Study On The Relationship Between Entropy Scale And Permeability Characteristics In The Fracture Network Through Rock Mass

Mining,tunnel,oil and gas,and water resources and hydropower engineering rock mass is generally involves the fissure seepage problem,natural rock mass affected by the long-term geological processes and engineering disturbed,contains a lot of internal cracks,full penetration ability are relatively weak,rock fracture network structure formed groundwater seepage channels,control of rock mass permeability properties has a key role.At present,the permeability evaluation of fracture network is mainly based on the statistical parameters of fracture geometric characteristics.It is difficult to quantitatively characterize the influence of spatial distribution and connectivity of fracture network geometric characteristics on anisotropy of permeability coefficient,size effect and distribution characteristics of free surface of rock mass.This paper takes rock fracture network as the research object,introduces the concept of geological entropy based on the information entropy theory,proposes the connectivity index of fracture network-entropy scale,and analyzes the correlation between permeability coefficient,REV and free surface distribution and entropy scale under different geometric characteristics distribution conditions of fracture network.The influence of spatial distribution of fracture network on permeability coefficient,size effect and free surface distribution of stable seepage was investigated.The main contents and conclusions of the study are as follows:(1)In order to analyze the influence of geometric feature distribution of fracture network on its connectivity,entropy scale,a quantitative characterization index of fracture network connectivity,was proposed based on Geological entropy theory,and a calculation method of fracture network entropy scale was established.A program for calculating the entropy scale of fracture network was developed by Microsoft Visual C++ platform,and the positive correlation between the entropy scale of fracture network and the permeability coefficient was verified by comparing three simplified fracture network models with traditional connectivity indexes.Based on the entropy scale calculation of a large number of fracture networks,the relationship between geometric feature distribution and entropy scale of fracture networks is analyzed.The results show that the fracture trace length is negatively correlated with the entropy scale,and the fracture spacing and opening are positively correlated with the entropy scale.The Angle change of orthogonal fractures has little influence on the entropy scale,and the entropy scale can be used to describe the disorder of the distribution of the geometric characteristics of the fracture network including trace length,spacing and opening.(2)In order to study the influence of entropy scale on permeability coefficient and size effect of fracture network,the mathematical relationship between permeability principal value and permeability tensor and coordinate transformation formula of fracture network were derived based on equivalent permeability tensor analysis.Combined with the flow balance equation and cubic theorem,the solution matrix and equivalent permeability coefficient of fracture network were established.Furthermore,the seepage head calculation program of fracture network is developed by Microsoft Visual C++,and the entropy scale and permeability coefficient of fracture network are analyzed under different trace length,spacing,dip Angle and opening.The results show that the smaller the fracture trace length and the larger the distance are,the more significant the difference of fracture distribution is,and the more uneven the distribution of fracture head and flow rate is.The permeability of the fracture network is greatly affected by the opening.When the total volume rate is constant,the opening of the fracture network decreases gradually with the increase of the trace length,and the entropy scale and permeability coefficient are inversely proportional to the trace length.On the contrary,when the total volume rate is constant,the fracture opening increases gradually with the increase of spacing,and the entropy scale and permeability coefficient are proportional to the spacing.The change of fracture inclination Angle has little influence on entropy scale and permeability coefficient.The relation between entropy scale and permeability coefficient of fracture network satisfies quadratic polynomial approximately.(3)In order to study the relationship between fracture network entropy scale and permeability characterization unit REV(Representative Elementary Volume),seepage ellipse method is adopted to calculate the permeability principal value and equivalent permeability coefficient tensor of fracture network models of different sizes,and the REV of fracture network seepage model is determined according to the quasi-tensor and quasi-constant properties.The entropy scale and REV variation of fracture network under different trace lengths,distances and inclination angles are analyzed.The results show that the entropy scale of fracture network is positively correlated with REV,which is obviously affected by the length and spacing of fracture traces.In a certain range,the larger the trace length and spacing are,the smaller the entropy scale HS and REV are.The Angle variation of orthogonal fracture has little influence on entropy scale HS and REV.(4)In order to study the influence of geological entropy of fracture network on the distribution of stable seepage free surface,the matrix form and numerical analysis method of stable seepage with free surface of fracture network were established based on the continuity equation and boundary conditions of stable seepage with free surface.A stable seepage test device for fracture network with free surface was developed,and the reliability and accuracy of numerical analysis method were verified by the test.The entropy scale and free surface variation of fracture network under different trace lengths,distances and inclination angles are analyzed.The results show that the trace length and distance have great influence on the entropy scale and free surface.The smaller the entropy scale is,the more dense the crack connected path is,and the lower the free surface descending rate is.The smaller the standard deviation of entropy scale is,the smaller the fluctuation of seepage free surface of fracture network with the same geometric parameters is.The influence of fracture dip Angle on entropy scale and free surface position is small,but mainly on local entropy and local shape of free surface.The larger the local entropy is,the more uniform the fracture distribution is,and the smoother the free surface is.