Study On Seepage Characteristics Of Micro-fracture In Rock Mass Based On Lattice Boltzmann Method

Natural rock mass is composed of intact rock and fissures.In rock fissures,the process of groundwater migration is fissure seepage.The stability,seepage characteristics and weathering degree of rock mass will be affected by fissure seepage,and seepage will be strongly restricted by fissure width.In natural rock mass,there are a large number of micro-fissures whose width is less than 1 mm.A series of engineering accidents have proved that micro-fissure seepage will affect unconventional projects such as large water resources and hydropower projects,deep-buried tunnels,low-permeability reservoirs and deep mines.In addition,with the decrease of fracture width,the interface characteristics such as surface roughness,surface wettability and specific surface area on seepage flow are greatly enhanced,viscous force and interfacial force are gradually dominant.Because of the strong interfacial force,micro-fissures has the characteristics of poor water conductivity and strong water holding capacity,and its water-rock reaction time is longer,which also leads to stronger chemical weathering of micro-fissures Because of the characteristics of micro-fissure seepage,the research results are scarce.Therefore,it is of great engineering significance and theoretical value to study micro-fracture seepage.Due to series of micro-scale effects,the numerical method based on classical Navier-Stokes equation is no longer applicable.Lattice Boltzmann Method(LBM)based on Boltzmann Equation has mesoscopic background.It has the characteristics of clear physical meaning,simple boundary treatment and high calculation efficiency.Many scholars use LBM to study micro-fluids.Based on LBM,the seepage characteristics of micro-fissures are analyzed by the improved Shan-Chen(SC)pseudopotential model in this paper.The main contents are as follows1.In this paper,the SRT and MRT lattice Boltzmann method,multiphase flow theory basis and SC pseudopotential model are analyzed.The results show that:CDMRT can solve the dynamic instability of SRT with high density ratio and high velocity fluids.?In order to realize the coexistence of two phases in a fluid system,the non-ideal fluid equation of state must be included.In addition,the thermodynamic consistency should be satisfied.?The interface of multiphase fluids is formed by surface tension.It is not necessary to track the phase interface when it is simulated by diffusion interface model.?The principle of SC model uses intermolecular force to introduce the equation of state of non-ideal fluid,and achieves two-phase separation.2.By analyzing the limitations of SC model and introducing an improved scheme,the improved SC model is validated by numerical simulation.The results show that:?The classical SRT-SC can not satisfy the thermodynamic consistency and has limitations in pseudo velocity,large density ratio and dynamic simulation.?The improved SC model can reduce the pseudo-velocity,satisfy the thermodynamic consistency and realize the dynamic simulation of large density ratio two-phase flow.3.Based on the MRT improved SC pseudopotential model,It has been studied that the effects of influencing factors(crack width,liquid length,viscosity coefficient,etc.)on the seepage of micro-fractures under different wettabilities.Through simulation,we can get:CD The contact angleo directly affects the fluid slip.The larger the contact angle ? is,the more significant the velocity slip and the greater liquid velocity u.?The smaller the crack width W is,the more significant influence of the contact angle 0 on the liquid velocity u.? In hydrophobic micro-fractures,the liquid velocity u does not change with the liquid length;In hydrophilic microcracks,the liquid velocity u is proportional to the liquid length,but the slope decreases gradually.?The liquid velocity u is proportional to the driving force F and inversely proportional to the viscosityv.4.Using the MRT improved SC pseudopotential model,it has been ananlyzed that the effects of geometric characteristics of rough elements(rectangular,trapezoidal and triangular)and wall wettabilities on micro-fracture seepage.Through simulation,we can get:CDwhen the height of rough element increases,the liquid velocity u in hydrophobic(?=145.5°,117.2°)micro-fractures decreases firstly and then increases,while the liquid velocity u in hydrophilic(?=93.0°)micro-fractures decreases continuously.However,when the hydrophilicity of the wall is further strengthened(?=67.0°),the liquid begins to accumulate between the rough elements and forms drag-reducing liquid film,which makes the liquid velocity u accelerate again.?With the increase of the spacing between rough elements,the liquid velocity u in the strong hydrophobic(?=145.5°)micro-fractures increases continuously.However,the liquid velocity u in weak hydrophobic(or hydrophilic)(?=117.2°?93.0°?67.0°)micro-fractures continues to decrease.Because the larger the spacing between rough elements is,the easier the liquid will be immersed inside the rough elements,thus affecting the liquid velocity.When the rough elements spacing is large,the strong hydrophilic(?=67.0°)wall cannot accumulate liquid.?The influence of geometric characteristics of rough elements with different shapes on seepage flow is same,however the influence of different shape rough elements on seepage velocity u is different.