Study On The Evolution Law Of Water Pressure Fissure In Intermittent Jointed Coal Seam

Coal is a typical fractured rock mass medium.The coal body contains a large number of joints and fissures,and the effective connectivity of the joints is low,which results in the high strength and low permeability of the coal.In this paper,the Jurassic low permeability hard and thick coal seam in Northern Shaanxi Province is taken as the object.The fracture expansion law of intermittent joint fracture under hydraulic fracturing is studied by the comprehensive methods of laboratory experiment,theoretical analysis,numerical simulation and field test.The main contents and results are as follows:(1)The physical and mechanical properties of coal and rock are measured,and the distribution form of coal fissures is analyzed.The failure law of intermittent jointed rock mass is studied,and the main transformation types of intermittent joint fracture of low permeability coal rock are determined.(2)The fracture initiation and expansion model of water pressure fissure in intermittent jointed joint is established by fracture mechanics,and the expression of the initiation orientation,the initiation pressure,the extension length and the expansion direction of the intermittent joint fracture under hydraulic fracturing are derived.The law of initiation and propagation of intermittent joints under fracturing is analyzed.At the same time,the criterion of tensile failure,tensile shear failure and shear failure of geese superimposed fissure rock bridge under water pressure is also given.(3)The initiation and propagation of hydraulic fractures in intermittent joints under the action of hydraulic stress field,surrounding rock stress field and mining stress field are studied.The effective stress range of fractured boreholes is analyzed,and the initiation sequence of hydraulic fractures in intermittent joints is determined.It is found that water pressure promotes crack initiation and propagation of intermittent joints,and the maximum principal stress controls the direction of crack propagation.When the side pressure coefficient is greater than 1,the initiation and extension pressure of intermittent joints decreases,and the crack growth length of airfoil increases.The local stress concentration will change the direction of crack initiation and propagation of wing crack.The mining stress causes the increase of the initiation pressure of the intermittent joint of coal and the decrease of the expansion pressure.And the crack propagation length of the intermittent joint wing is larger than that under the static load,and the connection effect of the coal fracture is good.The effective stress range of fractured boreholes is 3 times the pore diameter,and the intermittent joints located in 3 times pore diameter give priority to crack initiation.(4)The characteristics of network formation of fracture of coal body under fracturing are studied.The network evolution law and seepage characteristics of fracture under different fracturing methods are analyzed,and the best fracturing method for controlling the network of coal fracture is given.(5)The engineering application research shows that the fracture network is formed after fracturing of the working face,and the density and connectivity of fracture line extend by 3?5 times.The structure of the coal seam is significantly improved,which realizes the energy saving and environmental protection mining of the coal seam.Face coal seam working load and hydraulic fracturing by interaction of presplitting region lump coal rate up to 61.47%,compared with the non pre split lump coal rate increased by 14.87%,after a large block of fracturing is relatively reduced,small and medium sized coal are centralized manufacturing,coal product structure reasonable interval distribution.