Closed Loop Geothermal System Design

In contrast to other renewable sources (e.g., wind and solar) that may only be intermittently available throughout a day or year, geothermal resources provide a carbon-free and sustainable source of energy for base-load operations during the whole year with only a minor land-use requirement. Geothermal resources can be used for electricity generation, ideally combined with direct heat, which is valuable in cold seasons/areas.
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\nHot Dry Rocks (HDRs) at various depths present the largest portion of potential geothermal resources across the world. Great depths in low temperature gradient regions and the absence of sufficient natural fluid and permeability present challenges to extraction of heat from HDRs. An Enhanced Geothermal System (EGS) has been the only practical approach to the heat extraction from HDRs. However, the interaction between the injected cold fluid and hot rock mass in the stimulated fractured reservoir commonly leads to substantial challenges, uncertainties, and risks such as high-magnitude induced seismicity, circulating fluid loss, aquifer contamination, and short-circuiting. 
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\nA relatively new method, known as the Closed Loop Geothermal System (CLGS) has recently been introduced as an alternative to the EGS approach, which can significantly reduce the risks associated with EGSs. In CLGSs, the fluid is circulated in a closed-loop well system and produces an approximately constant level of heat over a long time (> 30 years); however, a rapid decline in the outlet fluid temperature from the reservoir, one of the main drawbacks of CLGSs, is putting the commercial viability of such systems in doubt. No solution has yet been presented to prevent such a rapid drop in the produced fluid temperature from CLGSs. The final outputs of the whole system, i.e., both heat and electricity generation, coupled with the reservoir system, have not been studied. Furthermore, previous studies have neglected realistic assumptions, such as reservoir evaluation with a non-constant flux and fluid temperature along the wells, non-constant ambient temperature for electricity generation, etc. 
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\nThis paper-based PhD research is dedicated to designing the CLGS for an acceptable level of electrical power production, in addition to addressing the mentioned gaps, in three main objectives. The first objective, fulfilled in Chapter 2, is to develop a semi-analytical solution for a single-well CLGS to accurately and quickly estimate the production fluid (water) temperature over time for a wide range of practical reservoir parameters without requiring computational cost. Ten reservoir characteristics (including rock and fluid properties, well length and diameter, and injection flow rate) are reduced to three dimensionless parameters, in which realistic boundary conditions are assumed in the analysis, e.g., a non-constant fluid temperature and heat flux. An axisymmetric Finite Element Method (FEM) program is developed to evaluate the production fluid temperature though transient conduction in the rock mass and conductive-convective heat transfer within and between the rock and fluid. The Response Surface Method (RSM) and power laws are used to present the relationship between output fluid temperatures and times as a function of dimensionless parameters, providing an empirical approximate solution for the mathematical model that can be used in a way similar to analytical solutions. The empirical solution has the advantages of ease and expediency of evaluations, as is often required for the optimization of systems.
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\nThe second objective, addressed in Chapter 3, is to design a water-based multilateral-horizontal-well CLGS for optimum heat and electrical power production, by coupling a FEM reservoir simulator with a surface Organic Rankine Cycle (ORC). The design parameters are the number of wells, interwell spacing, well length and diameter, injection flow rate, energy losses along the wells, and ORC working and cooling fluid mass flow rates. The study evaluates how these parameters affect the evolution of the spatial distribution of the temperature of the rock mass, the circulation fluid temperature leaving the reservoir, and the rate of electrical power generation with seasonal variations in surface air temperature. Moreover, the impacts of non-constant ambient air surface temperature and non-constant fluid temperature on electrical power generation are assessed - two variables not previously well studied together but having significant impact on system performance. 
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\nThe third objective, realized in Chapter 4, is to assess heat and electrical power production from a CLGS with supercritical CO2 (SCO2) or water circulating in a hexagonal array of horizontal wells in HDR. The analysis is based on a coupled Finite Volume Method (FVM)/FEM/ORC model, evaluating compressible temperature-dependent pipe flow of water and SCO2 in the wells and transient conductive-convective heat transfer within and between the rock and fluid, and electricity generation at a non-constant ambient air temperature. The performances of the system are evaluated for different numbers of wells and total injection and ORC mass flow rates to determine the best range of these parameters for optimum heat and electrical power production. 
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\nTo sum up the research findings, temperature evolution of the reservoir over a wide range of realistic properties is not influenced significantly by the rock and fluid properties, injection flow rate, or well length and diameter, being a function of time only for constant initial rock mass and injection fluid temperatures. Therefore, in a CLGS with a hexagonal array of horizontal wells, the distance between wells beyond which the performance of a hexagonal array of wells is practically indistinguishable from that of a single isolated well is 200 m over 30 years; i.e., the produced fluid temperature is practically unaffected by heat extraction from neighboring wells, regardless of reservoir properties. When using smaller interwell spacing (100 m), the system shows only a small impact (\\sim1%) after 30 years of heat production. The circulating fluid in CLGS experiences rapid temperature rise, decline and transition periods in less than a year before becoming approximately stable for a long time. Circulating SCO2 instead of water yields higher production temperatures but less energy content because of the lower volumetric heat capacity of SCO2 under production conditions. Both water-based and SCO2-based systems are able to generate considerable amounts of combined heat and electricity (3-4 MW) for over 30 years, which are valuable in northern Canadian communities. Water-based CLGS may be more attractive than SCO2-based system, as fewer horizontal wells are needed for the same level of power generation (lower drilling costs). Operation costs of SCO2-based systems would be reduced by thermosyphoning. The system produces the highest electrical power output during cold months and the lowest electrical output during warm ones.

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