Aquifer-well thermal and fluid dynamic model to design a downhole heat exchanger

Downhole heat exchangers (DHE) eliminate the problem of geothermal fluid di sposal because only heat is taken from the well. To design DHEs, it is nece ssary to know how aquifer fluid interacts with well fluid Experimental test s carried out by the authors indicate that there is a limit to the heat flu x that can be obtained by a DHE in a well: this limit arises only from the thermal and fluid dynamic interaction between the well and aquifer The two- dimensional thermal and flowfields in the aquifer were also determined usin g the finite element method. It was found that the heat flux limit is due t o the short circuit effect in the aquifer between cold and warm fluids, res pectively, leaving and entering the well. A characteristic parameter, terme d the "short circuit factor" was introduced to a simple model with lumped p arameters. This factor depends on the average temperatures of the well inle t and outlet fluid and on the unperturbed aquifer temperature. A simplified model developed by the authors allowed the maximum heat flux obtained by t he DHE in the well to be determined using knowledge of the short-circuit fa ctor as the starting point.