| Title | Sustainable Heat Farming of Geothermal Systems: a Case Study of Heat Extraction and Thermal Recovery in a Model EGS Fractured Reservoir |
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| Authors | Daniel Sutter, Don B. Fox, Brian J. Anderson, Donald L. Koch, Philipp Rudolf von Rohr, and Jefferson W. Tester |
| Year | 2011 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Thermal drawdown, recovery, modeling |
| Abstract | To address the question of renewability of Enhanced Geothermal Systems (EGS) a conduction-dominated, model EGS reservoir was evaluated as a representative “worst case” to estimate heat extraction during production and thermal recovery following shut down. In the model system water is injected at specified rates and temperatures into a single rectangular fracture surrounded by an infinite amount of impermeable hot rock. During the extraction phase, water moves along the fracture extracting heat from the adjacent rock matrix leading to local cooling and thermal drawdown of the reservoir. When the water injection is stopped, conductive heat transfer from the surrounding hotter rock regions leads to thermal recovery of the cooler zones in the reservoir. The rate of recovery is controlled locally by the temperature gradient that is induced during the thermal drawdown. A two-dimensional mathematical model was developed to describe heat transfer for both extraction and recovery. Regarding the recovery, an advanced analytical approach was developed that is capable of describing the temperature during recovery at every position along the fracture. Our approach leads to the same result for the temperature at the inlet position, as presented in earlier research using a different approach. In addition, numerical simulations were carried out using the TOUGH2 code to study the importance of the assumptions employed in the analytical description and to extend the applicability of the model by enabling simulation of operating cycles with alternating extraction and recovery times. The effect of neglecting heat conduction in the rock in the direction parallel to the flow in the fracture was analyzed by comparison of the analytical model to the TOUGH2 simulations. For a fixed fracture area, low flow rates can result in thermal drawdown localized around the fluid inlet with heat conduction in the parallel direction becoming significant. |