Electricity generation in an Enhanced Geothermal System (EGS) attracted more and more the researchers’ attention since the past fifty years. EGS consists of extracting thermal energy from Hot Dry Rock by injecting water under high pressure in an artificially created reservoir for electricity production. Deep geothermal energy is a sustainable, clean and renewable energy. Geothermal energy is also an alternative to fossil fuels. The different processes modeling in EGS is a very complex task and it requires a lot of research, numerical developments and large computing times. In this thesis, several models were developed for more realism. The geothermal reservoir considered is anisotropic in permeability, heterogeneous and water loss is taken into account. The profitability of the EGS depends on several factors, the analysis of which is very important in order to improve the system performance. Several factors studied in this thesis influence the performance and the profitability of the system: the reservoir heterogeneity of EGS, the inevitable water losses in the reservoir towards the surrounding formation, the well spacing, the fracture aperture, the well inclination, the direction of regional flow, the layout of the injection and production wells when several them are used for optimal electricity generation, the selection of the secondary fluid in the Organic Rankine cycle (ORC) coupled to the reservoir and the strategies of injecting the fluid into the reservoir and its recovery which requires in-depth analysis.
In previous studies, the injection of the fluid into the EGS reservoir is based on experiences from the oil and gas industry. The injection strategy is a key factor in the success or failure of an EGS. It is important to explore all possibilities for injecting and recovering the fluid. The analysis of injection and recovery strategies avoids premature thermal breakthrough which can shorten the reservoir lifespan. The sustainability and profitability of an EGS is of great concern to interest governments and investors. It is therefore important to provide the industry with improved management of fluid injection strategies in the EGS reservoir in order to maximize the heat mining. It is for this reason that the first part of this thesis is focused on six scenarios in order to study the injection and recovery strategies of the fluid with discrete fractures model over 40 years. Under the conditions and all parameters considered in this thesis project, the results showed that the best scenario is the first one with effective electric power of 9.65 MWe. In this scenario, the production temperature increases from 167.9 °C to a maximum value of 170.4 °C, then decreases as the injection continues and it reaches 166 °C at the end of production. The impedance of the reservoir flow is approximately 0.22 MPa/(kg/s). A sensitivity analysis was carried out by studying the influence of the fracture aperture, the fracture thermal conductivity and the distance between fractures on the production temperature and the effective electric power. For the fracture aperture values greater than 0.1 mm, there is a reduction in the production temperature, the effective electric power and the thermal breakthrough time. A distance between hydraulic fractures of 25 m gives interesting results and this value is highly recommended. The results obtained strongly depend on the physical properties of the geological formation and the fluid thermodynamic properties. The methodology adopted in this study can be implemented for any EGS reservoir.
In an EGS reservoir, the pre-existence of natural fractures, small cracks, and the variability of petrophysical properties make the reservoir strongly heterogeneous. The pre-existence of small cracks and the injection of the fluid under high pressure cause inevitable water losses to the surrounding formation. These two phenomena (heterogeneity and water losses) are difficult to quantify and they make modeling complex. The variability of the permeability in all directions means that the reservoir is anisotropic in permeability. Thus, to evaluate the electricity production potential of EGS, considering the permeability anisotropy, the reservoir heterogeneity and water losses are essential for real sustainable production. In this second part of this thesis project, the reservoir heterogeneity, the inevitable water losses and the anisotropy in permeability are considered. A sensitivity analysis over a 40-year period was carried out in order to quantify the effects of water losses and the reservoir heterogeneity during the whole process. The impact of the well inclination angle and the distance between the injection and production wells on the production temperature, the effective electrical power was also studied. By neglecting water losses in EGS modeling, the system performance is overestimated and the system lifespan may arrive rather than expected. Assuming that the reservoir is homogeneous when in fact it is heterogeneous, the performance of the system is overestimated which makes the prediction doubtful. In order to quantify the effect of the reservoir heterogeneity on the performance of the system, three heterogeneous cases (H1, H2, H3) and one homogeneous case (H4) were adopted. A combination of reservoir layers was proposed and the petrophysical properties are calculated based on weighted average. Seven (7) comparison criteria were established: the production temperature (comparison of its variation throughout the extraction process), the production temperature drop at the production well which should be between 5% and 15%, the relative difference between the average effective electric power in the homogeneous case and that obtained in the three different heterogeneous cases studied, the production pressure, the reservoir flow impedance which is the pressure drop required to circulate 1 kg of water from the injection well to the production well, the average effective electric power and the heat extraction ratio. Seven (7) comparison criteria were established: the production temperature (comparison of its variation throughout the extraction process), the production temperature drop at the production well which should be between 5% and 15%, the relative difference between the average effective electric power in the homogeneous case and that obtained in the three different heterogeneous cases studied, the production pressure, the reservoir flow impedance which is the pressure drop required to circulate 1 kg of water from the injection well to the production well, the average effective electric power and the heat extraction ratio. Of all the criteria established, only the heat extraction ratio makes it possible to determine the level of heterogeneity of the reservoir. The heat extraction ratio varies from 0 to 39.4%, 0 to 41.1%, 0 to 42.9% and 0 to 45.3% for cases H1, H2, H3 and H4, respectively. It is found that the case H1 is more heterogeneous. The inclination of the well has little influence on the production temperature. The higher effective electric power is obtained in the case of vertical wells in which the effective electric power increases from 9.47 MWe to 9.83 MWe in 19.3 years and decreases from 9.83 MWe to 9.20 MWe. The greater the distance between the injection and production wells, the larger the reservoir must be, and the water losses become significant. The maximum effective electric power is obtained for vertical wells with a well spacing of 540 m, the thermal breakthrough time is 25.6 years and the average effective electric power is 9.7 MWe. The thermal breakthrough time increases with increasing the well spacing of the well. The increase in water loss from 0% to 10% for a well spacing of 500 m results in a reduction in the energy efficiency of the improved geothermal system from 2.4- 2.2 to 2.3- 2.1. The direction of the regional flow from the production well to the injection well improves the EGS performance.
The third part of this thesis project addresses the possibility of using several injection and production wells to maximize heat extraction from the reservoir. The layout of the injection and production wells plays a crucial role in the assessment of the electricity generation potential by considering the reservoir poroelasticity. In this third part of the project thesis, a design and an optimal choice of several injection and production wells were implemented. Nine (9) different wells configurations were designed for the heat mining considering for reference the conventional doublet system with one injection well and one production well (case 1), one injection well and two production wells (case 2), one injection well and three production wells (case 3), three aligned injection wells and one production well (case 4), three aligned injection wells and two aligned production wells (case 5), three aligned injection wells and three aligned production wells (case 6), two aligned injection wells and one production well (case 7), two aligned injection wells and two aligned production wells (case 8), two aligned injection wells and three aligned production wells (case 9). All these configurations have been studied in order to extract a sufficient amount of energy and allow the fluid to reach a larger volume in the heterogeneous reservoir. From the simulated results, it is found that a five-spot well layout with three injection wells and two production wells, described in case 5, provides the best EGS performance. Increasing the injection fluid flow rate from 75 l/s to 125 l/s with an injection temperature of 60 °C leads to a decrease of the electric power from 10.40 – 9.40 MWe to 6.84 – 4.53 MWe because higher the volume flow rate, greater the water losses. The average production temperature after a 40-year period is 164.8 °C and reduced by 3.1%, which is acceptable. The injected fluid temperature and the hydraulic gradient have a low influence on the production temperature and the heat extraction ratio. The effect of the hydraulic gradient on the produced fluid temperature is noticeable for a hydraulic gradient value greater than 10 mm/m.
The fourth part of this thesis project is reserved for the selection of the appropriate secondary fluid for the production of electricity by coupling the geothermal power plant and the reservoir over 40 years of operation time. Twenty ORC working fluids were considered in order to select the best one based on the energy efficiency and the exergy efficiency. The results show the working fluids have a significant effect on the reinjection geofluid temperature and slight effect on the production temperature. The reinjection temperature varies from 56.5 °C to 108.4 °C depending on the ORC working fluid used. The best performing working fluid was R1233zd(E), a hydrofluoroolefin refrigerant, non-flammable with zero impact on ozone depletion and very low Global Warming Potential. Under the operating conditions of the EGS reservoir, a combination of a maximum turbine inlet pressure of 1.6 MPa, a maximum refrigerant mass flow rate of 50 kg/s and a condensing temperature of 25 °C prevents droplets formation in the turbine. This combination gives better performance with an energy efficiency of 15.8 –16.0%, an exergy efficiency of 43.5 – 44.6%, a turbine inlet temperature of 121.3 – 156.3 °C, a net electric power of 1.97 – 2.32 MWe. The energy efficiency and exergy efficiency decrease with increasing condenser temperature.
| Date | 13 Nov 2020 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Louis Lamarche (Supervisor) |
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