Study On The Evolutions Of Damage And Permeability Of Deep Coal Based On CT Visualization

The seepage characteristics of coal rock are closely related to its microstructure.The size and distribution of pores and fractures in the coal rock directly affect the migration or existence of internal fluid.For example,CBM exists on micro-pore surfaces in a condensed and near liquid-like adsorbed state,as well as in fracture networks in a free flow state.Both the original and mining-induced pores and fractures in coal seams,as the main storage carriers and transport pathways for CBM,evolve with the stress conditions of the surrounding rock and the coal during coal seam mining,especially deep coal seam mining,and then directly affect the flow state of the fluid(gas or water).Therefore,the study of pore-fracture networks and their evolution as well as permeability evolution is essential.And,it has important guiding significance for the exploitation of coalbed methane as well as the prevention and control of gas outburst and water bursting in mine.When studying the evolution of pore-fracture networks with stress in coal samples,most of the previous research methods have two difficulties: First,the contradiction between measurement accuracy and sample size,for example,lower measurement accuracy for the large-scale samples or mighty small test samples with high measurement accuracy.Second,the study errors result from the diversity of experimental coal samples.It is difficult to study the evolution behaviour of the pore-fracture networks under different stress conditions for the same coal sample.Focusing on the aforementioned research background and scientific questions,coal samples from Pingdingshan Coal Mine No.12 are chosen as research items,which is the deep coal sample buried approximate 1000 m beneath the ground surface.First of all,a method for porosity calculation was proposed based on the grayscale distribution image.Combining grey level histogram,reversely deriving measured porosity,the binarization image for fracture-matrix-inclusion of coal can be proceeded and fulfill the reconstruction of real coal sample.Secondly,a simplified Sierpinski-like model was proposed based on the self-similar fractal theory under the uniaxial compression conditions on the basis of results of CT scanning and MIP experiment.The characteristic of pore-fracture networks under different measurement accuracy and evolution with axial stress changing was investigated.Furtherly,permeability evolution changing with axial stress was analyzed and evaluated.Under triaxial compression conditions,damage evolution at different stages of flow-solid coupling was studied according to the evolution of acoustic and energy characteristics in cyclic loading seepage experiment,in which techniques of acoustic emission and CT scanning was utilized.The impact of different axial deviatoric stress and hydrostatic pressure on the evolution of permeability was investigated and the mechanism of the influence of confining stress on fracture morphology and evolution of permeability can be revealed.Thirdly,based on the CT visualization technology,inputting the 3-D model reconstructed by CT data into numerical software,the numerical simulation of seepage and cyclic loading can be performed under axial,triaxial,different confining stress conditions by giving proper physical and mechanical parameters and constitution equations.The results were compared and verified with experimental results which were received by the same sample.The work of dissertation is mainly performed from the following aspects:(1)Focusing on the CT image containing inclusion and fracture coal matrix,a method was proposed to visualize the real 3D coal sample.But in the context that the pore cannot recognized due to low pixel of CT image,firstly,according to the linear relation of gray value and density of material,a method based on the CT image gray distribution was proposed to calculate the porosity,which can reach microscopic pores.After that,reversely deriving the threshold of coal matrix and fracture,the fracture-matrix-inclusion of coal can be visualized.Through comparing with the results of OTSU and maximum entropy method,it can be proved that the method proposed in this dissertation has better effects and highly accuracy because the threshold between matrix and inclusion of coal is calculated by the experimental results.Finally,proceeding the CT image and reconstructing the coal sample to visualize the real coal sample.(2)Under the condition of uniaxial compression,combining with the mercury injection experiment,X-ray ?CT scanning experiment,and the proposed simplified sierpinski-like self-similar fractal model,in the same coal sample under different measurement precision of pore-fracture network with the evolution characteristics of axial stress has carried on the detailed description.Firstly,the evolution of fracture networks in coal during loading was investigated using X-ray micro-Computed Tomography(X-ray ?CT)with a uniaxial compression facility.The physical and geometrical parameters in ten levels,such as porosity,fracture aperture,fracture density,and connectivity,were obtained by processing the 3D ?CT data.Along with the self-similar characteristics of the 3D fracture networks,a simplified Sierpinski-like fractal model was proposed to further examine the evolution of the pore-fracture networks with higher measurement accuracy.The results showed that the developed fractal model had the ability to characterize the geometrical characteristics of the pore-fracture networks at various measurement scales.With the decreasing measurement scale r,the porosity and volumetric fractal dimensions of the pore-fracture networks increased,which were approximately linear and logarithmic curves with respect to log(1/r),respectively.At the measurement scale of 50 ?m,the longitudinal cleavage fractures,which were parallel to or perpendicular to the joints,were generated during uniaxial compression.The fracture porosity,as well as the fracture density,increased with increasing axial stress.The fracture growth process could be divided into three stages: the initial stage,the stable growth stage,and the accelerated growth stage.At the measurement scale of 0.008 ?m,the pore-fracture evolution in the coal sample under uniaxial compression occurred in four phases,namely,the pore compaction phase,the production phase of new fractures,the stable development phase of main fractures,and the accelerated production and development phase of the fractures.At the measurement scale of 0.5 ?m,the coal permeability varied in a V-shaped curve with axial stress.(3)Under triaxial compression conditions,damage evolution at different stages of flow-solid coupling was studied during cyclic loading seepage experiment according to the evolution of acoustic and energy characteristics in cyclic loading and unloading seepage experiment,in which techniques of acoustic emission and CT scanning was utilized.The impact of different axial deviatoric stress and hydrostatic pressure on the evolution of permeability was investigated and the mechanism of the influence of confining stress on fracture morphology and evolution of permeability can be revealed.Firstly,seepage experiment(hydraulic-mechanical coupling)during cyclic loading was conducted,during which the permeability was measured through transient method with different axial stress and under corresponding hydrostatic pressure conditions,respectively.At the same time,characteristic parameters of AE were recorded during the whole process of cyclic loading through acoustic emission 3D positioning real-time monitoring system and scanning the coal sample after experiment.After that,according to the change of characteristic parameters of AE(events,range,accumulative ringing count and accumulative energy)with time and acoustic emission damage positioning,combining results of CT scanning and 3D reconstruction,damage evolution during cyclic loading seepage experiment and the change of energy and permeability with stress were analyzed.Finally,the effect of confining stress on the form of coal failure and permeability evolution was revealed from geometry and mechanics aspects.The research results show that damage broken in coal mainly occurs in every cyclic peak stress before the peak stage,the post-peak unloading stage and permeability measured process.The spatial structure of the fracture shows that there are three basic forms of destruction in the coal: single shear planes,cross V-shaped failure planes and multiple parallel shear planes.Elastic energy density can be used to describe the amount of initial fracture,dissipated energy density reflects the generation,extension and through of fractures.In the state of axial deviatoric stress,the change of permeability with axial stress represents V shape of curve in the pre-peak stage,while the variation can be divided into two kinds of conditions at the post-peak stage:(1)monotonically increase;(2)increase first decrease then,representing V shape.In the state of hydrostatic pressure corresponding to different axial deviatoric stress,the trend of permeability versus axial stress represents V shape,which is decreasing first and increasing then.The relation between permeability and volumetric strain is close to the piecewise linear function,and the demarcation is the strain of dilatation point.From the geometric,the change of fracture dip with confining stress can directly reflect the permeability evolution along axial direction of coal.From the mechanics,the direction of confining stress is perpendicular to the flow direction of the fluid.Hence,confining stress tends to close the fractures parallel axial direction during the extension of fractures.(4)Based on CT visualization technique,three dimensions geometric model of CT reconstruction was introduced into numerical simulation software,numerical simulation of uniaxial compression is carried out,damage evolution law of simulation was further compared with uniaxial compression experiment.The results show that the numerical simulation results based on CT reconstruction coal can fit the experimental results well.On this basis,the damage evolution laws of three axial compression under different confining pressures were studied through numerical simulation by one coal sample,the results were compared with experimental results,the results show that the stress-strain curve of numerical simulation is more consistent with the theory curves and can describe the damage evolution law very well for The errors caused by the dispersion of experimental samples are eliminated.