An efficient computational model for deep low-enthalpy geothermal systems

In this paper, a computationally efficient finite element model for transient heat and fluid flow in a deep low-enthalpy geothermal system is formulated. Emphasis is placed on coupling between the involved wellbores and a soil mass, represented by a geothermal reservoir and a surrounding soil. The finite element package COMSOL is utilized as a framework for implementing the model. Two main aspects have contributed to the computational efficiency and accuracy: the wellbore model, and the 1D-2D coupling of COMSOL. In the first aspect, heat flow in the wellbore is modelled as pseudo three-dimensional conductive-convective, using a one-dimensional element. In this model, thermal interactions between the wellbore components are included in the mathematical model, alleviating the need for typical 3D spatial discretization, and thus reducing the mesh size significantly. In the second aspect, heat flow in the soil mass is coupled to the heat flow in the wellbores, giving accurate description of heat loss and gain along the pathway of the injected and produced fluid. Heat flow in the geothermal reservoir, and due to dependency of fluid density and viscosity on temperature, is simulated as two-dimensional fully saturated nonlinear conductive-convective, whereas in the surrounding soil, heat flow is simulated as linear conductive. Numerical and parametric examples describing the computational capabilities of the model and its suitability for utilization in engineering practice are presented.