Study On Numerical Methods For Coupled Fluid Flow And Heat Transfer In Fractured Rocks Of Doublet System

Geothermal energy is a great potential for clean and sustainable energy and doublet system is the primary composition in deep geothermal energy utilization. Due to the highly heterogeneous and complexity of naturally fractured reservoir, it is valuable and necessary to study the methods of numerical simulation for fluid flow and heat transfer coupled processes in fractured rocks. In this dissertation, the characterization and modelling of fluid flow and heat transfer processes in fractured rocks is studied, such as the calculation of heat exchange between fracture water and rock matrix, the characteristic of fluid flow and heat transfer coupled processes, an efficient and accurate fractured continuum method for fractured rocks, an artificial truncated boundary for unbounded transient seepage problems and at last practical numerical simulations in doublet scale. The main achievements of this dissertation are as follows:1. Two hypotheses for the calculation of heat exchange between rock matrix and fluid in the fracture are compared. It is found that the same results will be obtained when the convective heat transfer coefficient is large enough. A line element in the plane analysis of flow and heat transfer between rock matrix and fracture is realized in the commercial finite element software COMSOL and validated by comparing with analytical solution.2. A Matlab code which is used to stochastically generate the plane fracture system according to the feature of nature fracture reservoir is implemented and the characteristic of coupled mechanism of flow and heat transfer processes is studied by discrete fracture model using the fracture line element.3. A fracture continuum method is suggested for modelling of fluid flow and heat transfer coupled processes in fractured rocks. The efficiency and accuracy of fracture continuum method is validated by comparing to the discrete fracture model. The suggested fracture continuum method is advantageous in the middle scale analysis in nature fracture reservoir such as doublet system.4. A new artificial truncated boundary approach for numerical simulation of transient seepage problems in unbounded domain is suggested. It is a time dependent flux boundary related to the time derivative of hydraulic head at the boundary which makes the implementation much easier comparing with the infinite element method. The solutions of artificial truncated boundary models agree well with analytical solutions and solutions of large mesh model of axisymmetric problems and plane problems.5. Two ways to simplify the three dimensional doublet system to a plane model is compared. As a result, cutting the three dimensional reservoir in horizontal direction is suggested. Based on the proposed fracture continuum method and the artificial truncated boundary approach, numerical simulations in doublet scale which considers the fracture distribution are realized, and the influence of the position of the wells are studied. It is illustrated that appropriate positions of geothermal wells and reasonable development strategies can be decided by solutions of numerical simulation.