| Hot Dry Rock(HDR),as a significant source of clean and renewable energy within the Earth’s crust,holds potential for optimizing China’s energy composition and reducing greenhouse gas emissions.Enhanced Geothermal Systems(EGS)are the most effective method to harness HDR,with reservoir modification posing the key technical challenge.This modification involves a complex thermo-hydro-mechanical(THM)coupling process,initiated by injecting cold water into deep underground hot fractured rock masses to induce shear slip in existing fractures.The production process,including cold water injection,circulation,and extraction,raises multiple issues: thermal shock in high-temperature crystalline rocks,the mechanical behavior of thermal-induced fractured rock mass,the characteristic of flow and heat transfer in fracture,the impact of shear displacement on heat-flow migration,and the behavior of heat transfer in discrete fracture network under stress.Addressing these aspects,this study combines experimental,theoretical,and numerical simulation approaches to explore the mechanical response,permeability characteristics,and heat transfer evolution in fractured rock masses under THM coupling.This research offers valuable insights for the design and safety of deep geothermal energy development.The main works of the study are as follows:(1)A thermo-mechanical(TM)coupling model at the mineral scale for thermal-treated rocks is established.It clarifies the formation mechanism of thermal cracks in granite under thermal treatment,and reveals the patterns of mechanical property degradation and failure mode transition in granite under thermal treatment.Triaxial compression mechanical tests are conducted on granite after different temperature heating and cooling treatments.Meanwhile,adopting the finite discrete element method(FDEM)and grain-based Model(GBM),we simulate the formation process of thermal cracks in granite following heat treatment and the triaxial mechanical behavior of heated granite.The process of thermal crack formation in rocks under thermal treatment is described at the mineral scale,and the results of laboratory experiments is generally well in agreement with the numerical simulations.The results show that the formation of thermal cracks in granite under high temperature is mainly due to the differential thermal expansion coefficients of different mineral particles,with the temperature threshold for thermal crack formation being between 500°C and 550°C.The formation of thermal cracks creates internal defects in the rock,leading to the degradation of its compressive strength and elastic modulus.When the thermal treatment temperature of granite exceeds 500°C,both the peak strength and elastic modulus of the granite significantly decrease.Under uniaxial conditions,the mechanical failure mode of granite treated above 500°C shifts from splitting to conjugate shear failure.The results also show that when the thermal treatment temperature exceeds 1000°C,the triaxial stress-strain curve of granite after cooling exhibits ideal elastic-plastic characteristics.(2)A method for thermomechanical coupled shearing of rock fractures under constant normal stiffness(CNS)and constant normal load(CNL)mechanical boundary conditions is presented.It provides insight into the mechanical mechanism behind thermal-induced fracture shear slip,and quantitatively describes the evolution of fracture aperture throughout the slip process.Based on the hard contact and ideal elastic-plastic models,a simulation study of thermal-induced fracture shear slip under Constant Normal Stiffness(CNS)and Constant Normal Load(CNL)mechanical boundary conditions is conducted,exploring the influence of fracture roughness on this process.The correctness of the simulation method is validated through comparison with the laboratory experiment data from DECOVALEX-2023 Task-G.The study reveals the mechanism of thermal-induced fracture shear slip,analyzes the effect of mechanical boundaries and roughness on the mechanical response of thermal-induced fracture shear slip,and quantitatively describes the characteristics of fracture aperture evolution during the shear slip process.The analysis of the Mohr circle changes on the fracture surface during heat extraction and heat recovery processes shows that during thermal extraction,the injection of lowtemperature fluid into fractures causes thermal contraction of the rock mass,reducing the normal stress and increasing the shear stress on the fracture surface.During thermal recovery,thermal expansion of the rock mass enhances the normal and shear stresses on the fracture surface.These processes ultimately lead to the reactivation and slip of the fracture.The results also illustrate that increased normal stiffness inhibits fracture slip behavior,while increased vertical stress promotes continuous slip of the fracture.Additionally,both the shear and normal displacements of the fracture increase with the increase in fracture roughness.Moreover,plastic deformation of the fracture surface cannot be ignored in the analysis of shear slip,as it can lead to a significant underestimation of fracture closure during shear slip,with errors up to 46%.(3)An analytical model focusing on flow and temperature distribution in single fracture,incorporating contact factors,is developed.It delves into the effects of contact area and its spatial distribution on the macroscopic flow behavior and heat transfer characteristics in fractures.A three-dimensional parallel plate fracture model,incorporating contact bodies,is developed.Using the finite element method(FEM),the flow and heat transfer process in these three-dimensional parallel plates is analyzed.The study specifically examined how the contact area and its spatial distribution affect the seepage heat transfer process.To investigate this,uniform and normal distribution patterns for the spatial distribution of contact bodies are used.The results illustrate that,given the same contact area,the spatial distribution pattern of contact bodies critically influences the formation of flow paths in the fracture.Notably,the uniform distribution pattern yields smoother flow paths,enhancing both permeability and heat transfer efficiency compared to the normal distribution.Furthermore,this study involves modifying existing temperature analytical models to develop a new analytical model for the temperature distribution in single fracture,factoring in contact elements.This model accounts for the effects of contact area and seepage path on the seepage heat transfer process.Its predictive results closely align with numerical solutions,affirming its accuracy and utility in this domain.(4)Evolution of heat transfer coefficients in fractures under shear displacement is quantitatively evaluated.Using numerical models,direct shear tests on rough fractures is simulated,revealing the evolution of fracture surface morphology,aperture,and contact area under direct shear.On this basis,combining with flow and heat transfer model,the effect of shear displacement on the heat transfer coefficients of fractures is quantitatively evaluated.Based on hard contact and ideal elastic-plastic models,simulation studies of direct shear tests on three-dimensional rough fractures under different mechanical conditions are carried out.The correctness of the simulation method is validated by comparison with direct shear test data from the POST project of KTH Royal Institute of Technology.Subsequently,the finite element method(FEM)is used to solve Navier-Stokes and heat transfer equations to simulate the flow and heat transfer processes in fractures after shearing.The study describes the direct shear behavior of rough fractures,revealing the evolution of surface roughness,aperture,and contact area of the fracture during direct shearing.It analyzes the impact of shear displacement on the evolution of flow and heat transfer in rough fractures,and quantitatively characterized the evolution of heat transfer coefficients in fractures under shear.Results show that increased normal stress leads to the degradation of fracture surface height and an increase in the contact area,resulting in a decrease in fracture aperture.However,the degradation of fracture surface and aperture increase with the increase in shear displacement,while the contact area decreases.Meanwhile,the heat transfer coefficient increases with the increase in normal stress and Peclet number but decreases with increasing shear displacement.Moreover,the plastic deformation of the fracture surface during shearing significantly affects the flow and heat transfer process;neglecting the plastic deformation of the fracture surface can lead to an underestimation of heat transfer efficiency,with errors up to 40%.(5)A thermo-hydro-mechanical(THM)coupling method is developed to investigate the flow and heat transfer process in fracture network.It elucidates the effect of in-situ stress on the permeability and heat transfer process of fracture network,and provides a quantitative assessment of heat extraction efficiency in fracture networks under various in-situ stress conditions.Based on the rigid-body-spring-method(RBSM)and the discrete fracture network(DFN),simulation studies are conducted to investigate the effect of different in-situ stress conditions on the permeability of fracture networks.Building on this,the finite element method(FEM)is adopted to reveal the evolution of anisotropic flow-heat migration in fracture networks under in-situ stress.This study considers the nonlinear deformation of fractures under stress and the shear dilation effect,describes the distribution characteristics of anisotropic fracture aperture in fracture networks under in-situ stress,and illustrates the evolution of heat transfer processes in fracture networks under stress.It evaluates the heat extraction efficiency of fracture networks under in-situ stress.The results show that when the ratio of horizontal to vertical stress is not sufficient to cause shear dilation effects under in-situ stress,the nonlinear normal deformation of fractures is the primary factor affecting flow and heat transfer.The reduction in fracture aperture occurs uniformly across the entire model area,reducing the heat extraction efficiency of the fracture network.As the stress ratio increases,the shear dilation effect gradually becomes the dominant mechanism,with a few fractures almost parallel to the direction of the maximum principal stress increasing in aperture,enhancing the heat extraction performance. |
PDF Full Text Request