| Title | A Heat Extraction Prediction for Multiple Fractures in a Closed-loop Circulation Enhanced Geothermal System |
|---|---|
| Authors | Bisheng WU, Andrew P. BUNGER, Xi ZHANG, Robert G. JEFFREY and Cameron HUDDLESTONE-HOLMES |
| Year | 2014 |
| Conference | Stanford Geothermal Workshop |
| Keywords | optimization, enhanced geothermal reservoir, multiple parallel fractures, dipole wells, semi-analytical solutions |
| Abstract | In this paper, we present a semi-analytical model to calculate fluid temperature when it is circulated in an injection-production loop through multiple fractures. Maximizing the output heat and operational time of an enhanced geothermal system (EGS) is always of great importance. We propose the use of multiple fractures to provide connectivity among wells and thus target a sufficiently large volume of rock. Although the temperature of each fracture varies with depth, the duration for higher temperature output can be prolonged by using a number of fractures rather than a single-fracture reservoir. In addition, considering temperature changes within the closed-loop circulation system consisting of injection wells, fracture reservoir, and production wells can increase the applicability of the model. In particular, the model deals with heat advection and the heat exchange between the host rock and the fluid in both wells and in the fractures. The temperature continuity between the well and the fracture is satisfied. By using potential flow theory, the computation of temperature in the fracture plane is simplified significantly to be dependent on one variable. Also heat conduction in the host rock is treated as one dimensional. These simplifications lead to a set of ordinary differential equations (ODEs) in Laplace space, which can be solved analytically, and Newton’s iteration scheme is implemented to efficiently obtain the solutions in physical space of temperature changes for the EGS system. Numerical results confirm that thermal energy extraction rate and the lifetime of fracture reservoirs can be optimized. The results also demonstrate that the effect of the fluid viscosity on the output temperature of the EGS can be ignored. Parametric studies are performed to identify the controlling parameters to provide guidance for the operation of an EGS circulation test. |