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518 Chapter 5 Subsurface System Design Issues and Approaches 5.12 Longterm Reservoir Management Because no resource of commercial size has been tested for more than a few months, there are few data to make conclusions about the longterm management of an EGS resource. Flow rate per production well, temperature, and pressure drop through the reservoir and wells govern the energy that can be extracted from a well system. The nature of the fractured reservoir controls the reservoir life and the amount of heat that can ultimately be recovered from the rock volume. The pressure drop and wellhead temperature are both controlled by the nature of the fracture system, the surface area that the fluid is in contact with, and the fracture aperture and path length. While a long path length is desirable for a long reservoir life, a long path length would be likely to result in higher pressure drop. On the other hand, if there are many paths for the fluid to take, high flow rates and high fracture surface area can both be achieved with lower pressure drop, better heat exchange, and higher total heat recovered from the rock volume. The amount of temperature drop in the active reservoir that can be tolerated by any given system is largely a matter of project economics. If there is no temperature decline, then the heat is not being efficiently removed from the rock. If there is too much temperature decline, either the reservoir must be replaced by drilling and fracturing new rock volume, or the efficiency of the surface equipment will be reduced and project economics will suffer. The amount of decline in circulating fluid temperature that powergeneration equipment can tolerate is a matter of economics. While a given plant may be able to run with temperatures as much as 50% lower than the initial design temperature (in degrees Celsius), the net power output may fall below zero if there is not enough power to operate the pumps for the circulation system. With current technology, there are a number of options for operating the reservoir that might work to manage the reservoir long term. For the purpose of economics, a 10% drop in temperature means that the system can still operate without too extreme a reduction in efficiency, while extracting heat from the rock and maintaining some rock temperature for future heat mining. Sanyal and Butler (2005) use a drop of 15% in their analysis of recoverable heat from EGS systems, so a 10% drop is probably conservative. The temperature drop at East Mesa exceeded 10% in some parts of the field, and changes to the wellfield system were able to restore some of this without huge expense. This amount of temperature drop would be significant enough to trigger some intervention. Longterm reservoir management demands that models predicting longterm temperature, pressure, and fluid chemistry behavior be validated with data collected from operating the reservoir (for example, see Dash et al., 1989). These models can be used to predict the reservoir behavior prior to operation and then make changes in well pressures, flow rates, and in the reservoir fractured volume to maintain the produced fluid temperature. The circulation system consists of injection pumps, production pumps, the surface conversion system, the wells, the fractured reservoir, and the piping to move the fluid around. Each of these elements involves frictional pressure drop, which needs to be accounted for in the economics of the project because it represents an energy loss to the system. The wellbore size must be planned to accommodate the lowest pressure drop, while also controlling the cost of the well. The production wells most likely will be pumped using downhole pumps and so must be designed to accommodate the pump diameter needed for the pumps and motors – usually about 24 cm (9.5”) – and maintain a sufficientdiameter downhole to reduce pressure drop for the high flow rates.PDF Image | Subsurface System Design Issues EGS vs. Hydrothermal Pool
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