Experimental Study On Hydraulic Properties Of Fractured Rock Mass In Complex Stress State

Natural rock masses usually contain fractures or complex fracture networks due to perturbations resulting from geological movement and human activities.Fluid flow in fractured rocks is involved in many engineering activities,such as nuclear waste disposal,geothermal energy extraction,oil and natural gas production and CO2 sequestration.Understanding the fluid flow behavior through fractured rock mass,which varies with the in situ and disturbed stresses,is fundamental and of great importance to ensure performance and safety of these engineering projects.In this paper,the fractured rock mass was taken as the research object.By using the self-developed apparatus for stress-dependent fluid flow tests of fractured rocks,a series of hydraulic experiments on fractured rocks subjected to various loading conditions were carried out using the combined method of indoor laboratory test,theoretical analysis and numerical simulation.Correlation between the nonlinear flow characteristics in fractured rocks and the pre-existing fracture geometry,shear displacement,and the loading conditions was discussed respectively,which plays a certain positive role in riching the hydraulics theory of fractured rocks.?1?By using the high resolution rock CT scanning system,three-dimensional reconstruction of internal fracture characteristics of granite samples after different stress paths was carried out.The hydromechanical tests indicate that relationship between volume flow rate and the pressure gradient of the granite samples after uniaxial compression presents obvious nonlinear characteristics,which can be described by the Forchheimer's equation.Both the coefficients a' and b' show an increasing trend with the increase of confining pressure ?s.With increasing pressure gradient,the transmissivity decreases gradually.Relationship between volume flow rate and pressure gradient for granite samples after conventional triaxial compression and unloading before peak of triaxial compression all shows an approximate linear feature.The equivalent permeability coefficient of the samples all decreases with the increase of confining pressure ?s,while the variation characteristics present a certain difference with the increase of confining pressure ?₃.?2?A high precision apparatus for stress-dependent fluid flow tests of fractured rocks was developed,and water flow tests on 3D rough-walled fractures with different mechanical displacements were carried out.The fluid flow behavior of rough-walled fractures can be well fitted using both the Forchheimer's and Izbash's laws.Both the linear and nonlinear coefficients in Forchheimer's law experience a reduction trend during shear.The value of ?,which is a parameter in Izbash's law,decreases approximately 2?3 orders of magnitude,while the value of m,which is the other parameter in Izbash's law,keeps in the range 1.35?1.80.A polynomial function is proposed to evaluate the transmissivity considering the Reynolds numbers applied on fractures during shear.The transmissivity,critical hydraulic gradient and equivalent hydraulic aperture of the rock fractures all increase gradually with the increase of shear displacement.?3?Stress-dependent fluid flow behavior of plate granite specimens containing fracture networks can be well decribed using both the Forchheimer's and Izbash's laws.Both fitting coefficients a and b in Forchheimer's law experience an increase trend with the loading level,while decrease with the increase of included angle or number of intersections of the fracture networks.With the increase of included angle or number of intersections,transmissivity of the rock fracture specimens all increases gradually,while both the critical hydraulic gradient and critical Reynolds number generally display a reduction trend.With the increase of lateral pressure coefficient,hydraulic properties of the rock fracture specimens weaken gradually,while both the critical hydraulic gradient and critical Reynolds number are increased.Compared with the loading condition of Fx=Fy,for different lateral pressure coefficients,the larger the?Fy-Fx?,the more obvious the difference between the hydraulic properties.?4?A theoretical model for stress-dependent hydraulic properties of fractured rocks was set up.A series of numerical calculations using COMSOL Multiphysics software on hydraulic properties of a single fracture and fracture networks were respectively performed.For a single fracture,with the increase of time,the average transmissivity first increases gradually before attains a stable value.The average transmissivity for single fractures in stable state decreases gradually with the increase of fracture angle and the overburden pressure,while experiences an increase with the increase of inlet water pressure and initial fracture aperture.Both included angle and number of intersections of the fracture networks have an effect on the effective normal stress,fracture aperture,fluid pressure and flowing channel of the fractures.With the increase of included angle or number of intersections of the fracture network,the overall flow velocity flowing out of the models all presents an increasing trend.