| In recent years,the problem of seepage in fractured media has attracted wide attention.Fractures and fracture networks are the key migration channels for water,pollutants and oil and natural gas in the groundwater system,as well as the key path for radioactive waste leakage from low-permeability rock reservoirs.Studying the permeability characteristics of single fractures is the basis for studying the hydraulic characteristics of rock mass and the simulation prediction of seepage state.For a single fracture,the geometrical characteristics of the fracture and the filling of the fracture have a significant impact on its permeability.It can be seen that the comprehensive and accurate quantitative characterization of the fracture geometry aperture and the relationship between the geometric aperture and the hydraulic aperture are not only important for the accurate evaluation of the permeability characteristics of the fractured rock mass,but also for the effective prediction of the permeability characteristics of fracture rock mass at engineering scale.It is of great scientific significance and practical value to analyze the characteristics of fluid transport and the mechanism of seepage action in the fracture,and to lay a foundation for the evaluation of groundwater pollutant transport and hydrogeological conditions in the site of high-level radioactive waste disposal site.In this paper,the methods of observation measurement,numerical simulation and physical experiment are proposed,and the hydraulic parameters and geometric parameters of the fracture are combined organically.Based on the quantitative characterization of the geometric aperture,the monte-carlo method was used to generate the single fracture models with random aperture.By using lattice Boltzmann method for different single fracture model to simulate the seepage,combined with the real fracture permeability test,comparing with the results of real fracture seepage model,the relationship between fracture geometry parameters and hydraulic parameters is studied,the effect and mechanism of aperture on seepage flow in a single fracture are illustrated.The main research contents and results are as follows:(1)Based on digital image processing technology,a digital extraction method for aperture is proposed.It is proved that the method has strong applicability and accuracy,and it can realize accurate,fast digital extraction.Using this digital extraction method,the aperture of different types and directions are accurately measured and statistically analyzed.The results show that the generalized extreme value distribution has strong applicability to the probability distribution fitting of aperture,and the probability distribution model of the same sample in horizontal and vertical directions was basically the same.Most experimental variation curves of fractures show a linear or parabolic shape at the origin.On the mm scale,the aperture correlation between various points is good.The experimental variation curves of most fractures conform to the cavitation effect model and show the fluctuation of a certain period.For a single tensile fracture,the experimental variogram models are different in different directions,showing strong spatial anisotropy.(2)It is found that the hydraulic aperture of the model is all smaller than the mechanical aperture by means of lattice Boltzmann method.The spatial distribution of aperture has a greater impact on the hydraulic aperture than the probability distribution of aperture.Therefore,in the process of fracture modeling and seepage simulation,the generation of a model with the same probability distribution as the actual fracture will be more accurate.As the mechanical aperture becomes larger and larger,the variation of the aperture has a decreasing influence on the overall seepage of the aperture,and the aperture is closer to the smooth and straight fracture,and the hydraulic aperture is closer to the mechanical aperture.For the fracture model in this paper,when the value of the mechanical aperture is greater than 26 grid units,the actual width is 0.86mm,and the reduction coefficient is greater than 0.9,and it can be considered that the critical value of the mechanical aperture under the influence of the aperture distribution is approximately 0.86mm.(3)Using the device independently developed by the research group,the permeability test was carried out on different types of single fracture taken from the pre-selected site area of high-level waste disposal in China.The results show that the permeability coefficient of unfilled fracture is between 10-2m/s and 10-3m/s in the rock samples,the permeability coefficient of filled fracture is between 10-6m/s and 10-7m/s.Filling fracture permeability is far less than an unfilled fracture permeability,showing that pack cementation is one of the key factors influencing the fracture permeability.The simulated hydraulic aperture is between 0.1mm and 0.7 mm,and the test hydraulic aperture is between 0.001mm and 0.120mm.Combined with the filling and cementing of the fracture,the ratio of the simulated hydraulic aperture to the experimental hydraulic aperture is found,unfilling hydraulic aperture and the simulation test of hydraulic aperture ratio in single digits,filling simulation hydraulic aperture of fracture reached as many as hundreds of hydraulic aperture width ratio,shows that the cementation filling within the fracture greatly reduces the hydraulic aperture of fracture in practical value,the influence degree to the two orders of magnitude.(4)The concept of frequency hydraulic aperture was introduced,and it was found that the equivalent hydraulic aperture for a single fracture sample in this test could be taken as b20?b30.With the increase of dm/?,dh/dm is increasing gradually,and the amplitude of the increase is decreasing and reaching 1.With the decrease of dm/? the sensitivity of the hydraulic characteristics of the fracture to the roughness increases,leading to the increase of the error of cubic law.The corresponding formula was obtained by fitting:dh2/dm=1-e(-0.38896×dm/?),get the modified cubic law formula without considering filling and cementation:Q=gJb/12v((1-e(-0.38896×dm/?))·dm)3. |
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