Three-dimensional Propagation Of Hydraulic Fracture From Regional Fracturing In Hard Roof

The three-dimensional propagation mechanism of hydraulic fractures in the hard roof area of coal mine is systematically studied by combining theoretical analysis,numerical simulation,laboratory test and underground test.The research contents of this paper include six aspects:(1)the initiation characteristics of hydraulic fractures;(2)3D propagation characteristics of fractures around boreholes;(3)the propagation characteristics of hydraulic fractures across layers in layered rock;(4)interaction between hydraulic fractures and natural fractures;(5)interference mechanism of multi-fractures;(6)Underground hydraulic fracturing test in hard roof area.The main conclusions are as follows:(1)When there is an angle between the fracture and the direction of the principal stress and under non-isobaric conditions,after the initiation of hydraulic fractures,they all deflect to the direction of the larger principal stress.The crack initiation angle decreases with the increase of water pressure and lateral pressure coefficient.With the increase of crack inclination angle,the crack initiation Angle increases first and then decreases.Crack initiation a Angle increases with crack length.The critical water pressure decreases with the increase of crack length.With the increase of crack inclination and lateral pressure coefficient,the critical water pressure increases gradually.The critical water pressure increases monotonically with the fracture toughness increasing.(2)The change of borehole orientation leads to various changes in fracture propagation morphology around the borehole,fractures tend to propagate along the plane of the minimum vertical principal stress.Under the condition of low stress ratio,multiple fracture branches are easily generated in the borehole,and there is no obvious dominant direction for fracture propagation.With the increase of stress ratio,the main fracture branch decreases gradually,and the fracture branch tends to expand along the direction of the maximum principal stress.Under the condition of prefabricated grooving fracturing,the hydraulic fracture will gradually deflect to the plane perpendicular to the minimum principal stress after extending along the grooving.The ultimate propagation direction of hydraulic fractures is not controlled by the initiation mode,and the main propagation direction of hydraulic fractures is controlled by the geostress field.(3)Whether hydraulic fractures can propagate in adjacent layers depends on the stress intensity factor at the fracture tip and the water pressure in the fracture.When the adjacent layers have high fracture toughness,high horizontal principal stress or low elastic modulus,the fracture propagation in the adjacent layers is hindered,while the fracture propagation in the own layer is enhanced.The shear slip of interlayer interface leads to the blunt of fracture tip,and the opening of interface leads to the fluid loss.The combined effect inhibits the hydraulic fracture from crossing the interlayer interface.Fracture morphology is approximately elliptical.Lithologic differences may promote or inhibit fracture propagation,resulting in irregular morphology.(4)The fracture tip stress field and fluid intrusion may activate the natural fracture,leading to its opening or shear slip.The opening of natural fractures provides fluid flow space,and the shear slip reduces the stress concentration at the hydraulic fracture tip,which leads to the restriction of fracture propagation.With the increase of cohesion,friction angle,stress difference,approach angle,fluid viscosity and injection rate,the area of shear activation decreases gradually,and hydraulic fractures tend to cross natural fractures.The process of hydraulic fractures cross natural fractures can be divided into five stages:approaching,intruding,hydraulic pressure accumulation,discontinuous propagation and continuous propagation.The variation of dominant fracture modes at each stage shows that hydraulic fracture propagation follows the principle of least resistance.(5)The opened fracture will generate induced stress around it,which will reduce the degree of stress anisotropy,and even cause the direction of the principal stress to reverse,so as to make the adjacent fracture deflect or bifurcation.Under low stress difference,hydraulic fracture is strongly disturbed by induced stress,which is manifested as a high degree of fracture bending and may induce secondary fractures.Under high stress difference,the degree of interfracture interference is significantly reduced,and the fracture propagation surface is more straight.High elastic modulus will enhance the interference between adjacent fractures,cause severe fracture deflection and induce secondary fractures.In this paper,a new staggered fracturing method is proposed,in which fractures are arranged on both sides for simultaneous fracturing.After the fracturing is completed,new fractures are fractured in the middle,and the superimposed induced stress generated by the fractures on both sides is used to force the middle fracture to divert.(6)The hydraulic fracturing test results in the hard roof area of Hujiahe Coal Mine show that the spatial distribution orientation of regional fracturing macro fractures is controlled by the in-situ stress field.The stress field near the test site is characterized byσH>σv>σh.The main fracture tends to propagate perpendicular to theσh direction.The overall fracture morphology is close to the vertical direction,the average strike of the main fracture is N95°E.Fractures radiate outward from the initiation section as the center.The average area of the main fracture is about1.5×10~4m~2,and the average fracture propagation rate in the zone is about 152m~2/min.The water pressure curve consists of three typical stages,namely,cracking initiation stage,falling stage and stable propagation stage.The average peak fracture pressure of regional fracturing is about 29.1 MPa,the average extension pressure is about 23.2MPa.