Enhanced Geothermal Systems : Modelling Heat and Mass Transfer in Fractured Crystalline Rock

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Title: Enhanced Geothermal Systems : Modelling Heat and Mass Transfer in Fractured Crystalline Rock
Author: Piipponen, Kaiu
Other contributor: Helsingin yliopisto, Matemaattis-luonnontieteellinen tiedekunta, Fysiikan laitos
University of Helsinki, Faculty of Science, Department of Physics
Helsingfors universitet, Matematisk-naturvetenskapliga fakulteten, Institutionen för fysik
Publisher: Helsingfors universitet
Date: 2017
Language: eng
URI: http://urn.fi/URN:NBN:fi-fe2017112252532
Thesis level: master's thesis
Discipline: Geophysics
Abstract: Geothermal energy is a growing industry and with Enhanced Geothermal System (EGS) technology it is possible to utilize geothermal energy in low heat flow areas. The ongoing EGS project in Southern Finland provides a great opportunity to learn and explore EGS technologies in a complex environment: hard crystalline rock, high pressure and low hydraulic permeability. This work describes physics behind an EGS plant, as well as basic concept of EGS, give examples of some existing plants and make calculations of how much power a plant in Finland can produce. In order to plan and build a successful plant, suitable parameters for the system are determined by modelling. The modelling is done analytically and numerically. Physical properties governing the EGS models are conductive and convective heat transfer and rock hydraulic properties that allow fluid flow. Hydraulic permeability is discussed in detail, because it is the key parameter in EGS: rock is stimulated in order to enhance permeability in order to make fluid flow possible through interconnected fractures. It is a spatially correlated parameter and it is distributed lognormally making fluid flow highly channelled. Modelling of heat and mass transfer aims to parametrize an EGS plant in the conditions of Southern Finland. The parameters governing heat transfer with fluid flowing in the geothermal reservoir are size of the reservoir and fluid velocity, which depends on matrix permeability. The larger the reservoir the more hot contact area fluid encounters and the better it heats up, the slower the flow, the longer time fluid stays in the reservoir and therefore heats up more. High flow rates cool the reservoir rapidly. However, a large reservoir is difficult to achieve, maintaining enhanced permeability requires relatively high fluid flow rates and the higher the flow rate, the more power the plant produces, so slow flow is not economically feasible. Analytical models are done with Matlab and numerical models are done with finite-element software COMSOL Multiphysics. Numerical models benchmark the analytical solutions and use spatially correlated permeability to modify fluid flow pattern and see how temperature in the reservoir changes with changes in fluid flow. The results show that creating large reservoir that could operate for 20 years with desired power production is unrealistic. Total output fluid flow required to produce over 1 MW of power is 10 kg/s. At such rate there is a risk that the reservoir cools and output fluid temperature is not sufficient for power production. In case of heterogeneous permeability connectivity of the reservoir is not as good as in case of homogeneous permeability and there is a risk that total fluid flow in the reservoir is slower and therefore less power produced.

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