Study On Deformation And Destruction Mechanism Of Surface Wells Under Mining Influence And Mining-induced Methane Drainage

Efficient methane extraction in coal production process can not only prevent methane within gob from pouring into coal mining face,ensure safety production in coal mine,but also change waste into treasure,alleviate the shortage of conventional oil and gas supply and improve the atmospheric environment in China,and produce enormous economic benefits.In this paper,theoretical analysis,two-dimensional and three-dimensional similar simulation tests and field measurements were used to study the deformation and failure mechanism of surface drilling under the influence of mining and the spatial distribution characteristics of air flow in gob.Based on the above research results,an efficient surface methane extraction technology in gob was proposed.The main research results were as follows:The two-dimensional simulation experiments were used to simulate the shear dislocation caused by the bending and subsidence of overlying strata during the advancing process of downhole working face.By analyzing the maximum relative horizontal and vertical displacement distribution of drilling,the damage characteristics of surface drilling during mining were studied,and the high-risk period,high-risk location and drilling failure types of drilling damage are determined.When the working face pushes through 157 m of drilling,the overall horizontal and vertical displacement of drilling reaches the maximum: 70 m,125 m,195 m and 240 m above the coal seam,the horizontal displacement of drilling reaches 60-75 mm,and the vertical displacement reaches 85 mm.In high-risk failure period and high-risk failure location,drilling failure is easy to occur at the interface of hard-soft composite strata in overlying strata of coal seam,in thick rock strata and in thick-thin-thick composite strata.The accuracy of the above research results is verified by field drilling and extraction data and borehole peeps in the mining process of the working face.The characteristics of fracture distribution in gob were studied by two-dimensional simulation experiments.The caving zone is divided into complete penetration zone,partial compaction zone and stable compaction zone.The fracture zone is divided into fracture producing zone,fracture developing zone and crack closure zone.A self-built three-dimensional loading multi-element gas seepage test system was used to simulate the formation of gas flow space in gobs by coal seam excavation.The distribution of gas in the airflow space of gobs was simulated by injecting multi-component mixed airflow,and the distribution of gas concentration in different spatial positions was obtained at fixed points.The spatial distribution characteristics of gas flow and enrichment in the gob were studied.The gob space can be divided into lower enrichment zone,transition area,upper enrichment zone and no flow zone from bottom to top,and the regional boundary is "V" shape.The dynamic changes of gas flow and concentration in directional observation wells with variable trajectory during mining were studied by field measurement.The gas flow space can be divided into blocking zone,enrichment zone and exhaustion zone from bottom to top.In the enrichment zone,the methane concentration decreases linearly with the increase of coal seam height.Based on the above theoretical research results,the precise optimization technology of the mined-out area well position is put forward: when the drilling well location is about 42-54 m away from the return roadway,and the bottom of the drilling hole passes through the upper enrichment zone,and enters the bottom caving zone to realize the co-production of methane in the "double enrichment zone";the "low-medium-high" variable negative pressure efficiency maximization control and extraction technology is put forward: when the surface drilling is in the smooth flow stage,low negative pressure methane extraction with-20~-30 kPa is used to extract high methane mixture;medium negative pressure methane extraction with-30~-60 kPa is used when surface drilling is in the blocking stage;high negative pressure methane extraction with-60~-80 kPa can ensure high methane extraction flow in the exhausting stage of surface drilling.The "low-medium-high" variable negative pressure regulation is used to control methane extraction in drilling,which can maintain the maximum efficiency of methane extraction.