| Coal seam gas occurrence in China is characterized by"Three high and one low"?high stress,high gas pressure,high gas content and low permeability?.Coal reservoir structure is complex and there are many heavy deformations in coal seams.The coal are loaded generally under pressure with three directions,and the pressure in three directions are different for the effect of geological conditions such as dip angles,coal thickness and occurrence conditions and the mechanical turbulence such as tectonic movement.The stress environment is often in the state of unequal Three-dimensional pressure.In addition,the behavior of coal excavation,drilling and gas drainage will also lead to stress redistribution and local stress concentration,the corresponding stress state of the surrounding rock is also under the state of three-dimensional unequal pressure,that is,in true triaxial stress state.Meanwhile,there are obvious differences in coal fracture propagation characteristics and permeability evolution in different direction because of the variable coal mass strength and mechanical properties effected greatly by inequality during the coal forming in China,coal structure,fracture development and deformation degree.Therefore,it is urgent to carry out the research on the seepage characteristics of different coal beddings of loaded coal under true triaxial condition.In this paper,the research methods of combining theoretical analysis,experimental studies,numerical simulations and field applications were used to analyze the fracture microstructure before and after coal loading and morphological characteristics of different bedding coal samples before and after stress from the microscopic point of view.Experimental studies are conducted on seepage characteristics of loaded coal under the condition of True Triaxial.Qualitative and quantitative relationship between seepage characteristics and the maximum principal stress,the intermediate principal stress,the minimum principal stress,the effective stress in different beddings are formulated.The coupling model of coal rock damage and seepage flow and the dynamic evolution model of loaded coal permeability is presented,considering the effect of Klinkenberg,the compression deformation of gas pressure,the expansion of adsorption and the effect of effective stress.The numerical simulation and field application of the damage and seepage characteristics of coal rock are carried out afterward.Research results have important guiding significance to the optimization of the gas extraction design.The main research contents of this paper are as follows:First of all,from the microscopic point of view,the microstructure of coal before and after loaded and the morphological characteristics of different bedding coal samples before and after loaded were obtained.By mercury-intrusion method and industrial analysis,the basic physical parameters of experimental coal samples were obtained,such as:porosity,specific surface area,pore volume,moisture,ash,volatile,limiting adsorption constant a/b,etc.The acquisition of those basic physical parameters laid the foundation for the follow-up experiments and simulation studies.By using the scanning electron microscope in the level of nanoscale,it was found that the compact structure of coal samples was damaged before and after loading.A large number of cracks are generated,the original holes are further connected and a large number of shear cracks were observed.By using high resolution transmission electron microscope,the microstructure of the coal was tested before and after loading.The results showed that the number of pores and fractures increased obviously after the coal body damaged by the load,which was conducive to growth,development and penetration of crack,and the permeability of coal significantly increased ultimately.Secondly,uniaxial compression experiments using the TAW-2000KN electro-hydraulic servo machine of the different stratification of coal samples showed that for the coal samples of vertical bedding,uniaxial compressive strength and elastic modulus were great,while poisson's ratio was small.For the coal samples of vertical bedding,uniaxial compressive strength and elastic modulus were small,while poisson's ratio was great.For coal on Parallel bedding,uniaxial compressive strength,elastic modulus and poisson's ratio are between that of vertical bedding and oblique bedding.The SH-II acoustic emission system was used to test the acoustic emission signal in the process of monitoring,and we knew that the vertical,parallel and oblique bedding coal samples had obvious acoustic emission signal mutation at 200s,130s and100s.Acoustic emission mutation point of vertical parallel and oblique bedding coal samples of appear at 60%,41%and 33%of peak stress respectively.According to the true triaxial loaded coal seepage characteristic test,the initial permeability of coal samples of vertical,parallel and oblique bedding were 0.0181,0.1352,0.0822m D,and the value of vertical stratification loaded initial permeability of coal samples was only13.5%of that of parallel bedding,and the value of vertical stratification loaded initial permeability of coal samples was only 13.5%of that of oblique bedding;the permeability of coal samples in later stage of Vertical,parallel and oblique bedding were 0.00384,0.00635,0.00739m D,reduced by 78.9%,95.3%and 78.9%respectively.The relationship between the permeability of coal and maximum principal stress?1,intermediate principal stress?2 and minimum principal stress?3 and effective stress all met a exponential function,k=a+bexp??-c??i????i=1/2/3/e?.With the increase of stress,the permeability decreased,while it decreased by 74.1%?16.9%and 9.0%respectively for vertical stratification coal samples in the middle of loading of maximum principal stress and principal stress and minimum principal stress loading phase;it decreased by58.8%?27.8%?13.4%respectively for parallel stratification coal samples in the middle of loading of maximum principal stress and principal stress and minimum principal stress loading phase;it decreased by 67.4%?21.3%?11.3%respectively for oblique stratification coal samples in the middle of loading of maximum principal stress and principal stress and minimum principal stress loading phase.At constant intermediate principal stress?2 and minimum principal stress?3,increasing the load of maximum principal stress?1 until the coal samples failures,coal samples of different beddings deformation were characterized by permeability falling sharply in the begin.With the further increase of stress,permeability declined at a significantly slow speed.But for coal samples of different beddings of coal samples the border between compression to increase have obvious difference.The maximum principal stress of C-D border in vertical,parallel and oblique bedding coal samples were 25,22,19 MPa respectively.According to the Crack evolution and the seepage characteristics of different stratification of coal sample of the numerical simulation results,the vertical,parallel and oblique bedding coal in the initial stages of loading,along with the load stress and pressure,the original porosity and bedding fractured surface closed continuously,especially for the bedding cracks near the surface pressure effect areas.When the maximum principal stress increased to 28,24,22MPa respectively,the original pore fissures of coal samples of different beddings and further expansion of crack initiation of surface and the new,local area appeared.Instability and failure of coal samples appeared.Vertical,parallel and oblique beddings coal samples breaked in the form of shear failure mainly,but for the different bedding coal samples precast bedding face,the final form of the crack evolution were different from each other.From Acoustic emission images of the coal samples compressive of vertical stratification,tensile damage mainly appeared in precast bedding cracks near the surface.In the consolidation stage and elastic stage until the plastic phase of coal samples destruction,acoustic emission signal increased gradually,and the acoustic emission signal was mainly in bedding on both ends of the fracture surface and the surrounding,but parallel bedding coal samples at this stage showed no compressive damage;in the oblique bedding coal samples,the middle of the 3 precast bedding article damaged the most obvious among them,so the acoustic emission signal was detected the most significantly there.Simulation results of oblique bedding instability and failure of coal samples were consistent with the test results.Gas seepage pressure of different bedding of coal samples changed with the stress field.In the early stage of the load stress,gas seepage steadied uniform in coal samples.The pressure gradient in coal sample gas seepage was close to a parallel straight line.With the increase of load to coal samples,sporadic crackings appeared.Pressure field gradient curve had obvious changes where there had cracks,and pressure of gas flow field changed with the fracture,liked the wavy uneven distribution.The pressure of gas flow field changed more obviously when coal samples appeared obvious damage,especially in the vicinity of a new fracture.The dynamic evolution model of the porosity loaded coal mass was formulated on the basis of considering the deformation of the coal subjected to mainly the gas adsorption expansion deformation and the gas pressure compression deformation.Based on the analysis of three kinds of deformation mechanism in the process of damage and deformation,combining with the dynamic variation model of porosity,the effective stress formula was obtained.Then based on the relationship between coal permeability and porosity,the dynamic evolution model of the permeability of coal was established.Finally,Based on the seepage characteristic of gas in coal seams,considering the Klinkenberg effect,coal matrix compressibility by gas pressure,coal matrix swells and effective stress,a fluid-solid coupled model for gas flow to boreholes was developed.Then the model was applied to 29031working face in one coal mine.The results showed that in the vicinity of the extraction boreholes,model with Klinkenberg effect led to gas pressure droping faster than that without Klinkenberg effect,and the larger distance from the extraction boreholes,the smaller impact of Klinkenberg effect.Under a given time period,the relationship between the negative pressure and effect drainage radius met a power function,y=3.05x0.011,with a correlation coefficient of0.95.Considering the actual conditions in 29031 working face,a appropriate negative pressure of 15k Pa was obtained for gas drainage in coal seam to achieve the reasonable effect.Layout interval of boreholes had a significant impact on the effect of gas extracting.And gas pressure declined quickly with small intervals.Taking into account the highly complex and imbalance in 29031 working face,a margin of 30%should be taken,so the layout interval of boreholes was set to be 6m.In this case,“the blank zones”as well as high cost were avoided.Results of numerical simulation showed that after 180d of drainage in the coal seam,the pure methane flow rate extracted from coal seam decreased from 7.16m3/min to 3.58m3/min,which satisfied our demand.Further analysis of the extraction effect of angles between boreholes in seams and bedding plane showed that the angles had a major impact on the effect of gas extraction.The effect of the oblique gas extraction boreholes was much better than that of parallel boreholes.This phenomenon was consistent with observations of true triaxial test results for seepage properties of coal containing bedding.Accordingly,gas extraction boreholes for coal seam gas extracting should be perpendicular to bedding plane,in order to achieve the best effect of the gas extraction.This study greatly help not only the design of gas drainage layout,prevention of coal and gas outburst,reduction of"greenhouse effect",but also high and safe production operations in coal mines. |
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