Experimental and Numerical Study of Heat Flow under Low-Enthalpy Hydrothermal Conditions

Abstract

Energy and its management and environmental impact constitute one of the most important issues in the 21st century. Since fossil fuels are environmentally hazardous and sooner or later are going to be depleted, there is a pressing need for alternatives. Renewable energies, such as solar, wind and geothermal energy are vital sources of energy that are clean and abundantly available. Wind and solar energy sources, in spite of their several advantages, are naturally intermittent. They might not be available at times of peak energy demands and abundant at times of no demand. On the contrary, geothermal energy is available at all times. This makes geothermal energy sources a plausible alternative to fossil fuels. Several types of geothermal energy sources are available, including high, intermediate, and low-enthalpy which have different applications. In countries with low thermal gradients and relatively high permeable aquifers, such as the Netherlands, geothermal energy can be used for space heating using hydrothermal heating plants. A prerequisite to safe, economic and viable geothermal systems is a good understanding of the geology and the physical processes in the sub-surface. In a hydrothermal system, heat conduction and convection takes place in a rather highly disproportionate geometry. This combination of physical processes and geometry make numerical analysis of such a system complicated and resource-consuming. Hence, in developing numerical tools for geothermal systems, important efforts are devoted to tackling the discretization of two main issues: geometry and heat convection. Deep geothermal systems consist of very slender boreholes embedded in a large soil mass. This geometrical peculiarity exerts an enormous computational burden, as a combination of very fine elements (cells) and coarse elements (cells) is normally needed to discretize the physical domain. For three-dimensional systems, this normally requires hundreds of thousands to millions of elements, necessitating parallel computing using multiple processor computers and making the CPU times unrealistic for engineering practice. Additionally, heat flow in a hydrothermal system involves density and viscosity variation with temperature, and thermal dispersion. These phenomena make the problem non-linear and must be well understood and taken into consideration in optimizing a geothermal system. In this thesis, these physical and geometrical issues have been studied experimentally and numerically. The objectives of this thesis are: 1. To investigate the variation of the formation fluid density and viscosity, with temperatures typically existing in hydrothermal conditions. 2. To investigate thermal dispersion due to heat flow in a porous domain. 3. To establish a discretization technique that covers all important features of the hydrothermal system geometry and physical processes, and, at the same time, is computationally efficient such that it can be run on a normal PC (500 MHz, 4GB RAM). 4. To formulate a prototype model for a preliminary estimation of the reservoir lifetime by knowing its porosity and initial temperature for different design parameters, namely, discharge, well spacing and injection temperature. The outcome of the experimental-numerical study in this thesis emphasizes the significance of several manmade and physical parameters on the system lifetime. In conducting a viable design of a hydrothermal system, these parameters need to be carefully evaluated. The proposed prototype model can be utilized in the preliminary phases of a project, from which the project lifetime and consequently the cost and the amount of the extracted energy, can be estimated.