Mechanism Investigation Of Coal And Gas Outburst Based On Characteristics Of Coal Microstructure And Disturbed Stress

Coal and gas outburst is a dynamic disaster encountered in process of underground coal exploitation.An outburst evokes tons of coal and thousands of cubic meters of gas ejected from a working face in a short time.Studies have been conducted to unravel the outburst mechanism in the past 150 years,and significant progress has been made.So far,still no one single theory can fully explain the entire process of an outburst.In consideration that the nature of outburst-prone coal is not completely understood,the theories of multi-factor mechanism can be constantly improved with the updated knowledge of coal properties.Based on such background,this dissertation makes full use of multidisciplinary knowledge,including surface physical chemistry,elastic-plastic mechanics,seepage mechanics and mining science.Starting with microstructure characteristics of coal,theoretical analysis,laboratory-scale experiment,numerical simulation and instance analysis were combined to investigate the micro process of coal and gas outburst in coal mining environment.The surface morphology,open and closed pore distribution characteristics of coal with different rank were studied.Effective stress and elastoplastic characteristics of gas-bearing coal were analyzed,and the evolution law of permeability under mining-disturbed conditions was obtained.Pore failure models under conditions of stress concentration and coal body instability were established respectively.Outburst occurrence induced by pore failure was studied and explored in deep sense.Associated with outburst experiments in laboratory and two typical outburst cases,the outburst mechanism in microscale were investigated.An outburst preparation and trigger model were proposed and implemented in FEMLIP.（1）Base on the image processing method,the SEM images of coal sample were identified,analyzed,and calculated.The fractal dimensions of pore shape and pore distribution were used to characterize the complexity of coal surface.There is some inherent relationship between the two fractal dimensions and coal rank.The coal surface tends to be low complexity while its rank is close to medium.The low-temperature CO₂ adsorption,low-temperature N₂ adsorption method,mercury injection method,and small-angle X-ray scattering were used to test the pore structure distribution of 8 coal samples with different gas occurrence.Comprehensive characterization of open pores with diameters of 0.34～11000 nm shown that the nanopores（diameter≤100 nm）of coal are the typical features of pore structure,accounted for 92.7%of the total pore volume and more than 98%of the total specific surface area.The poor connectivity between the micropore and the macropore,because of low mesoporous distributions,leads to the“bottle neck”effect of gas migration.In the SAXS test range,the proportion of closed pores accounted for 14%～79.9%and its contribution to specific surface area was22.9%～86.3%.As the degree of metamorphism increases,the volume of micropores exhibits a U-shaped distribution law that decreases first and the increases.From the perspective of the change of adsorption potential and surface free energy during the microscopic adsorption process of coal,the physical effects of adsorption heat and coal deformation were analyzed from macro and micro aspect.（2）Based on the adsorption deformation experiment of coal samples,the dual effective stress characteristics of coal were analyzed and an effective equation were put forward.Effective stress of coal is affected by both adsorption expansion and pore pressure.Adsorption swelling effect changes the contact between coal particles and affects the load performance of coal body,while the pore pressure of free gas shares part of the external load.Gas-bearing coal is known as a natural damage material.Elastoplastic characteristic of soft coal was studied by conducting the acoustic emission test in condition of uniaxial compression,creep and relaxation.During the plastic failure process,coal underwent significant pre-peak hardening and post-peak softening,and coal failure was with accumulated large plastic deformation eventually.Combining with strain-softening/hardening model,friction criterion description for coal is of great suitability.（3）Triaxial seepage test system was constructed to conduct the axial compression and unloading confining pressure seepage experiment of coal samples.Permeability evolution under disturbance conditions was analyzed and explored.An exponential function equation of the first effective stress invariant and permeability is used to describe the evolution of permeability.Combined with numerical simulation of the mining stress evolution in the working face,the spatial evolution law of the permeability in work face is studied.Seepage behavior is sensitive to the stress state of coal body in coal face and exponential growth of permeabilities emerges in distressed zone.The variation of coal strength results in abnormal changes of high permeability zone（distressed zone）and low permeability zone（stress concentration zone）,ranging from 5 to 7 m and 4 to 9 m in front of the working face,respectively.The mining-disturbed scope of permeability of coal body was more than 60 m in front of the working face.（4）Based on nano-indentation test,fracture toughness distribution in the range of600μm×600μm was measured on the coal sample.The distribution nonuniformity of micro fracture toughness was found and that of values ranged 0.0104 to 0.1178 MPa·m^1/2.Weibull Statistical fitting shows that the feature parameter is 0.078 and shape parameter is 0.32.With the nano-indentation creep test,the micro stiffness increased instantaneously with the increase of the loading speed,but eventually stabilized with the load stabilizing.Considering the mining stress distribution law,"II"crack stress intensity factor（slip crack）was used to describe the pore failure under stress concentration,and"I"crack stress intensity factor（open crack）was used to describe the pore failure under the condition of coal body instability.As the pore size decreases,the smaller stress intensity factor produced,the less likely to be damaged.（5）The process of pore collapse followed with gas releasing is defined as“microburst”.Microburst is used to explain the outburst process:a high gas pressure gradient emerges while coal failure at work face develops the highest permeability.The instense microburst results in destruction of pores,which means the range extension of zones influenced by high-pressure gradient.More microbursts occur and produce the chain reactions.As released gas energy gathers,the coal body is destroyed and outburst is initiated.While the coal body is being damaged continuously by in-situ stress and gas flow,and microbursts are occurring,the occurring outburst is at development stage.When local stress stops damaging the coal body and the pressure gradient decreases,the number of microbursts decline.Outburst terminates when the microburst fades,gas energy decreases,and outburst obstruction increases.（6）According to the description of outburst process,the conditions for the outburst initiation are about two aspects:on one hand,enough gas energy can produce pore microburst and accumulate energy to produce macroscopic destruction to coal body;the other hand,instability of coal in stress concentration region is the triggering condition for a high gas pressure gradient.The microscopic destruction of coal pores—microbursts is the source of instant release of a large amount of gas,initiating and sustaining outburst.Mining-induced fracture increasing pressure gradient instantaneously can lead to microburst in coal body and create conditions of outburst initiation.（7）Based on middle scale coal and gas outburst experiments,the macro process of outburst was analyzed from the gas pressure,coal temperature,outburst coal propagation,and particle size distribution during outburst process.From the viewpoint of energy conservation,the energy forms of the outburst included elastic strain energy,gas expansion energy,internal energy of coal,breakage work,throwing out work and gas-flow loss energy,and each was calculated respectively.Gas potential energy accounts for more than 90%of total energy,and initiated and sustained outburst occurrence and development.The damage of coal pores caused by gas during outburst was analyzed from the reduction of the diameter of the outburst coal and the change in the pore structure.It was found that the opening of closed pores could promote the occurrence of outburst.Based on two typical outburst cases,the Red International Coal Mine Outburst in the former Soviet Union and Zhongliangshan Mine Outburst in China,the law of microburst caused by gas pressure gradient was analyzed,and the process of outburst was described.（8）A material hypothesis about gas-bearing coal is put forward.Gas-bearing coal is a granular material in process of coal exploitation and meso-contact mechanical characteristics between particles can show the stress state of gas-bearing coal.Based on the idea of solid-fluid conversion,gas pressure,called energy scale,and second-order work,considered as a conservative failure criterion,are combined as the criteria for solid-flow conversion.A unified model for describing the consistent process of outburst is established herein.Based on the FEMLIP numerical simulation algorithm,the outburst process was numerically simulated by calibrating coal parameters with uniaxial compression test and constructing tunnelling model.The investigation shows that FEMLIP can overcome the shortcomings that finite element method only suites to solve minor deformation problem,and second-order work can describe the failure of gas-bearing coal.Simulation of outburst process conforms to the four-stage description,preparation,trigger,development and termination,and show the tremendous application potential of FEMLIP.