| In recent years,with the rapid development of our society and the innovation of mining equipment and technology,the mining activities of coal resources began to transfer to the deep.The hard rock roof is not easy to collapse,and it is easy to form a large-area suspended roof at the side of the goaf.In the deep coal seam,the large-area suspended roof exacerbates the stress concentration on the hydraulic support and the coal wall of the working face,which greatly affects the safety production of the mine.In order to improve the stress condition of coal seam roof,in the process of shallow coal resource development,the way of roof cutting and pressure relief is often used to improve the stress state of surrounding rock.Then for deep coal seam,its stress environment is complex,and the stress structure and state of coal seam roof rock mass are quite different from that of shallow coal seam.Whether the existing shallow roof cutting and pressure relief methods and technical parameters can meet the control requirements of the hard roof of deep coal seam is an engineering and technical problem faced by a miner,and it is also an urgent scientific problem to be solved in the process of deep coal mining in China.Therefore,based on the high stress of deep mine,this paper explores the control method and mechanism of coal seam hard roof in deep mine,which has important scientific and engineering significance for the safe,green and efficient mining of deep mine.Based on the stress state of surrounding rock of kilometer deep mine,taking the hard roof(sandstone)of coal seam as the research object,and based on the TCHFSM-I true triaxial fracturing seepage simulation device of the Key Laboratory of in-situ modification Education Department of Taiyuan University of technology,this paper explores the fracture initiation,extension and expansion law of tight sandstone under different high stress,different liquid injection rate and different fracturing fluid viscosity,and quantitatively expounds the fracture initiation pressure and fracture propagation of sandstone under different conditions Relationship between crack initiation time.In addition,with the help of acoustic emission monitoring technology,the fracture initiation and propagation mechanism of fracturing fracture is revealed based on the ring count,energy evolution and other parameters of sandstone samples during fracturing.Finally,based on ABAQUS and other numerical simulation software,the physical model under the condition of engineering scale is established.Based on the parameters such as fracture propagation width and propagation length,the effects of fracturing fluid viscosity and liquid injection rate on fracture initiation and propagation are further explored,and compared with the indoor test,so as to reveal the fracture propagation law of deep well high stress sandstone fracturing.Based on the research results of this paper,the main research contents and conclusions are as follows:(1)Through the TCHFSM-I true triaxial fracturing seepage simulation device,the hydraulic fracturing experiment of tight sandstone is carried out to explore the fracture initiation and propagation law of hydraulic fracturing of tight sandstone under different injection rates(20 m L/min,50 m L/min,100 m L/min),and reveal the evolution law of injection pressure and the dynamic response characteristics of acoustic emission.The results show that:(1)the liquid injection rate significantly affects the characteristic parameters such as fracture initiation,propagation law and fracture initiation pressure of tight sandstone fracturing.With the increase of liquid injection rate,the greater the fracture initiation pressure of tight sandstone,the shorter the fracture initiation time.That is,when the liquid injection rate is small(20m L/min),the average crack initiation pressure of the test piece is 34.52 MPa and the crack initiation time is54 S.when the liquid injection rate increases to 50 m L/min,the average crack initiation pressure of the test piece is 36.62 MPa and the crack initiation time is 27 S.when the liquid injection rate further increases(100 m L/min),the average crack initiation pressure of the test piece is39.51 MPa and the crack initiation time is 19 s.(2)When using clean water fracturing,different liquid injection rates have little effect on the fracture initiation and propagation form,but when the liquid injection rate is large,the fracture opening is large.In addition,the direction of crack extension is perpendicular to the direction of minimum principal stress,and there is no significant difference.(2)Based on the 100 mm cube sandstone specimen,the effects of different fracturing fluid viscosities(1 m Pa?s,50 m Pa?s,100 m Pa?s)on the initiation and propagation of fracturing fractures are studied,and the initiation and propagation mechanism of fracturing fractures is discussed.The results show that:(1)under the same stress state(40 MPa,20 MPa,35 MPa),the fracturing fluid viscosity has a great influence on the fracturing pressure and time of sandstone,that is,with the increase of fracturing viscosity,the greater the fracturing pressure and the shorter the fracturing time of sandstone,which is consistent with the action law of the increase of fluid injection rate on the fracturing pressure and time.(2)Under the same stress state(40 MPa,20 MPa,35 MPa),when the sandstone specimen is fractured with clean water fracturing fluid(viscosity 1 m Pa?s)and the liquid injection rate is 20 m L/min,50 m L/min and100 m L/min respectively,the average fracturing pressure of sandstone is 34.52 MPa,36.62MPa and 39.51 MPa,and the fracturing time is 54 s,27 s and 19 s;When the viscosity of fracturing fluid increases to 50 MPa?s,the average fracturing pressure of sandstone is 34.04MPa,35.51 MPa and 38.94 MPa,and the fracturing time is 47.67 s,28.33 s and 18 s;When the viscosity of fracturing fluid is further increased to 100 MPa?s,the average fracturing pressure of sandstone is 37.46 MPa,38.21 MPa and 44.21 MPa,the fracturing pressure increases by 3.42MPa(10%),2.69 MPa(7.58%)and 5.27 MPa(13.53%),the fracturing time is 42 s,21.67 s and13 s,and the fracturing time decreases by 5.67 s(11.89%),6.66 s(23.5%)and 5 s(27.78%).(3)Under the condition of different fracturing viscosity,the final extension shape of fracturing fracture is still perpendicular to the direction of minimum principal stress,but the greater the viscosity of fracturing fluid is,the greater the fracture opening is,that is,the more obvious the fracture development is.(3)Based on the evolution law of initiation pressure and initiation time of tight sandstone with different stress states,different liquid injection rates and different fracturing viscosities,with the help of origin data processing software,the quantitative relationship expression of initiation pressure and initiation time of tight sandstone under different influencing factors is established:(1)Under the condition of clean water fracturing(viscosity 1 m Pa?s),the fitting relationship between liquid injection rate and fracturing pressure is y=0.07798x+32.2654,and the fitting degree R~2=0.93736.The fitting relationship between liquid injection rate and crack initiation time is y=76.25-1.35417x+0.00792x~2,and the fitting degree R~2=0.99112.(2)Under the condition of self-made thick oil fracturing fluid(viscosity 50 m Pa?s),the fitting relationship between liquid injection rate and fracturing pressure is y=0.06199x+32.6515,and the fitting degree R~2=0.72659.The fitting relationship between liquid injection rate and crack initiation time is y=66.028-1.028x+0.00547x~2,and the fitting degree R~2=0.9813.(3)Under the condition of self-made thick oil fracturing fluid(viscosity 100 m Pa?s),the fitting relationship between liquid injection rate and fracturing pressure is y=0.08854x+34.9527,and the fitting degree R~2=0.70296.The fitting relationship between liquid injection rate and crack initiation time is y=61.861-1.119x+0.00631x~2,and the fitting degree R~2=0.9786.(4)Based on acoustic emission nondestructive testing technology,the dynamic initiation and propagation process of fractures in sandstone fracturing process is characterized by comparing and analyzing the digital information such as acoustic emission ringing count in the fracturing process of sandstone samples.That is,at the initial stage of fracturing,the dynamic ringing count is less,and the local count fluctuates.When the fracture of sandstone specimen is generated after fracturing,the ring count suddenly increases to about 7～9 times of the initial stage of fracture.After the fracture of sandstone specimen,the increase range of ring count decreases significantly,or even does not increase,which indicates that the fracturing fracture has penetrated the whole sandstone specimen.(5)Using ABAQUS extended finite element numerical simulation software,based on the basic mechanical parameters and fracturing experimental results of tight sandstone,this paper explores the opening and length of fracturing fracture initiation and propagation of tight sandstone under the condition of engineering scale.The results show that:(1)When the liquid injection rate reaches a certain value,the fracture initiation pressure will increase significantly.Through the comparison of fracture development patterns under different liquid injection rates,it is found that the half width and half length of fractures increase significantly.The simulation results are consistent with the physical test results.There is a high similarity between them.(2)When the viscosity of fracturing fluid changes from low viscosity fracturing fluid to high viscosity fracturing fluid,the simulation results show that the width of the fracture increases significantly and the length of the fracture decreases significantly,that is,it is easier to develop wide and short fractures under the condition of high viscosity fracturing fluid and narrow and long fractures under the condition of low viscosity. |
PDF Full Text Request