Study On Seepage Property And Geological Entropy Of Three-dimensional Fracture Network In Rock

The fractured rock mass is composed of impermeable rock blocks and fissures.due to the weak permeability of the rock blocks themselves,water can only flow in the cracks.The fracture network formed by the mutual intersection of the fractures has a controlling effect on the permeability of the rocks.The fracture network formed by intersecting fractures is the main flow path of groundwater in rock mass,and the fracture network in engineering rock mass is complex.The geometric characteristics and connectivity of fracture network have an important influence on its permeability.At present,the traditional three-dimensional fracture connectivity index mainly takes the fracture density,the number of fracture intersections,the average intersection length of fractures and the fracture area as the main parameters.It is difficult to quantitatively characterize the influence of the geometric characteristics and connectivity of the fracture network on the anisotropy of rock permeability coefficient and the free surface distribution characteristics of unstable seepage.In this paper,the three-dimensional rock fracture network is taken as the research object.Based on the principle of information entropy,the geological entropy theory of threedimensional fracture network and the connectivity index entropy scale are proposed.Based on the theory of geological moisture,the index directional entropy which characterize the spatial distribution anisotropy of three-dimensional fracture network is proposed.The variation law of three-dimensional fracture surface density,dimensionless percolation density,entropy scale and permeability coefficient under different fracture trace length,dip angle,spacing and opening degree is analyzed.The relationship between direction entropy and permeability coefficient in different directions is determined,and the influence of geometric characteristics of fracture network on entropy scale and free surface distribution of unsteady seepage is analyzed.The main contents and conclusions of the study are as follows :(1)In order to comprehensively quantify the influence of fracture trace length,spacing,dip angle and aperture on the connectivity and permeability of fracture network,basing on the principle of information entropy,the geological entropy theory and connectivity indexentropy scale of three-dimensional fracture network are proposed.The calculation method of three-dimensional entropy scale of fracture network is established,and the calculation program of three-dimensional entropy scale of fracture network is compiled by Microsoft Visual C + +.By comparing the three-dimensional entropy scale and traditional threedimensional connectivity index of four simplified fracture network models,the positive correlation between entropy scale and permeability coefficient is revealed,and the rationality of entropy scale to evaluate the connectivity and permeability of threedimensional fracture network is verified.Entropy scale can also predict the permeability of fracture network.The effects of different geometric characteristics such as trace length,spacing,dip angle and opening on the entropy scale of fracture network are analyzed.The results show that the trace length of fracture is negatively correlated with the entropy scale,and the spacing and opening of fracture are positively correlated with the entropy scale.The entropy scale can be used to describe the disorder distribution of geometric features such as trace length,spacing and aperture of fracture network.(2)In order to study the relationship between the entropy scale of the fracture network and the permeability coefficient,a numerical solution method for the equivalent permeability coefficient of the three-dimensional fracture network is established based on the flow balance equation and boundary conditions of the three-dimensional fracture network.The variation laws of the entropy scale of the fracture network,the threedimensional fracture areal intensity the dimensionless percolation density and the permeability coefficient under different trace lengths,spacings and dip angles are analyzed.The results show that the smaller the fracture trace length and the larger the spacing,the more significant the difference of fracture distribution,the more uneven the distribution of water head and flow in fracture network,and the less influence of dip head distribution and flow distribution.Different entropy scales and permeability coefficients are inversely proportional to the fracture trace length;the entropy scale and permeability coefficient are proportional to the spacing.The change of fracture dip angle has little effect on entropy scale and permeability coefficient.The mathematical relationship between entropy scale and permeability coefficient satisfies the quadratic polynomial.when the volume ratio is constant and the influence of aperture is considered,the fracture areal intensity and dimensionless percolation density cannot quantitatively characterize the influence of fracture network geometric characteristics.(3)In order to study the seepage anisotropy of three-dimensional fracture network of rock mass,based on the theory of geological entropy,the index-direction entropy which characterizes the anisotropy of spatial distribution of three-dimensional rock mass is proposed,and the calculation method of direction entropy of three-dimensional fracture network is established.By comparing the directional entropy and permeability coefficient in x,y and z directions of three simplified fracture network models,the positive correlation between directional entropy and fracture permeability coefficient is revealed,and the rationality of directional entropy to characterize the anisotropy of three-dimensional fracture network is verified.The seepage anisotropy of rock slope fracture network is analyzed by using the directional entropy theory.The results show that the three strata have the largest entropy and the strongest permeability along the z direction.The directional entropy of No.I rock layer along the y direction is the smallest and the permeability is the weakest.The directional entropy of No.II rock stratum and No.III rock stratum along the x direction is the smallest,and the permeability is the weakest.The directional entropy and permeability coefficient of different rock strata are positively correlated.(4)In order to study the influence of geometric characteristics of fracture network on three-dimensional entropy scale and free surface distribution of unsteady seepage,a numerical solution scheme of unsteady seepage in three-dimensional fracture network is established by using variational inequality method,combining continuity equation and free surface boundary condition.The reliability of the numerical method for unsteady seepage analysis is verified by comparing the analytical solution and numerical solution of the classical orthogonal model.The variation law of entropy scale and free surface of fracture network under different trace length,spacing and inclination angle is analyzed.The results show that under the condition of considering the influence of opening,the smaller the trace length of the fracture and the larger the spacing,the larger the entropy scale of the fracture,the better the connectivity of the fracture,the larger the inflow and overflow flow through the upstream and downstream,and the faster the free surface of the fracture rises.The dip angle of the fissure has little effect on the entropy scale,but the smaller the dip angle of the fissure model,the easier it is to form the dominant flow path,the faster the water flow velocity in the fissure,and the faster the free surface rises.