Study On Nonlinear Fluid Flow Characteristics In Single And Three-dimensional Granite Fracture Networks

Quantitative description of deformation and failure behavior of single rock fracture and the influence of geometric characteristics such as fracture surface roughness,fracture aperture and fracture number on fluid flow characteristics in multi-fractured rock mass are the basis of understanding hydraulic coupling characteristics of fractured rock mass.In this paper,granite samples with single fracture are made,and the permeability evolution law of fracture under different normal stress is tested by high-precision hydraulic coupling experiment system under different shear dislocation conditions.After obtaining the fracture surface topography data by high-precision 3D profiler,the deformation behavior and flow process of granite single fracture were simulated by using the calculation program of fracture compression deformation meter independently developed based on MATLAB and COMSOL software,then analysis the influences of normal stress and shear displacement on these two processes.independently developed based on MATLAB.64 numerical models of multi-fractures are established,and the Navier-Stokes equation is solved by COMSOL to simulate the flow state of fluid in three-dimensional multi-fractures.The 3D printing and other modern technologies are used to build multi-fracture flow models with different geometric characteristics,after high-precision seepage test,CT scanning is used for reverse modeling and numerical simulation,which verifies the effectiveness of numerical simulation of fluid flow,the influences of fracture roughness,opening and number on k_vand k_icoefficients in Forchheimer equation,coefficients in k_v-k_irelation equation and critical hydraulic gradient J_care studied.In the study of fluid-structure coupling of single fracture,the fracture surface failure area obtained from the test is basically consistent with the numerical simulation results,which verifies the reliability of the fracture deformation calculation program.The relationship between normal stress and shear displacement and mechanical opening is power function decreasing and exponential function increasing,respectively.The increase of shear displacement will lead to more concentrated contact surface in fracture and increase of fracture flow capacity.Inertia coefficient B and critical hydraulic gradient J_cin Forchheimer equation can quantitatively describe the nonlinear flow characteristics of fractures.B and J_chave a power function decreasing relationship with shear displacement and a linear increasing relationship with normal stress.According to the study of multi-fractures,the geometric characteristics of fractures,such as roughness and fracture number,have significant influence on the critical hydraulic gradient J_c,the inertia term permeability k_vand viscosity term permeability k_iin Forchheimer equation,and the influence relationship between them can also be expressed by corresponding functional relationship.Although it is known that k_vand k_ican be expressed by power function k_i=?×k_v~?,under different geometric conditions,the values of?and?in the function will change.There is a linear relationship between coefficient?and geometric features,and there is a nonlinear relationship between coefficient?and geometric features such as opening degree.These results show that the nonlinear fluid flow in fractures can be predicted when the geometric characteristics of fractures are known.