model of drainage boreholes water yield based on water exchange between aquifers and boreholes fully considered the laminar and turbulent flow characteristics in the borehole during drainage.The water yield of one main drainage hole and the water levels of two observation holes calculated by the model were consistent with the actual situation.The model had good reliability and could determine the proportion of aquifer water in inclined borehole in compound water filled aquifer.With the increase of elevation angle of drainage borehole,the water yield per unit length of borehole showed a trend of increasing first and then decreasing.When the elevation angle was 60°,the water yield per unit of borehole was the maximum,with a stable water yield of 165 m3/h and the main water yield layer was the first member of Zhiluo Formation,accounting for about 81% of the total water yield.In addition,with the increase of the number of boreholes in a single drilling field,the total water yield of boreholes increased continuously,and the water inflow of boreholes per unit length showed a gradually decreasing trend.After the number of boreholes reached 5,the total water yield was basically stable,and the stable total water yield of boreholes was about 440 m3/h.In the range of 600 m working face under 150 m drilling field spacing,within 60 days,the total water yield from the borehole decreased from 1 700 m3/h to 170 m3/h under the 780 m water head,a decrease of 90%,and decreased from 1 530 m3/h and 1 350 m3/h to 153 m3/h and 135 m3/h respectively,with a decrease of 90%,under the 830 m and 880 m constant water head.The total water yield from the drilling field was affected by the drainage of the surrounding drilling field and gradually increased from both ends to the inside.The total water yield from the boreholes at both ends was about 309 m3/h,and the total water yield from the boreholes in the middle drilling field was about 142 m3/h under the 780 m water head.The time of the working face water head drop to 780 m,830 m and 880 m was 150,125 and 100 d respectively.As time went on,the range of different depths gradually increased.In addition,with the extension of the drainage time,the cross section presented a descending funnel centered on the inclined borehole and its range gradually increased,while the plane presented a descending funnel centered on the whole drilling field and its range gradually increased.The study results can provide a reference for the optimal arrangement of pre drainage holes and the reasonable development of drainage project before coal seam mining.