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Chapter 5 Subsurface System Design Issues and Approaches The economics of hydrothermal systems let us know that we need to target very high flow rates. High 54 flow rates are only possible if the reservoir has high transmissivity. However, high transmissivity can come from a single fracture with a large aperture, or from a large number of fractures with small apertures. We could have both high flow rates and low pressure drop, if a large number of fractures density changes due to boiling or by downhole pumping. The amount of possible pressure drop is controlled by the natural permeability and other properties of the rock that make up the reservoir. The permeability, or ability to conduct water to the well, may result from cracks in the rock or from connected pore spaces; but, from whatever cause, in a hydrothermal system the permeability is high. While permeability is a property of the rock only (related to the interconnectedness and size of cracks or pores), the transmissivity, which includes the crosssectional area that the fluid flows through on its way to the well, can be influenced by well design. Measured transmissivities in geothermal systems are very high (greater than 100 darcymeters is common), compared to oil and gas reservoirs with transmissivities often around 100 millidarcymeters. In an EGS system, the natural permeability is enhanced – or created when none exists – through stimulation. Stimulation can be hydraulic, through injecting fluids with or without controlling the viscosity at higher or lower rates and pressures; or chemical, by injecting acids or other chemicals that will remove the rock or the material filling the fractures. The stresses on the rocks and the elastic and thermal properties of the rocks in the potential reservoir, along with the design of the stimulation, control the extent of the enhanced or created fractures and their ultimate transmissivity. The natural rock properties and stresses on the rocks then become metrics for the formation of an EGS reservoir. with small fracture apertures gave high transmissivity. We need to understand how the type of rocks, the stresses on those rocks, and the design of the stimulation interact to develop a connected fracture system. Rocks that are critically stressed to the point where they will fail, shear, and movement during stimulation should produce fractures that will stay open and allow for fluid circulation. Rocks with at least some connected permeability through either fractures or pore spaces are more likely to result in a connected circulation system after stimulation. If there is some significant porosity in the rocks before stimulation, there will be some water stored in the reservoir for future production. Taken together, all of these metrics define the outcome of stimulation and, thus, the economics of the project. The fracture system not only needs to be connected and have high transmissivity, but it also must allow injected cool water to have sufficient residence time to contact the hot rock, so that it will be produced from the production wells at or close to the formation temperature. The amount of temperature drop in the production fluid that can be tolerated by different power plant equipment then will determine the life of that part of the reservoir for a particular conversion technology. Under given flow conditions, the longer the life of the reservoir, the better the economics. At the best sites for developing a reservoir, the rocks will be stressed so that when they are stimulated, open fractures will be created. However, if there is too much preexisting permeability or if the stimulation produces a preferred pathway of very open cracks that the injected fluid can take to the production wells, the created or enhanced fractures may allow water to move too quickly, or short circuit, from the injection wells to the production wells without heating up enough to be economic.PDF Image | Subsurface System Design Issues EGS vs. Hydrothermal Pool
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