Study On The Evolution Law And Influencing Factors Of Hydraulic Cracks In High-stress Coal

When hydraulic fracturing the coal body,the expected results are often not obtained due to stress and other factors.In this paper,through laboratory tests and numerical simulations,the influence of stress and other factors on coal hydraulic fracture initiation and expansion is studied,thus providing some guidance for the on-site construction of hydraulic fracturing.The main work is:(1)The laboratory hydraulic fracturing test studied the initiation and propagation characteristics of coal hydraulic cracks under different stress conditions.The result is that the higher the stress on the coal body and the higher the cracking pressure,the harder the coal body is to crack,and the cracking point is transferred from the middle of the bare hole to the bottom.The angle between the crack initiation direction and the maximum horizontal principal stress direction gradually decreases.But when the stress is too high,the coal body will produce complex hydraulic cracks.(2)The growth rate of force cracks has a tendency from fast to slow.As the stress on the coal body increases,not only the angle between the direction of the crack and the direction of the maximum principal stress decreases,but also the number of cracks generated also decreases.(3)In addition to stress,there are other influencing factors.The larger the permeability coefficient of the coal body,the longer the crack initiation time;the larger the particle gradation,the more unstable the propagation of hydraulic fractures,so the direction of crack propagation becomes unpredictable;the worse the homogeneity of the coal body,the rate of crack propagation The faster,as the coefficient of homogeneity increases,the number of hydraulic fractures decreases,and the effect of the coefficient of homogeneity on hydraulic fracturing gradually decreases.(4)In actual engineering,the more difficult the hydraulic fracture propagation of coal body under greater stress,the more hydraulic fractures may be generated along the layered structure when the fracturing drill hole is arranged near the coal wall,or even directly disintegrate the coal body.As the distance between the fracturing borehole and the coal wall increases,the difficulty of expansion appears from small to large to small.Increasing the flow rate of fracturing fluid can improve the fracturing effect,but if it is too large,it may cause cracks to propagate near the roof and affect the stability of the roof.