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Chapter 5 Subsurface System Design Issues and Approaches 5.1 Scope and Approach: EGS vs. Hydrothermal Reservoirs 5.2 Formation Characteristics – Porosity, Permeability, Fracture Distribution, Density and Orientation, and Connectivity 53 Geothermal electric power and heat production from hydrothermal resources has been commercialized since 1904, leading to a large body of experience on what constitutes a good hot water resource. In terms of thermal energy, a kilogram of hot water at temperatures of 150°C to 300°C has a low energy content compared to a kilogram of hydrocarbon liquid. This occurs because only the sensible and latent enthalpy of the geofluid can be used, rather than the stored chemical energy released during combustion of a hydrocarbon fuel. Therefore, for a producing geothermal well to be comparable in energy content to an oil well, high mass flow rates of hot water are needed. Typically, 50 to 150 kg/s or more per production well, depending on its temperature, are required to make a geothermal project economical. Resource temperature and flow per well are the primary factors in defining the economics of a geothermal resource. The increasing cost of drilling deeper wells trades off against the increased thermodynamic efficiency of higher temperature. Eventually, an Enhanced Geothermal System (EGS) will reach an optimum depth after which drilling deeper wells will not be more economical. However, studies by Tester and Herzog (1991) have shown that the optimal depth for minimum costs is on a fairly flat costversusdepth surface for most geothermal gradients. The insensitivity of project cost to depth, in the neighborhood of the optimal point, permits a range of economically acceptable depths. Hydrothermal projects are based on resources with naturally high well productivity and high temperatures. They rely on having high flow per well to compensate for the capital cost of drilling and completing the system at depth, and they need very high permeability to meet required production and injection flow rates. Typically, in a successful hydrothermal reservoir, wells produce 5 MW or more of net electric power through a combination of temperature and flow rate (see Chapter 7). For instance, a well in a shallow hydrothermal reservoir producing water at 150°C would need to flow at about 125 kg/s (2,000 gpm) to generate about 4.7 MW of net electric power to the grid. Thus, as a starting target for EGS, we assume that the fluid temperature and production flowrate ranges will need to emulate those in existing hydrothermal systems. A number of resourcerelated properties – temperature gradient, natural porosity and permeability of the rock, rock physical properties, stresses in the rock, water stored in the rock, and susceptibility to seismicity – control the amount of the heat resource in the earth’s crust that can be extracted. These factors, taken together, not only control the physical process of extracting the heat, but also ultimately play a major role in determining the economics of producing energy (see Chapter 9 for details). In the example above, to determine the economics of the hydrothermal project, the well depth, the temperature, and the flow rate need to be defined. While it is clear that the flow rate of the fluid and its temperature control the rate of energy produced, it is not evident what controls the reservoir production rate. In a natural system, wells flow due to pressure drop at the well, caused either byPDF Image | Subsurface System Design Issues EGS vs. Hydrothermal Pool
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