On wellbore heat transfer and fluid flow in the doublet of Enhanced Geothermal System

Enhanced geothermal system which utilizes geothermal energy beneath the ground surface with a depth of several thousand meters has aroused intense interest recently. Wellbores in Enhanced Geothermal System extends several kilometers from the ground surface, providing large heat transfer areas between the flowing fluid and the surrounding formation. Various techniques and approximations for predicting flowing process in the wellbore have been presented in literature. Most studies assume steady one-dimensional flowing without considering heat transfer with the surroundings. In this paper, unsteady flowing within a vertical injection and a production well, together with wellbore heat transfer between the fluid and the surrounding formation was modeled. An analytical solution under certain assumptions is presented using the Laplace transform technique. The time-evolving results show that almost the entire pipe is under hydrodynamically fully developed condition. Transient effect is significant in the initial phase after injection and production. Approximate steady condition could be reached within the 30 years of operation under fixed injection and production conditions. (C) 2015 The Authors. Published by Elsevier Ltd.