Mechanism Of Permeability Enhancement By Multi-staged Fracturing In The Roof Of Outburst Coal Seam And Application

To achieve safe mining in outburst coal seam,most domestic coal mines adopt rock tunnel and cross-measure borehole for gas pre-drainage,but this method has defects such as large amount of tunnel engineering,high cost,low utilization rate of borehole and so on.Borehole instead of tunnel is put forward for regional gas control in broken soft and low permeable outburst coal seam.Multi-staged fracturing is one of the key technologies for borehole instead of tunnel,which can effectively transform reservoir and increase its permeability.However,due to the lack of systematic and in-depth research on multi-staged fracturing for gas drainage,the development of multi-staged fracturing lags behind and there are many problems,which to some extent restrict the application of multi-staged fracturing in low permeable outburst coal seam.In this paper,in view of the engineering background of urgent need to eliminate outburst in 39061 work-plane of Zhongmacun Coal Mine in Jiaozuo,by comparing fracturing in outburst coal seam and roof fracturing of outburst coal seam,roof fracturing of outburst coal seam is chose.Mechanism and effect of multi-staged fracturing in the roof of outburst coal seam were studied through laboratory tests,laboratory experiments,numerical simulation theoretical analysis and case analysis of field engineering.Through the above research,some conclusions were drawn in the following aspects.?1?The coalbed methane content,mineral compositions in coal,mechanical parameters of roof and floor and others factors are tested by rock samples collected from coal seam and its roof and floor.The characteristics of coal structure and developing characteristics of natural fractures in coal,roof and floor are analyzed.?2?Large-scale true triaxial hydraulic fracturing simulation experiment on coal rock is conducted by means of large-scale true triaxial hydraulic fracturing experimental device,effects of experimental stress,fracturing rate,fracturing frequency and natural fracture on hydraulic fracture are discussed.The difference of fracture width between homogeneous and heterogeneous sample is studied.Simulation experiments of fracturing position in the roof of outburst coal seam are carried out.The differences of fracture propagation path and geometry under different fracturing position are described,the optimum fracturing position under the given conditions is obtained by comparison.It shows that the water pressure collected by pressure sensor can reflect the fracture propagating path.Based on the above experimental results,the mechanism of forming fracture network in coalbed methane reservoir is discussed.?3?The effects of minimum horizontal stress,stress ratio,fracturing layer and elastic modulus of interlayer on fracture propagation were numerically simulated by RFPA^2D-Flow software.The numerical simulation results show that when horizontal stress difference is constant,initiation pressure and initiation time increase with the increase of minimum horizontal stress,which indicate that a higher water pressure is required to maintain fracture propagation when hydraulic fracture enters reservoir with high stress from reservoir with low stress.Induced stress produced by multi-staged fracturing causes the direction of maximum stress change,the fracture propagating path also changes,which is beneficial to forming fracture network.When fracture propagates from layer with low elastic modulus to layer with high elastic modulus,a higher water pressure is required.When hydraulic fracture propagates to adjacent layer at the same time,the fracture is easier to propagate in the layer with high brittleness index.?4?When induced stress difference generated by multi-staged fracturing in the roof of outburst coal seam is greater than horizontal stress difference is advantageous to the formation of fracture network.Fracturing distance of sequential fracturing and alternating fracturing are optimized.The impact of natural fracture and mechanical properties on forming fracture network during multi-staged fracturing are analyzed.The mechanical condition of hydraulic fracture in the roof of outburst coal seam passing through coal-rock interface are discussed.Mechanism of permeability enhancement by multi-staged fracturing in the roof of outburst coal seam is revealed.?5?Based on the characteristics of in-situ stress,lithologic characteristics of roof and floor,and fracture network stimulating technology in the surrounding rock of outburst coal seam,hydraulic jet multi-staged fracturing technology is useful for field engineering.The field engineering case indicates that hydraulic jet multi-staged fracturing technology is effective for gas control in the low permeable outburst coal seam,the maximum daily gas production is 2275 m³ and total gas production is nearly235000 m³ up to October.Because the directions of natural fracture in coal seam and its roof,maximum horizontal stress are consistent,the main fracture points to and communicate 3904 work-plane,resulting in the increase of gas drainage in 39042 air return way.