Numerical Simulations Of Complex Fluid Flow And Heat Transfer Through Rock Fractures

Complex fluid flow through rock fractures is quite common in the projects of hydropower station construction,enhanced geothermal system,large-scale efficient oil and gas exploitation and nuclear waste repositories.The fractures dominate the conductivity of the fractured rocks,which is influenced by the geometry evolution and variable hydraulic conditions.Without a good understanding of the fracture flow through single fractures would cause serious threat to the security of the projects.Besides,the heat transport and multiphase fluid flow are often encountered in the engineering field,which would influence the efficiency of energy exploitation.However,relative studies are restricted to the macro-scale study with simplifications of single fractures,a good knowledge of the complex fracture flow and heat transport is needed.In this thesis,fluid flow through single fractures with fractal geometries was investigated.Lattice Boltzmann method was coded and applied to simulate single phase fluid flow,convective and diffusive heat transfer and two-phase flow in rough fractures.With a combination of theory modeling,numerical simulation and mechanism analysis,fluid flow through fractures with directional shear dislocations,nonlinear fracture flow under different Reynolds numbers,the convective diffusion of heat transport process in rough fractures and two-phase displacement process in fractures were investigated.Main content and findings are:(1)The programs of complex fluid flow were developed based on the lattice Boltzmann method.Specifically,for single phase fluid flow through rough fractures,the single relaxation time method and multi-relaxation time method were used to solve the full Navier-Stokes equations with complex geometries.For convective heat transfer in rough fractures,the convection and diffusion equation was solved by the lattice Boltzmann model with double distribution functions.For two-phase flow in fractures,the Shan-Chen pseudo potential model with multi-relaxation time method was used to solve the multi-phase flow problem.Tested with classical examples,the lattice Boltzmann method was proved to be effective in modeling the complex fracture flow problems.(2)Considering the effect of directional shear dislocations on fracture flow,the fractal fracture models with different shear dislocations were constructed.Single relaxation method was used to conduct fracture flow simulations with these models.The effect of fractal geometry on fracture flow was investigated by analyzing the influence of roughness,directional shear dislocations and contact ratios= The results revealed that the hydraulic conductivity decreased with the increasing fractal dimension and contact ratio.The hydraulic conductivity was dominated by the fractal roughness.When the direction of dislocation shifted from the direction parallel to the flow direction to the direction perpendicular to the flow direction,the hydraulic conductivity increased.The dependence of the conductivity on the directional shear dislocations decreased with the increasing roughness.Considering the tortuosity of preferential channels,an improved model was developed to calculate the hydraulic conductivity of fractures during the closure process.(3)For fracture flow which deviates from the linear law under relative larger velocities,the multi-relaxation method was used to conduct fracture flow simulations under different Reynolds numbers.With the distribution information of streamlines in fractures,the mechanism and influence factor of non-linear fracture flow were analyzed.Based on Forchheimer equation,a quantitative evaluation method for the non-linearity of fracture flow was derived.The non-linear feature of fracture flow in different flow directions was investigated.The results revealed that the anisotropic degree and non-linearity of fracture flow were higher for fractures with larger fractal dimensions than for fractures with smaller fractal dimensions.The relationship between the non-linear coefficient and effective hydraulic aperture was developed and a new model was proposed to calculate the critical Reynolds number.(4)With Navier-Stokes equation and convective heat transfer equation,the numerical simulation of heat transfer process in rough fractures was carried out.During the heat transfer process,the evolution of temperature for fracture flow gradually slowed down.Based on the analysis of the aperture information,flow field and temperature distribution,fluid flow and heat transfer in rough fractures were presented to be channelized.The quantification methods of the overall and local heat transfer efficiency were derived.The results showed that,the overall heat transfer efficiency increased non-linearly with the Peclet number and the distribution of the local heat transfer efficiency was influenced by the irregular geometry.The amplitude of roughness had greater influence on the heat transfer efficiency than the local fractal roughness.(5)The research showed that the lattice Boltzmann method was effective in simulating the two-phase flow process.Two-phase displacement process in fracture channels was simulated with the immiscible multicomponent lattice Boltzmann method.The formation mechanism and existence form of residual phase in rough channels were carefully analyzed.The effects of capillary number,viscosity ratio,wall wettability and irregular geometry on the two-phase displacement efficiency were systematically investigated.The results showed that the efficiency of two-phase displacement in fracture channels was controlled by the capillary number.The residual saturation increased non-linearly with the capillary number.Under the condition of the same capillary number,the residual saturation increased with amplitude and frequency of roughness.Under the condition of lower capillary number,the wettability of the wall had significant influence on the displacement effciency.When the capillary number increased to a certain value,the influence of the wall wettability can be neglected.