Heat transfer investigations for optimal harnessing of enhanced geothermal systems
Keywords: Energy Resources Engineering.

Enhanced Geothermal Systems (EGS) offer the opportunity of exploiting the vast energy resources contained in hot impermeable rocks. In such rocks, the natural flow capacity of the system may not be sufficient to support adequate geothermal applications until it is enhanced by opening up existing fractures and propagating new fractures. Cold fluid is injected into the reservoir to exploit the energy resource, whose permeability has been enhanced. The increased permeability allows the fluid to circulate through the opened fractures to production or extraction well(s), thereby capturing and transporting the heat contained in the hot impermeable rock for power generation. Accurate prediction of the thermal performance of EGS depends on an understanding of how the heat transport is affected by the presence of the fracture(s) -- the primary flow conduit of EGS. These fractures may have aperture variability that could create channels and alter flow paths, affecting the availability of surface area for heat transfer. The overall goal of this study was to understand the fracture topology, investigate how it can impact flow and heat transport, and demonstrate ways Enhanced Geothermal Systems can be harnessed to optimize thermal performance. To achieve the goal of this study, a systematic fracture characterization approach was used, and numerical simulation models were used to study the physical processes that govern the interaction between the fluid and the rock during heat extraction from Enhanced Geothermal Systems. Using variogram modeling and Sequential Gaussian Simulation method, fracture apertures representing actual fractures were generated for lab-scale and field-scale investigations. Fracture characterization metrics such as the Joint Roughness Coefficient (JRC) and Hurst exponent were used in analyzing the data. Geometric anisotropy was a vital character of the generated fracture aperture distributions, which was seen to originate from the process of shearing or slip. Flow and heat transport relative to the direction of fracture shear was studied, with the perpendicular flow configuration being perpendicular to the direction of fracture shear. In contrast, the parallel flow configuration had flow in the same direction as the fracture shear direction. It was demonstrated in this study that the flow wetted surface area had a direct and significant contribution to the amount of heat extracted. For the lab-scale fractures, the JRC confirmed geometric anisotropy of the fracture aperture and was seen to have a direct correlation with the flow contact area. The lower the difference in JRC values between the perpendicular and parallel flow configurations, the more flow contact area expected in the perpendicular flow direction, which will lead to more heat extracted from the rock. From the variogram model parameters, it was deduced that high geometric anisotropy results in high differences in thermal drawdown and consequently a high difference in energy extracted. The thermal performance appeared to be better in the perpendicular flow configuration with a ratio of 70:30 for the lab-scale fractures. For the field-scale fractures, it was seen that most of the fracture aperture distributions with a geometric anisotropy ratio of 2 had Hurst exponents of fracture surface aperture variability found in nature. For all the fracture aperture distributions analyzed for the field scale, the perpendicular flow configuration resulted in better thermal performance than the parallel flow configuration with a ratio of 58:42. Furthermore, for the geometric anisotropy ratio of 2, the ratio was 70:30. The perpendicular flow configuration had the injected fluid move through tortuous flow paths. These tortuous flow paths contributed to more fracture surface area being contacted by the flowing fluid, leading to an improved thermal performance in that flow configuration. Throughout this study, temperature-dependent viscosity was used. However, a section of this study investigated the impact of using a constant viscosity in the thermohydraulic model. It was seen that for fractures with smooth, uniform apertures, for all temperature ranges and at the operating conditions being modeled, there was no significant difference between using a constant viscosity or a temperature-dependent viscosity in modeling an Enhanced Geothermal System. However, for fractures with spatial variations, it was determined that modeling with a temperature-dependent viscosity was necessary, especially for systems with high differences in reservoir and injection temperatures, and for fractures with high correlation lengths. The impact of thermal stresses on heat extraction was also investigated. An analog Enhanced Geothermal System, the Altona Field Laboratory, was also studied for thermo-mechanical influences. It was found out that the injection of hot water into the cold rock resulted in thermal stress generation and reduction in the aperture but did not cause significant changes to the temperature profile due to the small volumetric flow rate through the system. Also, anisotropic aperture distributions were studied to determine the impact of thermoelasticity on the heat extraction of Enhanced Geothermal Systems. It was shown that when thermoelasticity is taken into consideration, the thermal drawdown could either be improved or deteriorated depending on the nature of the aperture distribution. The impact of fracture aperture variability was investigated for Enhanced Geothermal Systems using supercritical CO2 as working fluids. It was established that CO2 as an EGS working fluid would result in better heat extracted from the system if the fractures are considered smooth, which agrees with related studies. However, where there is spatial variation in the fracture aperture, channeling could be detrimental to CO2, especially at high fracture correlation lengths and high mass flow rates, due to the high mobility of CO2. The following are the main contributions from this study. First, it has been demonstrated that heat transport is affected by the geometric anisotropy of fracture surfaces. It was determined that in most cases, flowing perpendicular to the direction of shear or slip results in more heat extracted due to more contact of the fluid with the rock while moving through tortuous flow paths. Secondly, the conditions under which a constant viscosity can be used in modeling EGS were determined. If the fractures are known to be smooth, have low correlation lengths, or have distributed surface areas, a constant viscosity can be used in the model, especially if the difference between the reservoir temperature and the injection water temperature is small. However, for anisotropic fracture surfaces, surfaces with high correlations lengths or high tortuosity, and when the difference between the reservoir temperature and injection water temperature is large, the use of constant viscosity could result in significant computational errors from the actual. Thirdly, it has been shown that thermal drawdown could either be improved or deteriorate when thermoelasticity is considered. This finding is different from studies previous studies that have looked into coupling thermohydromechanical processes for fractures with spatial variations and suggests that Enhanced Geothermal Systems may benefit from thermal stimulation. Finally, this work shows the first comparison between CO2 and water at a field scale considering fracture aperture variability. Recommended future work includes modeling of vertical fractures with spatial variations in fracture aperture to investigate how convection may impact the current findings; considering multiple fractures with spatial variations in the fracture aperture; considering non-Darcy flow in the simulation models; coupling geomechanics with the study of CO2 on fractures with spatial variations, and developing proxy models that are quicker to perform the thermohydraulic and thermohydromechanical simulations