| The microfractures developed in the coal seam are important bridges between the micropores which act as diffusion and the macro-fractures that act as seepage channels in the coal reservoir,and have an important contribution to the permeability of the coal seam.Therefore,a comprehensive understanding of the structural characteristics of microfractures in coal seams and its control mechanism for permeability is not only the theoretical basis for revealing the changes of coal reservoir permeability,but also has important practical significance for improving the production of coalbed methane.Based on typical Chinese CBM exploitation demonstration area,by integrating coal geology,coal petrology,fluid mechanics,fractal geometry,threshold segmentation,three-dimensional reconstruction,the pore network model extraction and other discipline theories and methods,through the physical experiment,numerical simulation and theoretical analysis,the three dimensional visualization models of the microfractures with different scales in different ranking coals were constructed.The structural characteristics and types of microfractures at different scales in coals were quantitatively characterized.The influence of coal rank on the evolution of microfracture structure in coals was revealed,and the seepage simulation of single-phase water flow and methane gas in the complex three-dimensional microfracture network was realized.On this basis,from the perspective of effective stress-microfracture-coal permeability,the anisotropy characteristics of coal permeability and the dynamic variation law of stress sensitivity under different stresses were systematically studied.Lastly,the controlling mechanism of the microfracture structure on coal permeability was discussed.Studies have shown that:?1?In the experimental coal samples,different types of microfractures were developed,and the morphological characteristics of microfractures are different in different ranking coals.The size of microfractures is not uniform in Xigou coal,and they are mostly curved with uneven edges,and locally connected into branches and partly in an isolated form.There are well-connected microfractures with the aperture between 4-8?m in Zhaozhuang coal.These microfractures provide important channels for CBM migration.The microfractures in Sihe coal are distributed around pores with short length,and small aperture?<1?m?,these microfractures are poorly connected.In the microscale,type B microfractures are mainly developed in the experimental coal samples,followed by type A microfractures,type C and type D microfractures are not developed.In the nanoscale,type D microfractures are developed in the experimental coal samples,but type C,type B and type A microfractures are not developed.?2?Coal rank has a significant influence on the structural characteristics of the microscale fractures.With the increase of coal rank,the average length,average width and average volume of the microscale fractures gradually decrease.The downward trend can be divided into three stages:rapid decline stage(0.59%<Ro,ran<1.25%),slow decline stage(1.25%<Ro,ran<2.25%),and stable unchanged stage(Ro,ran>2.25%).However,with the increase of coal rank,the average quantity of the microscale fractures gradually increase,in a sequence of anthracite?avg.37?>high-rank coals?avg.35?>medium-rank coals?avg.16?>low-rank coals?avg.10?.The average aperture of the microscale fractures is between 49.5 and 160.4?m,and shows a trend of first decrease and then slight increase with the increase of coal rank.The average shape factor of the microscale fractures ranges from 0.047 to 0.056,indicating that the cross-section shape of these microfracture is mainly irregular triangle or square.Moreover,with the increase of coal rank,the value of shape factor first increase and then decrease,reflecting that the surface roughness of these microfractures first decrease and then increase as coal rank increases.?3?The fractal dimension of microfractures is an important parameter which can reflect the porosity and predict the permeability of the fractures.The fractal dimension of the microscale fractures is between 1.64 and 1.78,and increases with the increase of coal rank,reflecting that the structure of the microfractures becomes more and more complicated with the increase of coal rank.The fracture porosity changes with the fractal dimension,and overall shows a“U-shaped”distrbution trend:the porosity first decreases from 2.12%to 0.94%,and then increases from 0.94%to 1.49%?corresponding to the turning point D=1.72?.In addition,it is found that the permeability of coal samples with high fractal dimension is generally low,and this relationship is particularly obvious in high-rank coal samples.The effect of fractal dimension on fracture porosity and permeability is mainly related to the change of microfracture structures during coalification.?4?Nano-CT scanning results show that the nanoscale fractures are connected to each other to form a microfracture network.The interconnected porosity is 1.96%and the predicted permeability value is 0.025 mD.The connected pore size is mainly distributed between 200 and 700 nm with the peak value in the range of 300-400 nm.The average coordination number of the connected pores is between 2 and 5,and the coordination number increases with the increase of the pore size,indicating that the larger the pores,the better the permeability.The shape factor of the connected pores mainly ranges between 0.047 and 0.059 with an average value of 0.051,reflecting that the cross-sectional shape of these pores is mainly triangular.The shape factor of the connected pores generally decreases with the increase of the pore size,indicating that the larger pores have higher roughness.The connected pore throat is mainly distributed in the range of 100-500 nm,and the length of the connected pore throat mainly ranges between 297and 449 nm,but there is no obvious correlation between the length of the pore throat and the pore throat size.The shape factor of the connected pore throat is mainly distributed between 0.043 and 0.048,with an average of 0.046,which reflects that the cross-sectional shape of the connected pore throat is mainly triangular,and there is also no obvious correlation between the shape factor of the pore throat and the pore throat size.?5?The axial pressure and confining pressure have different effects on the permeability of coal rock,and the stress sensitivity of the parallel and vertical bedding coal samples is different.?1?Under certain gas pressure and confining pressure,with the increase of axial pressure,the permeability of the experimental coal samples decreases regularly.According to the descending speed of permeability,it can be divided into four stages:1MPa-5MPa,5MPa-9MPa,9MPa-13MPa and 13MPa-17MPa.In each stage,the permeability damage rate of vertical bedding coal samples is higher than that of parallel bedding coal samples,and the permeability damage rate of both vertical bedding and parallel bedding coal samples decreases with the increase of axial pressure,and the difference of the damage rate between them gradually decreases in the whole testing process.In the process of progressive unloading of axial pressure,the permeability of the experimental coal sample all rebounded,but could not rebound to the original value.In addition,the irreversible permeability damage rate of the vertical bedding coal samples is higher than that of the parallel bedding coal samples.Under the action of the axial stress,the sensitivity of vertical bedding coal sample to stress is higher than that of parallel bedding coal samples.?2?Under certain gas pressure and axial pressure,the permeability of the experimental coal samples decreases regularly with the increase of confining pressure,and the downtrend are divided into four stages:2MPa-4 MPa,4 MPa-6 MPa,6MPa-8MPa and 8MPa-10MPa.The permeability damage rate of coal samples at each stage is different.With the increase of confining pressure,the permeability damage rate of parallel bedding coal samples and vertical bedding coal samples both decreases,and the difference of the damage rate between them gradually decreases in the whole testing process.In addition,under the same effective stress conditions,the permeability damage rate of coal samples caused by confining pressure is much higher than that caused by axial pressure.In the process of unloading the confining pressure step by step,the permeability of the experimental coal sample also rebound,but they could not return to the original value,and the irreversible damage rate of permeability under confinning stresses is higher than that under axial stresses.?6?The influence of effective stress on the permeability of coal rock is controlled by the aperture of microfractures in coals.The microcfracture compressibility exponentially decreases with the increase of the effective stress,not only the average microfracture compressibility in the different coal samples is different,the average microfracture compressibility of the same sample under different gas pressures is also different.Under the experimental gas pressure?0.6-1.6 MPa?,the average microfracture compressibility of vertical bedding coal samples and parallel bedding coal samples ranges between 0.032and 0.075 MPa-1,0.064 and 0.076 MPa-1,respectively.The average microfracture compressibility of the parallel bedding coal samples is greater than that of the vertical bedding coal samples,this phenomenon shows that the parallel bedding coal samples is more vulnerable to stress damage.The average effective stress sensitivity coefficient of the vertical bedding coal samples is distributed between 0.161-0.380 MPa-1,while the average effective stress sensitivity coefficient of the parallel bedding coal samples ranges between 0.331-0.399 MPa-1.The effective stress sensitivity coefficient of the parallel bedding coal samples is greater than that of the vertical bedding coal samples,which proves that the parallel bedding coal samples are more prone to stress damage.In addition,with the increase of gas pressure,the average microfracture compressibility of both the vertical and parallel bedding coal samples decreases,indicating that the increase of gas pressure promoted the increase of pore pressure and reduced the compression effect of stress on microfractures.Therefore,the average microfracture compressibility decreased.?7?Based on the three-dimensional nanoscale fracture network,combined with the digital core software and the numerical simulation method,the seepage simulation of single phase water flow and methane gas in the complex microfracture network was realized.The results show that the internal structure of the microfracture network is very complex with strong heterogeneity.The microfracture structure has obvious controlling effect on fluid seepage,and the average aperture of the nanoscale fracture network in the z-axis direction is the smallest.The single-phase water flow seepage in Avizo shows that the permeability values are different in different directions,among which the permeability value in the Y-axis direction is 0.291 mD,which is the highest,followed by the X-axis and Z-axis direction,with the value of 0.258 mD and 0.184 mD,respectively.The seepage simulation of single-phase water flow in Comsol shows that the permeability values in different directions are also different,among which the permeability value in the Z-axis direction is the smallest,with a value of 0.311 mD.The permeability value in the Y-axis direction and X-axis direction is 0.408 mD and 0.404 mD,respectively.The seepage simulation of methane gas shows that the permeability value in the Z-axis direction is the minimum,with a value of 0.269 mD under the gas pressure of 0.6 MPa,while the permeability in the X-axis direction is 0.466 mD. |
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