| 3D rough single fracture is the basic element of fracture network in fractured rock mass.To master the permeability characteristics of single fracture is the basis of determining the permeability characteristics of rock mass fracture network,which is of great significance for the safe and efficient implementation of engineering practice.Therefore,3D rough single fracture is a popular object in the study of rock mass fracture seepage law.However,the permeability theory of 3D rough single fracture is not perfect,which mainly includes the following aspects: the influence and mechanism of the matching characteristics between the two walls of the fracture on its permeability characteristics are not fully considered;The influence of hydro-mechanical coupling in the fracture on the permeability characteristics of the fracture is ignored,and the evolution of the permeability characteristics and the deep mechanism of the 3D rough single fracture under the hydro-mechanical coupling are not fully understood.Improving the above problems will help us to master and predict the permeability characteristics and evolution law of 3D rough single fracture more completely and accurately,which is of great significance to many engineering practice activities such as water conservancy and oil and gas resource exploitation.This paper targets the imperfections in the existing theories,takes the "seepage characteristics of 3D rough single fracture" as the main research object,establishes a complete research process based on the numerical simulation method,realizes the generation of random3 D rough surfaces with controllable rough characteristics by using the algorithm program written by itself,constructs a 3D rough single fracture model based on the customized rough.A series of seepage simulations under different conditions were carried out for the rough single fracture.The research work was carried out in the following three aspects: the matching characteristics of the 3D rough single fracture and its influence and mechanism on the seepage characteristics,the seepage characteristics of the 3D rough fracture under the effect of hydromechanical coupling,and the evolution law of the seepage characteristics of the 3D rough single fracture.The main research results are as follows:(1)The significant influence of the matching characteristics on the seepage characteristics and the mechanism are revealed.The fracture’s aperture distribution is controlled by the degree of fracture surface matching.The weaker the matching degree of the fracture is,the stronger the dispersion of the aperture distribution is,which leads to the drastic change of the aperture of the fracture along the flow direction,resulting in numerous "sudden expansion" and "sudden contraction" structures,which cause drastic changes in the flow velocity and flow direction of the fluid.Under the effect of fluid inertia,a large number of eddy current regions are caused near these structures,which seriously hinder the flow of the fluid.As a result,the permeability of the fracture is reduced,and the aperture of the perfectly matched fracture varies little along the path,and it does not have the structural foundation to cause the eddy current area,so the permeability is relatively strong.The perfectly matched fracture can be used as the upper limit of seepage capacity for all uniformly rough fractures with the same roughness and average aperture.(2)A complete set of methods is established to study the seepage characteristics of threedimensional rough single fracture,which improves the research efficiency.Under the hydromechanical coupling,the fluid flow in the fracture shows nonlinear characteristics.Forchheimer’s equation is still suitable to describe the nonlinear characteristics,but the influence of the hydro-mechanical coupling on its coefficient needs to be reconsidered.Not considering the hydro-mechanical coupling will lead us to ignore the change of the aperture of the fracture,underestimate the permeability of the fracture,and exaggerate the nonlinear flow intensity in the fracture.In addition,the larger the pressure gradient,the larger the error between the predicted value based on traditional simulation or experimental methods and the actual value.The hydro-mechanical coupling in the fracture can slow down the growth rate of fluid inertia strength,reduce the nonlinear degree of flow,and increase the permeability,because the hydro-mechanical coupling increases the aperture of the fracture.The hydromechanical coupling strength in the fracture is affected by the roughness of the fracture surface,the pressure gradient and the stiffness of the fracture,the latter two of which are more significant and critical.The rougher the surface,the larger the pressure gradient,and the smaller the stiffness of the fracture,the more obvious the influence of the hydro-mechanical coupling,and the greater the change of the aperture and permeability.(3)The seepage characteristics and evolution of three-dimensional rough single fracture under hydro-mechanical coupling were studied.The seepage characteristics of four different forms are summarized.From the perspective of the development and evolution process of fluid inertia effect and hydro-mechanical coupling effect in fractures and their competition mechanism,the internal causes and evolution rules of various seepage characteristics are analyzed,and their mathematical expressions are determined.According to four different seepage characteristics,the seepage characteristics and evolution rules of three-dimensional rough single fracture under hydro-mechanical coupling were found and summarized,and the development and evolution rules of fluid inertia effect and hydro-mechanical coupling effect in fractures in each stage were explained,as well as the influence of the competition mechanism between them on the fluid flow in fractures.The evolution law,internal reason and critical condition of the phase permeability characteristics of fractures are obtained.It is found that with the gradual increase of flow rate,the gradually enhanced hydro-mechanical coupling effect in fractures is the main driving force to promote the evolution of permeability characteristics of fractures.In this process,the expansion of mechanical aperture caused by hydro-mechanical coupling effect inhibits the growth rate of fluid inertia effect,so that the hydro-mechanical coupling effect is gradually in a dominant position in the competition with fluid inertia effect.Finally,the fracture permeability characteristics formed a phased evolution law. |
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