| Natural fractures in coal reservoirs are the main channel for gas migration and determine the permeability of the reservoir.Quantitative description of its structure and construction of a permeability model that considers the characteristics of the fracture structure are important for studying the mechanism of gas seepage and evaluating the permeability of the reservoir.The previous fracture network permeability model and the traditional reservoir fracture inversion modeling method mostly assume that the fracture is a parallel plate fracture,but the description of the real fracture details is insufficient.This paper takes the natural fracture network in coal reservoirs as the main research object.Through the combination of laboratory tests,theoretical analysis and numerical simulation,the quantitative characterization of complex fracture networks,stochastic modeling of rough fracture networks,and the mechanism of coal seam gas seepage are carried out,in order to reveal the development characteristics of natural fractures in coal reservoirs,and to explore the seepage law of coal seam gas in fractures.The main research results of this article are as follows:(1)The visual microscope observation tests of fractured coal and rock body at different scales are carried out,a set of image-based fracture information extraction and characterization program is established,the information of the obtained fracture network image is extracted,the fracture network is quantitatively characterized by indicators such as fracture rate,fractal dimension,opening,length,inclination,tortuosity and connectivity,and the quantitative relationship between fracture structure parameters is statistically and deeply explored,The structural characteristics of natural fracture network in coal reservoir are revealed.(2)Based on the power-law distribution relationship of natural fracture trace length in coal seam,combined with fractal geometry and the parallel plate cubic law,the permeability model of rough fracture network considering the characteristic parameters of fracture structure is deduced,and the correctness of the model is verified by experimental data.Compared with the traditional parallel plate fracture network permeability model,this model increases the tortuosity of natural fractures and the connectivity between fractures.The analysis results show that the permeability value predicted by this model is about 2 to 3 orders of magnitude lower than that of the traditional parallel plate fracture network model.The fracture network permeability model may overestimate the reservoir permeability.(3)The method of establishing the numerical model of coal seam gas flow based on the fracture network image is improved,and the porosity and permeability functions of the fracture network image are defined.By monitoring the flow velocity in different fracture forms,it is found that the contribution of different fracture forms to gas seepage is also different.The analysis shows that in the fracture network,the main contribution to the seepage is the adjacent type and the cross type fractures,while the contribution of the isolated type fractures minimum.(4)In view of the deficiency that the traditional discrete fracture network modeling assumes that the fractures are parallel plate fractures and ignores the geometric characteristics of real fractures,this paper provides a new rough discrete fracture network modeling method.Compared with the traditional method,this method fully considers the geometric characteristics and random distribution characteristics of fractures,and this method is not limited by size,rough discrete fracture network models with different scales can be established according to the actual fracture statistical information.According to the fracture statistics,a rough discrete fracture network model is established,and the factors affecting coal permeability are analyzed in combination with numerical simulation.The analysis of fracture parameters shows that the permeability of coal seams increases with porosity φf(0.0635～0.164)and fractal dimension Df(1.571～1.755),the maximum fracture branch length lbmax(0.0111～0.0249m),the connectivity c(0.808～2.789)and the proportional coefficient β(0.0057～0.086)increase Increases,and decreases with the increase of the fracture dip angle θ2(1.61°～88.39°)and the tortuosity fractal dimension DTf(1.0018～1.0195).(5)The seepage tests of single fracture coal and complex fractured coal under stress loading conditions were carried out,and the evolution law of permeability under different effective stresses was analyzed.①The single fracture seepage test shows that the fracture permeability gradually decreases with the increase of the effective stress,and the permeability presents a trend of decreasing with a negative exponential function.②The seepage test of complex fractured coal rock under true triaxial stress conditions shows that with the increase of principal stresses σ1,σ2 and σ3,the fractures in the coal are gradually compressed,the gas seepage channel becomes narrower,and the fractured coal.The permeability shows a decreasing trend with the increase of principal stressesσ1,σ2 and σ3,and a coal permeability model under true triaxial stress conditions considering the characteristics of the fracture structure has been established and verified by experimental data,the calculated results are in good agreement with the experimental values,and the obtained curve trend is consistent with the process of loading the three principal stresses.(6)The fluid solid coupling model of coal seam gas migration is constructed.By generating the two-dimensional discrete fracture network geometric model of coal seam in line with the engineering geological characteristics,the coal seam gas drainage simulation of discrete fracture network model is carried out.The results show that the gas pressure and velocity in the coal seam are non-uniform due to the influence of the heterogeneity of coal;With the increase of extraction time,the decline rate of gas pressure,flow rate and extraction flow gradually slows down,which reflects the phenomenon that gas extraction is easy first and then difficult. |
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