Research On Failure Strain And Permeability Evolution Of The Coal Ahead Of Working Face And Its Application

Currently, longwall mining with caving method is the main way to mine thick seam.Phenomenonof large gas emission exists widespreadly in fully mechanized cavingfacesof high gas mine, which limits the production efficiency and puts the mining workers at the risk. One of the reasons is that well-developed soft coal seam decrease the coal permeability and gas extraction efficiency; another is high intensity mining disturbs the extraction process, like hurried gas fore-pumping makes extraction design at the period of mining fail to fully consider the gas extraction in mining distressing zone. Gas drainagein mining distressing zone ahead of working face could increase the extraction effectiveness of draining while mining in the process of coal mining. Hence, in order to make extraction design and solve the gas abnormal effusing in working face, it’s of great theoretical guidance meaning to research the failure strain and permeability evolve of the coal in front of these faces.This thesis focuses on the working faces of Yu Wu Mining Company, and bases on rock mechanics, elastic-plastic mechanics and poromechanics, adopts the integrated method include the stress test, the tri-axial simulation experiment of coal mass deformation, permeability evolution modeling, Comsol numerical simulation and drainage engineering application, analyses the effect on coal deformation from the features of different coal structure and gas adsorption-desorption. Though the researchon the stress-strain-permeability transformation characteristic of coal ahead of working face, it could found the damage mechanism and permeability evolution regulation of these faces. In this thesis, the main research is as follows:(1)Research on coal’s pore structure and gas adsorption-desorption characteristicsIn N1102 working face, Tectonic soft coal often distributes inmining coal seam, and well-developed pore, higher metamorphic grade and stronger absorbability makes its gas content higher than hard coal, which could be found by comparing and analyzing the volatiles, gas content, gas absorption-desorption coefficient(a&b).Analyzing the pore feature of soft and hard coal collected at experiment site by mercury-injection method, conclusion could be found that for shaped coal, mesoporeaccounts for 85.87%, while the proportionof micro porous and transitional pore are14.13%, and corresponding data is 8.29% and 91.71% for raw coal. The fractal dimension of raw coal and shaped coal is that Dr=2.69,Ds=2.92, which shows that pore structure ofshaped coal is more complex than that of raw coal; Meanwhile, the pore volume and porosity of shaped coal is 6.24 and 6.89 times than the raw coal, which shows the pore of shaped coal develops better, its effective seepage channel is bigger. The permeability of shaped coal is far stronger than raw coal and is 41 times than it.Compared to raw coal, it’s quicker to reach and longer to sustain the steady state(80%~90% of the final desorption amount) of gas desorption process for hard coal,the reasonis that micro porous in raw coal blocks the gas desorption and diffusion, thus delays the gas completion desorption. While compared to hard coal, soft coal’s gas adsorption capacity is bigger, it has more gas desorption amount and higher gas desorption speed. The reason is that soft coal structure is based onmesopore, while micro porous and transitional pore of shaped coals,and larger specific surface area means lower absorbability; moreover, due to the geological tectonic movement, the metamorphic grade of soft coal is higher than hard coal.It found that raw coal’s deformation caused by gas adsorption and desorption could be divided into six stages as follows: gas vacuum contraction stage, inflation compressed stage, adsorption expansion stage, inflation with pressure relief stage, elastic recovery stage, and desorption contraction stage, meanwhile, shaped coal’s deformation excluding gas vacuum contraction stage and elastic recovery stage, the reason is that pore structure of raw coal is different from the shaped coal without coal body skeleton stress.(2)Conventional triaxial seepage experiment research of raw and shaped coalRaw coal’s conventional triaxial seepage experiment process includes initial compression stage, linear elastic deformation stage, plastic yielding stage, the stress drop stage and residual damage stage. Shaped coal’s experiment process includes initial compression stage, linear elastic deformation stage, plastic yielding stage and plastic softening stage. On the condition of same axial pressure and confining pressure, changes of the permeability of raw and shaped coal seem like a ―V‖ type with the pore pressure changing, and these two variables present a quadratic function relationship. On the condition of same axial pressure and pore pressure, there is an index function relationship between the permeability and confining pressure. Those results are under combined action of gas adsorption inflation effect, shrinkage effect of coal matrix and Klingberg effect.Comparing raw coal’s complete stress-strain curve and the shaped coal’s in the simulation experiment, it found that obvious macro failure happen in the raw coal sample, and permeability increases suddenly as the failure happening, axial strain and radical strain of shaped coal are bigger than raw coal. Critical gas pressure of Klingberg effect of raw coal is 1.1MPa as that of shaped coal is 1.4MPa, which illustrates that pore structure of coal body also plays an important role in gas seepage process besides axial and confining pressure.(3)Simulation experiment rsearch on deformation of the coal ahead of working faceThrough borehole stress measuring and numerical simulating by FLAC on N1102 working face, it found that vertical stress in the original rock stress zone of the working face is 10.23 MPa and its concentration coefficient is 2.23. The coal ahead of the working face is divided into slow pressurization area, rapid pressurization area and rapid unloading area according to the vertical stress distribution,and we derivate those mathematical expressions of vertical stress and horizontal stress in the corresponding area, and then determined the accurate loading scheme of axial and confining pressure, which ensure the correctness of the experimental results.Through the simulation experiments of raw and shaped coal, it found that there are obvious differences between nonlinear compression stage and linear elastic deformation stage in original stress area of raw coal, but shaped coal’s deformation appears nonlinear on the whole in the same area, the reason is the difference between pore structure of raw coal and shaped coal. In stress concentration area, volume deformation of raw and shaped coal change from compression deformation to expansion deformation gradually on the whole, but raw coal’s deformation is smaller than shaped coal. There exists essential difference between raw coal deformation and shaped coal deformation in rapid unloading area, macro failure happens obviously to raw coal and the coal body appears clear shear fractures. Meanwhile, shaped coal’s deformation enters creep stage and appears dilative shear failure slowly. Comparing conventional triaxial seepage experiment results with simulation experiment results, both two tyoes of coal samples’ failure and the corresponding power of the former experiment are weaker than that of simulation experiment. Effect coefficient of unloading confining pressure of raw coal is bigger than that of shaped coal, it illustrates that the initial outburst energy of soft coal is smaller, which reflects soft coal is prone to occur coal and gas outburst.Based on the equivalent mechanical model of the coal pore-fracture, this chapter puts forward that the raw coal deformation can be divided into matrix deformation, pore deformation and surface cracks. Considering the effect of the gas adsorption and desorption, it derived the constitutive equation of the raw coal which containing gas in the elastic and plastic deformation stage. It also puts forward the main coal deformation of shape-coal containing the matrix deformation and pore deformation, considering the effect of gas adsorption and desorption, deriving the constitutive equation of the shape-coal which containing gas in the elastic and plastic deformation stage.(4)Permeanility evolution model of the coal ahead of working face and its numerical simulationIn the coal deformation simulation experiment which is front of working face, the deformation of coal sample is influenced by the result of combined action of the ontology effective stress, structure effective stress and damage effective stress. But the leading role stress is different in the different stages of the simulation experiments, and it has certain corresponding relationship between the equivalent permeability of coal and the effective stress of the coal body suffered, for example, the nonlinear consolidation and the effective stress coefficient linear of elastic phase tend to be the original porosity ф, the effective stress coefficient range of the yield and plastic softening is(ф, фd), the effective stress coefficient range of the residual damage phase is(фd, фc).Considering the stress and strain deformation, pore pressure compression deformation and the gas desorption contraction deformation, it deduced respectively the mathematical equations of the porosity and permeability of the coal in front of the working face:Using the Comsol software to analyze the stress distribution of coal, gas pressure and permeability distribution of the working face, and the numerical simulation of stress distribution and field drilling stress test results are basically identical. The stress concentration peak is basicly near the 6.7 m; the gas pressure increased with working face distance, and gradually become the original gas pressure and the stress in the transition area of stress concentration and unloading occurred pelter, producing larger gas pressure gradient. The permeability reached the lowest at the stress peak, and the biggest permeability of the stress-relaxation area is about 2.5 times to the original rock stress zone.(5)Gas drainage engineering applicationAccording to the coal stress equilibrium equations of the unloading area in the limit stress state, and drawing lessons from different scholars for the theory calculation research of the unloading zone width, derived the width calculation formula of unloading zone containing gas pressure based on the coal stress state in front of the fully-mechanized face, and further analyzed the effect of different elements for unloading zone width. By the related geological parameters of the N1102 working face calculating the unloading zone width is about 4.6 m, according to the different specific parameters of the field, the values of the unloading zone width is 0 to 5 m.Based on the related theory of the active manometric method, designing the nitrogen injection test of the bedding boring, there is clear difference between the drill hole early unloading rate and the stable pressure value apart from the working face different distance, indirectly responsing the stress state, the fracture and the permeability change trend of the coal of the "three areas" in front of woring face, and the range of different parts is consistent with the theoretical calculation value, providing the field theory basis for the gas extraction design of different regions.Based on the theoretical calculation, the nitrogen injection test of the field drilling and the status quo of the N1102 face, designing the bedding borehole for the unloading area gas drainage, calculating the effective length of drainage borehole in the unloading area and optimizing the borehole layout, got the gas drainage flowduring the different drainage stage changing along with the advancing period of the working face.The drainage gas flow is about 2.8 times more than the normal pressure zone after entering the unloading area, so that it has higher practical application value for optimizing gas drainage design in unloading area.