| In response to coal mines are prone to crash disasters when hard roofs collapse,using hydraulic fracturing to weaken the hard roof,At present,the method of oriented hydraulic fracturing of coal mine is in the borehole along the drilling radial direction parallel to the rock layer for mechanical cutting,high-pressure water is injected to generate horizontal directional cracks within a certain range to achieve the purpose of reducing the thickness of the falling rock layer and reducing the strength of the roof.The disadvantage is that the roof cannot be caved in the designated place of the roadway.Therefore,vertical fracturing technology for prefabricated crack along the borehole is proposed.This article takes purple sandstone and limestone as examples,using a combination of theoretical analysis and the true triaxial physical simulation experiments,research on the initiation and propagation of hydraulic fracturing cracks in rocks with prefabricated crack.The study main contents and results are as follows:(1)Analysis of the stress state of circular hole with prefabricated cracks by means of linear elastic fracture mechanics,and a hydraulic fracturing mechanical model of circular hole with prefabricated cracks was established,the criterion of maximum circumferential tensile stress was used as the criterion for crack initiation.The crack initiation condition and crack initiation direction under the ground stress and high pressure water are analyzed.The calculation methods for the initiation pressure and initiation angle of cracks are given.By comparing the theoretical calculation results with the physical experiments,it is proved that the model can predict the crack initiation pressure and initiation angle of rock with prefabricated crack in hydraulic fracturing.(2)The true triaxial fracturing experimental device was used to study the effects of different prefabricated crack angles on the directional crack initiation and crack extension.The experimental results show that when the prefabricated crack length is 5mm and the horizontal stress difference is 2MPa,the fracturing cracks start from the tip of the prefabricated crack and are affected by the stress field,gradually deflecting to the direction of the maximum horizontal stress.As the prefabricated crack angle increases,the crack initiation pressure increases,the prefabricated crack angle increases from 0° to 90°,the crack initiation pressure increases from 9.23 MPa to 13.22 MPa,and the crack initiation angle increases first and then decreases,at 45°.Get the maximum value.With the increase of prefabricated crack angle,at the same deflection angle,the crack extension length decreases first and then increases;at 45°,it reaches the minimum value.(3)A true triaxial fracturing experimental device was used to study the influence of different prefabricated crack lengths on crack initiation and extension of directional fracturing was studied.The experimental results show that with the increase of the length of prefabricated cracks,the crack initiation pressure gradually decreases and the crack initiation angle gradually increases.When the prefabricated crack angle is 45°and the prefabricated crack length is increased from 5mm to 10 mm,15mm,and 20 mm,the crack initiation pressure is reduced from 10.49 MPa to 8.99 MPa,8.09 MPa,and 7.69 MPa,and the initiation angle increases from 24.1° to 25.8°,34.5°,43.9°.The longer the prefabricated crack,the faster the crack deflects toward the direction of the maximum horizontal stress,and the shorter the crack extension length is at the same deflection angle.(4)The true triaxial fracturing experimental device was used to study the effects of different lithologies on the directional crack initiation and crack extension.The experimental results show that as the fracture toughness increases,the crack initiation pressure increases.The greater the fracture toughness,the smaller the initiation angle and the slower the crack deflects towards the maximum horizontal stress,reaching the same deflection angle,and the longer the crack extension length. |
