| The coal-bearing stratum in China has experienced several strong tectonic movements after its formation.Under the effect of tectonic stress,extrusion,shearing,granulation and crumpling deformation occur continuously in the coal mass,as a result its original structure has been destroyed,and tectonic coal widely develops.Tectonic coal seam is always characterized as high in-situ stress,high gas,low mechanical strength and low permeability,therefore its gas extraction is rather difficult and coal and gas outburst accidents always occur.In this work,aiming at the problem of coal and gas outburst control and gas extraction in the driving face of the tectonic coal seam,typical outburst mines such as Xinjing coalmine,Sijiazhuang coalmine and Xinyuan coalmine in the Yangquan coalfield were studied.By combining theoretical analysis,laboratory experiments and field investigation methods,we first studied the tectonic coal evolution and the outburst disaster characteristics in the Yangquan coalfield,the mechanics and seepage characteristics of tectonic coal mass,and the microscopic pore structure characteristics and adsorption and desorption kinetics characteristics of tectonic coal.According to the study results,the efficient permeability-diffusion-migration increasing method for tectonic coal was obtained.On this basis,regional in-seam borehole gas extraction technology was put forward,and numerical analysis method was used to reveal the gas extraction enhancing mechanism of regional in-seam borehole hydraulic cavitating.Finally,a drilling and flushing integrated hydraulic cavitating technology system was built,and the cavitating parameters were optimized.After then,the gas extraction effect and cost of this new technology were investigated in the field.The main conclusions in this work are as follows:1)After the formation of coal-bearing stratum in the Yangquan coalfield,it has experienced three geological tectonic movements,therefore fold structures widely develop and superimpose on each other.During this process,the horizontal stress could be 1.42.0 times that of the vertical stress,as a result tectonic coal widely develops.Due to the fact that micro-cracks widely develop in the tectonic coal,its firmness coefficient is just 0.220.48,and its crushing work ratio is 12 orders of magnitude lower than that of the intact coal.Meanwhile,the coal-bearing stratum experienced magmatic thermal event in the late Mesozoic,as a result the Ro,max of the coal seam could be greater than 2.0%.Under the effect of thermal evolution by magma,a lot of gas generates.At the same time,the dense surrounding rock provides favorable condition for gas storage.Therefore,the gas pressure and gas content are rather high in the coal seam.In some local areas,the gas pressure and gas content could reach up to2.48 MPa and 24 m3/t,respectively.Moreover,under the high in-situ stress condition,the permeability in the coal seam is just approximately 0.0030.015 mD at the current mining depth.Considering that tectonic coal seam is characterized as high in-situ stress,high gas content and low mechanical strength,coal and gas outburst disasters always occur in its driving face.2)Compared with intact coal,tectonic coal has lower mechanical strength and deformation resistance.Its uniaxial compressive strength,cohesion and average elastic modulus are just approximately 23.62%,26.49%and 15.50%of those in the intact coal,respectively.At the same time,the post-peak failure form of tectonic coal is also different with that of the intact coal under conventional triaxial loading stress path.Multiple shear failure occurs in the tectonic coal,while shear failure occurs in the intact coal.Meanwhile,under the pre-peak unloading confining pressure stress path,the cohesions of tectonic coal and intact coal decrease by 49.0%and 38.8%,respectively.Moreover,due to the tensile shear failure occurs,the damage degree of the coal mass increases significantly.3)With the hydrostatic stress unloading from 24 MPa to 3 MPa,the permeability of the tectonic coal just increases by approximately 4 times,therefore it is impossible to significantly improve the permeability in tectonic coal just by stress-unloading without macroscopic damage.Meanwhile,under conventional triaxial loading stress path,the permeability of tectonic coal only increases by approximately 0.51.1 times during the coal damage process,so loading damage also fails to significantly improve the permeability in tectonic coal.However,under the pre-peak unloading confining pressure stress path,the permeability of tectonic coal can increase by 61.6111.0 times during the damage process,as a result unloading damage is the efficient permeability increasing method for tectonic coal.4)N2 adsorption experiments results for tectonic coal sample show that the adsorption/desorption isotherm tends to close in the coal sample with a particle size below 0.074 mm when compared with that with a size of 13 mm.Meanwhile,the pore structure fractal dimension decreases by 2.8213.43%,while the pore volume and BET specific surface area increase by 3.079.85 times and 4.0319.68 times,respectively.These phenomena suggest that the pore structure in the tectonic coal becomes much simpler,and the pore connectivity improves,and new pores develop during the damage process of coal matrix.With the decrease of the matrix size and the change of the pore characteristics during the coal matrix damage process,the gas adsorption and desorption kinetic properties improve significantly in the tectonic coal.Compared with the coal sample with a particle size of 1-3 mm,the adsorption constant a and b increase by 14.3143.45%and 6.259.58%in the tectonic coal sample with a size below 0.074mm,respectively.Meanwhile,the initial gas desorption speed increases by 1.504.19times,while the initial effective diffusion coefficient increases by 6.1113.83 times.Considering that the gas diffusion property could also be improved by matrix damage,unloading damage could not only improve the seepage property of the tectonic coal,but also improve its diffusion property.With the improvement of the seepage and diffusion properties of the tectonic coal,its gas migration property also improves.Therefore,unloading damage is the efficient permeability-diffusion-migration increasing method for tectonic coal.5)According to the efficient permeability-diffusion-migration increasing method for tectonic coal,regional in-seam borehole hydraulic cavitating gas extraction technology was put forward for the driving face.Numerical results show that the minimum principal stress?3 unloads,while the middle principal stress?2 and the maximum principal stress?1 load during the hydraulic cavitating process.As a result,unloading damage occurs in the tectonic coal.After unloading damage,the permeability in the surrounding coal could increase by 23 orders of magnitude,while the gas adsorption time could decrease from 1 d to 5 min.Considering that the coal mass obtains sufficient permeability-diffusion-migration increasing effort,the effective extraction radius of a single hydraulic cavity with a radius of 0.40.6 m could reach up to 2.653.45 m after 10 days of gas extraction.Moreover,under the multiple hydraulic cavities condition,the close hydraulic cavities may interact with each other,which results in the increase of the unloading damage range of single cavity and superimpose on each other.Therefore,the permeability-diffusion-migration increasing effort and gas extraction effect could be further enhanced in the coal mass.Given this,regional in-seam borehole hydraulic cavitating is to enhance the gas extraction by inducing unloading damage and permeability-diffusion-migration increasing in the surrounding coal.6)By adopting advanced drilling and flushing integrated hydraulic cavitating equipment,coal-water separation system,coal metering system,high concentration gas extraction system and low concentration gas extraction system,a drilling and flushing integrated hydraulic cavitating technology system was constructed for the regional in-seam borehole hydraulic cavitating gas extraction technology.On this basis,industrial tests were carried out in 6 roadways at the 3#coal seam in the Xinjing coalmine.According to the critical coal discharging index,the cavitating spacing was optimized.The cavitating spacing increases in the area with a small borehole spacing,while that decreases in the area with a large borehole spacing.After this,the problem of uneven gas extraction caused by the divergence characteristics of in-seam boreholes in the driving face has been solved.Moreover,advancing hydraulic cavitating technology,in which the stress-unloading effect of the hydraulic cavity could be adopted to help the drilling of the in-seam borehole,was put forward.After adopting this new technology,the spray orifice,pushing-drill and pipe-sticking frequency decreases during the drilling process of the in-seam borehole in the tectonic coal seam.In addition,the drilling speed and the drilling distance increase by 4.8 times and 20 m,respectively.7)Compared with the ordinary in-seam borehole gas extraction technology,the gas extraction effect in the driving face improves significantly after adopting the optimized regional in-seam borehole hydraulic cavitating gas extraction technology.The drilling quantity reduces by 75.0%.The gas extraction cycle decreases from 45 d to 10 d,and the gas extraction rate increases from 26.6%to 32.5%.Meanwhile,the residual gas content and K1 value also decrease during the driving process of the coal roadway.Moreover,due to the increase of the drilling speed and the drilling distance after adopting the advancing hydraulic cavitating technology,the driving speed of each driving team could increase from 58 m/month to 168 m/month under the double-lane driving condition.In addition,after adopting the new gas extraction technology,the amount of drilling work decreases greatly,so that the direct gas extraction cost of the coal mass in the driving face decreases by 35.1%.This work contains 127 figures,51 tables and 178 references. |
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