Numerical simulation of permeability heterogeneity in single- and two-phase flow systems to assess the performance of enhanced geothermal system
by Dejian Zhou
Date of Examination:2022-07-27
Date of issue:2022-11-09
Advisor:Prof. Dr. Martin Sauter
Referee:Prof. Dr. Martin Sauter
Referee:Prof. Dr. Ingrid Tomac
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EnglishThe development of Enhanced Geothermal System (EGS) technology leads to the possibility of an extensive application of geothermal energy, which is attractive because of its ability to reduce CO2 emissions and dependence on traditional fossil fuels. Different from the conventional porous geothermal reservoirs, the EGS reservoirs are located several kilometers underground and formed by artificial fractured zones and surrounding rock matrix. Due to the higher permeability and porosity, the artificial fractured zones determine the fluid flow and heat transfer in EGS reservoirs. Thus, the representation of fractured zones is crucial to investigating the multi-physical processes and energy performances in EGS reservoirs. On the other hand, working fluid is another factor affecting the EGS reservoir performance since the fluid properties, i.e., density and viscosity, play a role in reservoir pressure and heat production rate. This thesis, based on a commercial software, COMSOL Multiphysics, and an open-source research software, DuMuX, employs 1) fractured zone, 2) the discrete fracture network, and 3) the rough singe fracture to represent the natural fractures for the investigation of reservoir performances using scCO2 and H2O as working fluids. In the first part, a three-dimensional thermal-hydraulic-mechanical (THM) coupled model is established with COMSOL Multiphysics. The THM model is validated with analytical solutions and existing published results. The effects of coupled physical processes among multi-fractures are investigated. The results show that the growth of the number and spacing of fracture zone can effectively decrease the pore pressure difference between injection and abstraction wells; it also increases the production temperature at abstraction, the service lifespan, and the heat production rate of geothermal reservoirs. Then a discrete fracture model considering variable aperture fractures is presented and used to investigate the influence of fracture aperture distribution on EGS reservoir performance. The fracture apertures are randomly distributed within the networks, but constant for one single fracture. It is found that the coefficient of variation of DFN aperture strongly affects the performance of geothermal reservoir. The heat production rate and outlet temperature can be divided into three stages based on the value aperture variation coefficient. Furthermore, the average heat production rate is proportional to fracture density, but its effect is reduced by increasing the variation coefficient. Considering scCO2 as the alternative to H2O as EGS working fluid, the third part studies the possibility of using scCO2 as EGS working fluid by comparing the three specific EGS setups: 1) The combined scCO2-H2O-EGS; 2) scCO2-saturated EGS; and 3) H2O-saturated EGS. The results illustrate that the EGS using scCO2 as working fluid leads to a much considerably heat production rate and reservoir lifespan compared to H2O. In addition, H2O performs worse than scCO2 at a higher injection rate due to the considerably shorter reservoir service period. Finally, the roles of preferential flow path within the heterogeneous permeability field played on EGS performance are investigated. It is found that the preferential flow pathway significantly increases the fluid flow rate, such that the breakthroughs of scCO2 and temperature at production well are advanced. However, a larger number of preferential flow leads to a worse long-term energy performance. On the other hand, the sequestrated scCO2 mass presents an independent, but a proportional relationship with the length and number of preferential flow pathways, respectively. Furthermore, the reservoir using scCO2 as working fluid has a better energy performance than the reservoir whose working fluid is H2O.
Keywords: Geothermal energy; Permeability heterogeneity; Supercritical carbon dioxide; Discrete fracture network