| Fractures widely exist in rock media.Accurately describing the distribution,surface morphology,and topological relationships of fractures,and their control mechanisms on fluid flow are of great significance for the assessment of rock media,the safety production of underground engineering,and the safety and rational use of water resources.However,the complex structure of fracture networks and the wide range of fracture scales(from a few micrometers,even nanometers to several kilometers)cause the difficulties to describe and count the structure of fracture networks.Besides,the roughness of internal surfaces,the differences of aperture scale,mineral precipitation,slip between two surfaces of a profile,and erosions not only cause the difficulty in the selection of fracture apertures but also cause the variation of local apertures and therefore induces the nonlinear distribution of pressure gradient in rough fractures.Although the application of fracture segmentation methods solved some nonlinear problems,the segmentation is rigid.The segmented results depend on the randomness of profiles.Besides,the effects of segmentation numbers on the evaluation of permeability have scarcely been studied.Therefore,only by solving the segmentation of fractures and the accurate measurement of fracture apertures can we accurately evaluate the permeability of fractures and provide a basic guarantee for safe production.Considering the above difficulties,this thesis takes two-dimensional and threedimensional self-affine rough fractures as research objects.Firstly,we segmented rough fractures into multiple small components using the Cuckoo Search algorithm and solved the deviations between fractures and segmented results caused by the randomness of profiles and the start position of a measurement.With the fractal theory and the segment results,we quantitatively analyzed the surface roughness and its characterization.Then,we applied the laser cutting and 3D printing technologies to construct two-dimensional and three-dimensional rough fractures,respectively.The flow properties of fluids in rough fractures are studied using hydrological experiments and lattice Boltzmann simulations.The effects of surface roughness on fluid flow are analyzed quantitatively.Further,permeability models for rough fracture are proposed to evaluate the permeability,and they are verified using experimental data,simulation results and the methods from other research.The following results and conclusions are obtained in this study:(1)Using the Cuckoo Search algorithm,fracture profiles are automatically optimized into multiple segments,which improved the accuracy for evaluating fractal dimensions.A modified local cubic law is proposed to evaluate the effects of segmentation on the permeability evaluation by considering the fractal and roughness characterizations of profiles.First,using self-affine functions,we constructed a series of rough profiles.Then,instead of the traditional fixed-length measurement method,measurement with variable scales is applied to measure these rough profiles.The measurement results show that the trend lines are closer to the morphology of the rough profiles as well as better correlations among statistical parameters,which improved the accuracy to evaluate fractal dimensions.Since only one fractal dimension or one roughness is not capable to characterize the roughness of fracture profiles,we proposed a modified local cubic law considering the fractal and roughness characterizations and determined the effects of segmentation on the permeability evaluation.The experimental water flow through the laser cutting fracture models verified the segmentation method and the modified local cubic law.Basing on the velocity analytical solution through the cross-section of a rectangular tube,we achieved the dimensionality reduction of flow data.The method of transforming pseudo-three-dimensional flow data into two-dimensional data provides the basis of methods and data for the verification of 2D permeability models.(2)Basing on the different controlling effects of local apertures on the permeability,we proposed a segmentation method integrating the weight of local apertures.The segmentation effectively measures the nonlinear distribution of pressure gradient caused by the variation of apertures.With which,we accurately evaluated the permeability for fractures with variable apertures.The flow properties through aperture-variable fractures indicate that the non-uniformity of local apertures causes the nonlinear distribution of pressure gradient,and therefore causes the differential contribution to fluid flow.The proposed segmentation method integrating weights of local apertures can efficiently characterize the nonlinear distribution of pressure gradient and emphasize the control effect of smaller apertures on fracture permeability.The study reveals the mechanism of effects of variable apertures on the flow,and accurately evaluate the permeability of fractures with variable apertures.(3)The scale of fracture aperture and surface roughness determine the distribution of local apertures and therefore influence the flow through rough fractures.Their effects are inseparable,thus we proposed a scale parameter to characterize their integrating effects and determined the weaken effects of this parameter on fracture apertures.Using the self-affine function,we constructed a series of fractures with the same surface morphology but different vertical apertures.The scale effects of fracture aperture and surface roughness on the distribution of local apertures are analyzed with the concept of perpendicular aperture.The effects are characterized using A scale parameter,characterizing the effect,is proposed to modify the aperture of fractures.Finally,with simulation results,the scale of apertures and the local cubic law are verified.(4)Basing on the knowledge in fluid flow through 2D fractures in the previous study,we carried out the study on fluid flow through 3D rough fractures.Fluid flow through 3D rough fractures is studied using hydrological experiments with highresolution fracture models constructed using 3D printing technologies and fractal theory.The LBM simulations are verified using the physical experiments and the theoretical solution through rectangular-cross-section tubes.With the simulation results,we analyzed the effects of surface roughness on fluid flow.A modified local cubic law for 3D fractures is proposed basing on the optimized segmentation of fractures and series-parallel relationships between fracture components.3D selfaffine rough fractures are constructed by printing the fracture as two parts(the bottom part and the top part).This method solves the problem of cleaning the support materials in the printing process.Meanwhile,the problem of misalignment of the two parts is solved by using a tenon structure.Fluid flow through 3D fractures is experimented using a designed experimental setup.Experiments verified the validation of the numerical simulation method.Then,with the numerical simulation method,we analyzed the influence of flow directions on fluid flow and the main factors that affect the flow in rough fractures.An aperture correction model is proposed and it is verified based on the segmentation results,the series-parallel model,and the simulation results.There are totaly 68 figures,2 tables and 204 references in this paper. |
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