Rock Pre-grooving Under True Triaxial Stress Experimental Study On Directional Fracturing

Rock burst is still one of the biggest disasters in mine production in China.The thick hard roof is the main reason for this phenomenon.How to reduce the strength of roof strata and avoid large area roof hanging is the main problem that needs to be studied.Directional fracturing can divide the hard and intact roof rock into blocks,reduce the harm of large area suspension of the roof rock,and achieve the purpose of reducing or even eliminating rock burst.Based on this,this article uses a true triaxial crushing device to perform a pre-grooved directional fracture test.Study the law of crack propagation and rock mass development under the influence of different groove angles,different borehole diameters and different hydraulic fracturing media.The main conclusions of the article are as follows:(1)Use the rock true triaxial hydraulic fracturing device to carry out the pre-groove fracturing test of 100 mm cube mortar specimens.During the test,the acoustic emission device is used to count the time-varying data of the ringing count and explore different pre-grooves.The influence of angle on hydraulic fracturing fracture propagation reveals the characteristics of fluid injection pressure changes and acoustic emission dynamic changes.The test results show that:(1)The initiation pressure of the specimen gradually decreases with the increase of the prefabricated angle,and the fitting function between the two The relational expression is:y=-0.0384x+14.382;(2)Before the crack initiation pressure reaches the peak value,the acoustic emission ringing count fluctuates at a low level,and cracks are basically not generated;when the injection pressure reaches the peak value,the acoustic emission ringing count surges synchronously,and the specimen appears obvious Fissure;(3)Under the action of the stress field,the cracks all crack from the tip of the pre-groove angle and gradually deviate to the maximum horizontal stress direction;(4)When the same deflection angle is reached,with the increase of the pre-groove angle,the crack arc of the specimen The length change shows a trend of decreasing first and then increasing;at the same deflection angle,the arc lengths of the specimens with pre-grooving angles of 0°,30°,and 60°are 50.19 mm,27.27 mm,and 38.06 mm.(2)A 100mm cube mortar specimen was used to study the influence of different borehole diameters on the development of directional fracturing cracks.The test results show that:(1)The larger the borehole diameter,the greater the crack initiation pressure and pressure reduction rate of the specimen.For specimens with apertures of 6mm,8mm,and10mm,the fitting relationships between the notch angle and the crack initiation pressure are:y=-0.0384x+14.382?y-0=.0447 x+16.215?y=-0.0595x+18.25;(2)The sudden change of the acoustic emission ringing count during the test and the time when the specimen produces a crack The time point is basically the same;(3)When the same deflection angle is reached,the arc length of the fracture of the test piece gradually decreases with the increase of the bore diameter;the average arc length of the fracture of the test piece with the bore diameter of 6mm,8mm,and 10mm is 53.94mm,44.56mm,28.5mm.(3)Using a 6mm cubic mortar sample,the influence of different fracturing media on the development of directional fracturing cracks was studied.The test results show that:(1)The initiation pressure of the H₂O fracturing specimen is greater than the initiation pressure of the CO₂ fracture specimen;The average initiation pressures of H₂O and CO₂ fracturing specimens are 12.65 MPa and 10.56 MPa;(2)Comparing the two fracturing media,the acoustic emission ringing count data of the CO₂ fracture specimens are more and the fluctuation range is large,and the CO₂ fracture test There are many microscopic cracks in the specimen;(3)Under the same deflection angle,the H₂O cracked specimen has a larger fissure arc length than the CO₂ cracked specimen;at the same deflection angle,the arc length difference between the two is 0.07 mm respectively,13.06 mm,31.88 mm;(4)Under the same notch angle,the crack opening of the H₂O cracked specimen is greater than that of the CO₂cracked specimen;With the increase of the pre-groove angle,the difference in opening of the fractured specimens of the two fracturing media is 0.3 mm,0.68 mm,1.15mm,and 1.18 mm respectively;the average fracture opening of the H₂O and CO₂fracturing specimens are 1.3 mm and 0.51 mm.(4)Use numerical simulation software to study the crack propagation law of hydraulic fracturing under true triaxial stress state based on extended finite element simulation.Using the rock mechanics parameters obtained above,numerical simulation of single-hole and double-hole directional fracturing is carried out,and the law of propagation and evolution of cracks in the rock formation is analyzed.The simulation results show that the cracks between the double holes start to crack at the same time and deflect along the maximum horizontal stress direction,and then connect together to form a through crack.With the increase of the deflection distance,the crack width between the double holes increases rapidly and decreases again after a stable period of time.With the increase of the deflection distance,the deflection angles of the double holes first increase and then slowly increase and become stable.