Research On Hydraulic Fracturing Characteristics And Application Of Thick Coal Seam With Gangue

During the fully mechanized caving mining of thick coal seams containing gangue,the gangue increases the mining difficulty,and the mine pressure cannot fully break the top coal,resulting in a low top coal release rate.The use of hydraulic fracturing technology can weaken the mechanical properties of the coal seam containing gangue,reduce the overall strength of the coal gangue seam,make the top coal fully broken and collapse in time,reduce the coal seam rock burst and tile gushing disasters,wet the coal seam and improve the working face operating environment,the technology is safe,economical,environmentally friendly and efficient when applied to top coal caving mining.This paper takes the application of hydraulic fracturing technology in fully mechanized top coal caving in coal mines with gangue-containing thick coal seam as the engineering background.Aiming at the shortcomings of the current research,a combination of theoretical analysis,experimental research,numerical simulation and field tests is used to deepen the research.The instability mechanism and hydraulic fracturing characteristics of thick coal seam containing gangue were studied,and the research results were as follows:(1)According to the different controlling roles of the gangue layer in the coal seam in the movement of the coal-rock layer,the concept of "key gangue layer" is put forward.The key gangue layer plays a decisive role in top coal caving mining.When the thickness of the gangue is large,the crushing effect of the gangue in the top coal caving mining becomes poor,the fragmentation of the top coal is large,and the coal caving bracket cannot fully discharge the coal or it is difficult to discharge the coal.(2)After high-pressure water is injected into the coal seam containing gangue,the elastic energy accumulated in the coal-rock mass is mainly transformed into the dissipative energy that plastically deforms the coal-rock mass and produces crack expansion,and the coal and rock can release the elastic energy.The dissipated energy caused by crack propagation is proportional to the water pressure and the damage degree of the coal and rock mass.Therefore,high-pressure fracturing of coal and rock can promote the initiation and expansion of coal-rock joints and fissures,and increase the dissipated energy of fissure expansion,which can effectively increase the degree of coal-rock damage and improve the coal release rate.(3)The hydraulic fracturing test system of coal seam containing gangue was established,and the hydraulic fracturing characteristics were studied.The results show that the hydraulic fracturing process of thick coal seam containing gangue can be divided into two stages according to the cracking mechanism and the crack propagation mechanism.,namely the densification stage,the crack initiation and propagation stage of the thick coal layer containing gangue.During the water injection cracking in the thick coal seam containing gangue,when the hydraulic pressure reaches the rupture limit of the weak surface of the test block,the test block begins to crack.The weak side of the fissure ruptures and expands.When the confining pressure ratios are σ1/σ2=1,1.5,2,2.5,and 3,respectively,hydraulic fracturing is performed,and the rupture limits of the test blocks are different.When the confining pressure ratio is 1,the fracturing pressure limit is 7.45 MPa,and the confining pressure ratio is 3,the fracturing pressure limit drops to 4.98 MPa,that is,the larger the confining pressure ratio,the smaller the water injection pressure fracturing limit.(4)For the prefabricated horizontal and vertical fissure-containing thick coal seam test blocks,when hydraulic fracturing is performed,the prefabricated horizontal fissures test block has a pressure of 3.58 MPa,and the prefabricated vertical fissures limit is 4.13 MPa;The cracks spread along the prefabricated fissures,indicating that the weak surface structures such as crack joints generated by the prefabricated fissures have an important influence on the waterflooding of the coal seam containing gangue.The cracking limit of the prefabricated double-layer gangue test block is 7.48 MPa,which is 5.46 MPa compared with that of the single-layer gangue test block.It can be seen that the gangue inclusion makes the coal seam more stable and stronger.(5)The greater the water injection pressure and the length of the open-hole section,the more obvious the hydraulic fracturing effect of the coal seam containing gangue;the fracturing effect of different fracturing positions is: upper coal seam > middle coal seam > lower coal seam;the hydraulic fracturing effect of different gangue layers is: : single-layer gangue >double-layer gangue > three-layer gangue;the effect of hydraulic fracturing at different gangue positions is: median gangue > lower gangue > upper gangue;when directional cracking is used,the longer the cracking length is.The longer it is,the larger the damage range of hydraulic fracturing is in the thick coal seam containing gangue.(6)Through the on-site application of Pulongwan 5# coal seam,namely drilling arrangement-fracturing device installation-fracturing process implementation,a complete hydraulic fracturing top coal caving technology system for thick gangue-containing coal seams has been constructed,and a "drilling drilling" has been formed.The technical path of detecting gangue layer + pinpoint sealing and precise fracturing + porous coupling area fracturing”.After the implementation of hydraulic fracturing technology,the fragmentation of top coal was significantly reduced,the recovery rate of top coal in the working face was increased by 5%,the efficiency of top coal caving was increased by 20%,and the step distance of the working face was significantly reduced,which effectively reduced the impact of the roof.The risk of dust production on the working face is significantly reduced,which greatly improves the working environment of the working face.