Study On Oriented Fracturing And Propagation In Sandstone With Prefabricated Cracks

In the process of coal mining,if the coal seam is covered with hard and stable roof,it is easy to form a large area of the hanging arch at the rear of the working face.Once the hanging arch collapses,the hurricane and strong impact load are tending to be triggered,which will cause a serious mine accident.For the gob-side entry retaining and gob-side entry driving of non-pillar mining,the roadway side of the support side of the mined-out area is also easy to form a hanging roof,leading the roadside support load to increase,so that the roadway deforms,which is difficult to maintain.For the coal pillar entry retaining,the hanging roof of goaf increases the width of coal pillars,a serious reduction in mining rate.Therefore,there is a need for artificial methods to force the roof timely collapse with the advance of the working surface.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,resulting in horizontally oriented cracks within a limited range in order to delaminate hard roofs,decrease the overall strength,and reduce the disasters of weighting over great extent.The disadvantage is that the roof cannot be fully caved in the designated place of the roadway and stope,limiting its wide application.Due to the particularity of coal mining,it is required that the roof can be fractured according to the designated location and along a certain direction.The roof fracturing at present stage is difficult to achieve this goal.For this reason,the oriented fracturing technology of the vertical layer prefabricated cracks is proposed.In view of the influence factors of the final fracturing effect in this technical scheme,the influence factors of the final fracturing effect in this technical scheme such as prefabricated fracture azimuth angle,horizontal stress difference and injection rate on the crack initiation and extension law of the fracturing crack are studied in this paper.The study contents and results are as follows:(1)Using the true triaxial fracturing experimental device,the effects of the azimuth angle of the prefabricated crack on the fracturing morphology,the crack initiation pressure,and the crack propagation law of the fracturing are studied.Experimental results show: with the increase of the azimuth angle of the prefabricated fracture(the horizontal stress difference is 2MPa),the initiation pressure is linearly increasing.All the initiation position is at the tip of the prefabricated crack,forming a single symmetrical straight or turning crack,with the degree of bending varying with the azimuth of the prefabricated fracture: 60 °> 75 °> 45 °> 30 °> 15 °> 0 °> 90 °.Through acoustic emission,it is found that the variation law of the shear failure ratio with the azimuth angle of prefabricated fractures test is consistent with the change of crack diversion degree with azimuth angle of prefabricated fractures.As the azimuth angle increases,the crack extension length decreases gradually,reaching a minimum value at 45°.When the azimuth angle of the prefabricated crack is greater than 45°,with the increase of the azimuth angle of the prefabricated crack,the length of the crack increases firstly and then decreases,compared with the case of 45°.(2)Using the true triaxial fracturing experimental device,the effect of horizontal stress difference on the location of crack initiation,crack morphology,initiation pressure and crack propagation is studied.Experimental results show: with the increase of the horizontal stress difference,the azimuth angle interval that can provide the initiation position is gradually reduced.When the horizontal stress difference is less than or equal to 2MPa,the crack propagate along the prefabricated crack tip;when the horizontal stress difference is 3MPa,that is,close to the rock tensile strength of 3.2MPa,and the prefabricated fracture oriented in the 0 ~ 75 °,the crack propagates along the prefabricated crack tip crack;when the azimuth angle is 90 °,the crack propagates in the drilling wall along the maximum principal stress direction.For the horizontal stress difference being 4MPa,when the azimuth angle is 0 ~ 60 °,crack propagates along the prefabricated crack tip;when the azimuth angle is 75 ° and 90 °,the borehole wall is cracked in the direction of maximum principal stress.With the increase of horizontal stress difference,the initiation pressure gradually decreases.After fracturing,a single substantially symmetrical straight or turning crack is formed.For the same prefabricated fracture azimuth angle,with the increase of horizontal stress difference,the transition crack gradually evolves into a straight crack,the fracture diverts to the maximum horizontal stress direction faster,and the crack extension length is shorter when the same deflection angle is reached.(3)Using the true triaxial fracturing experimental device,the effects of different injection rate on initiation pressure,fracture morphology and crack propagation law of rock oriented fracturing were studied.Experimental results show: with the increase of the injection rate,the variation range of the initiation pressure value is smaller and the fracturing fracture tends to extend in the direction of the prefabricated crack.When the same deflection angle is reached,extension length of the fracturing fracture increases with the increase of the injection rate.(4)The hydraulic fracturing model based on cohesive fracture is established and used to simulate the crack initiation and propagation using a zero-thickness,cohesive interface element containing fissure water pressure.The variation law of the initiation pressure for the azimuth angle being 0° is simulated.The results of numerical simulation are compared with physical experiment results.It is found that the model can well reflect the initiation and propagation of hydraulic fracturing cracks in rocks.(5)The study of this paper proved that among the main influencing factors of vertical level oriented fracturing with prefabricated cracks,subject to certain ground stress,the crack can propagate in the direction of prefabricated crack firstly within a certain length by changing the azimuth of prefabricated crack and injection rate,and then controlled by the stress field to expand further along the direction of the maximum principal stress.By controlling the spacing between adjacent drilling holes,the deflected cracks in the adjacent drilling holes can be overlapped with each other within the allowable range so as to achieve the purpose of roof broke along the roadway and roof cutting.