Numerical Analysis Of Coupled Fluid Flow And Heat Transfer At Multiple Scales In Deep Geothermal Systems

Geothermal energy,as a kind of renewable energy,shows broad prospects with the invention and development of doublet system and hot dry rock system.Due to the complexity and uncertainty of the rock mass in geothermal reservoir,it is a challenge work to simulate the fluid flow and heat transfer coupled processes therein.In this dissertation,seepage and heat transfer processes at various scales in fractured rock mass are studied systematically,such as analytical solution of seepage and heat transfer processes for single fracture conceptual model,the role of fracture surface roughness in fractured rocks,fracture continuum method?FCM?in fractured networks,artificial boundary condition for unbounded problems,and tracer test-based analytical and numerical models.The main achievements of this dissertation are as follows:1.Based on the existing conceptual models and their analytical solutions in a two-dimensional domain,a new conceptual model characterizing thermal exchange between a borehole and ambient rock matrix is proposed.The analytical solution in Laplace space is derived by using integral-transformation method.After implementing Laplace inverse transformation with exponential sum approximation,the approximate solution is compared with the results of the numerical model,which proves the superiority of the solution.All these models and solutions can be devoted to thermal production evaluation in geothermal energy extraction engineering.2.Based on two empirical models relating hydraulic apertures to mechanical apertures and two distributions of Joint Roughness Coefficient?JRC?,the effects of local surface roughness of fractures on fluid flow and heat transfer processes at the macroscopic scales of the fracture networks are assessed.The results show that fracture surface roughness can affect the fluid flow and heat transfer processes in fracture networks to various extent,mainly depending on the empirical models of mechanical-hydraulic apertures.In other words,the role of fracture surface roughness in macroscopic fluid flow and heat transfer in fractured rocks is critical,when using a model of mechanical-hydraulic apertures that predicts significant reduced hydraulic apertures.Discrete fracture networks models with the normal distribution of JRC are less permeable than those with the lognormal distribution of JRC,using the fitting parameters of in-situ JRC data.3.A new fracture continuum model is proposed to simplify the discrete fracture model,which retains the heterogeneity of the original fracture network model.When the ratio of the rock matrix permeability and the fracture permeability is relatively large,the hydraulic interaction between the rock matrix and fracture network is considered and the above-mentioned FCM is corrected accordingly.4.A local artificial boundary condition?ABC?is proposed for numerical analyses of seepage and heat transfer processes in unbounded domain.The basic idea of the artificial boundary is to find the relationship between the Dirichlet and Neumann conditions on the truncated boundary.By using Laplace integral transformation,the function in Laplace space with respect to time is approximated numerically and high accuracy is achieved.Stability analysis,approximation order,and computational efficiency are also discussed.5.According to particle tracking method and the analytical solution of one dimensional advection-diffusion equation,the analytical solution of the tracer concentration at the extraction well is proposed.Based on the in-situ tracer test results,the simplified geometry model of geothermal reservoir is established,which can indicate the existence of the preferential seepage channel of the reservoir.Meanwhile,the effect of the heterogeneity of the permeability field is carefully investigated.The numerical models indicate that the correlation length of the permeability field can largely influence the randomness of the flow and heat transfer processes.The numerical method shows theoretical and practical value since similar work can be applied in many applications,such as oil exploration,underground nuclear waste repositories,and CO₂ sequestration.