| The fracturing of the hard roof of the coal mine is of great significance for preventing the impact of mine pressure and ensuring the safe mining of the mine.The particularity of coal mining requires that the roof can be fractured according to the specified position and direction.Therefore,the directional fracturing technology for prefabricated cracks in the vertical rock layer is proposed.Because the directional fracturing technology of prefabricated cracks on the vertical rock layer adopts bare hole fracturing,and the fracturing fluid is in direct contact with the borehole wall and the prefabricated crack face,the fluid loss phenomenon is obvious.In the process of fracturing,when the fluid loss reaches a certain level,the effect of directional fracturing will be affected,and there may be no cracking,even fracturing failure.In view of this phenomenon,the purple sandstone has been taken as an example in this paper,and theoretical analysis and true triaxial physical simulation test have been combined to study the cracking pressure and the law of crack extension of directional fracture with different fluid loss surface and different viscosity of fracturing fluid.Expand the law of influence of extension.The main research contents and research results are as follows:(1)According to the theory of linear elastic mechanics,fracture mechanics and fluid mechanics,a mathematical model of hydraulic fracturing is constructed at the tip of the prefabricated crack.At the same time,the effects of fluid loss on fracturing were considered,and the effects of fluid loss and viscosity on the cracking and expansion of hydraulic fracturing were discussed.(2)The premature crack surface and the wall surface of the borehole were coated with water repellent to control the fluid loss.The physical experiment of true triaxial directional hydraulic fracturing was carried out,and the influence of different fluid loss on crack initiation and extension was studied.The results showed that: With the increase of filter loss,the cracking pressure becomes longer and the cracking pressure becomes larger.Comparing the test results of uncoated waterproofing agent with the test results of prefabricated crack surface,hole wall surface and full coating waterproofing agent,the cracking pressure increased from 11.95 MPa to 12.21 MPa,12.64 MPa and 15.76 MPa,and the time required for crack initiation reduced from 318.1s to 198.2s,165.8s and 152.0s.The smaller the fluid loss is,the greater the length of the crack is when the crack reaches the same deflection angle.As the fluid loss increases,the peak value of the acoustic emission energy rate of the specimen gradually decreases,and the cumulative energy of sandstone from injection to destruction increases.(3)The effects of crack initiation and propagation extension of directional fractures of different viscosity fracturing fluids were studied by using the true triaxial directional hydraulic fracturing physics test.The results showed that: As the viscosity increases,the cracking pressure increases.When the viscosity increases from 1MPa·s to 10MPa·s and 40MPa·s,the cracking pressure increases from 15.76 MPa to 16.56 MPa and 17.10 MPa.As the viscosity of the fracturing fluid increases,the length of the crack increases gradually at the same deflection angle.The crack width increases linearly with y=0.0024 x +0.1355 as the viscosity of the fracturing fluid increases.(4)Based on the results of(2)and(3),the directional hydraulic fracturing physics experiment has been supplemented for studying the effects of directed fracturing on the co-effects of filtration failure and viscosity.The results showed that: As the viscosity of fracturing fluid increases,the cracking time becomes shorter,the filtration loss of fracturing fluid decreases,and the fracturing pressure gradually increases.Sensitivity analysis result of viscosity and fluid loss by the Morris method showed that the fluid loss has a greater influence on the cracking pressure and the length of the crack. |
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