Geothermal Energy from a Borehole in Hot Dry Rock - A Preliminary Study

Abstract

A simple procedure for calculating heat transfer between circulated water and hot dry rock in a single, concentric-annulus geothermal well is presented. Also presented are the results of an application of the procedure to a proposed well in Sweden.

The concept examined consists essentially of pumping water, at naturally-occurring temperatures, down a cylindrical well drilled several kilometers into the earth, and extracting it in a heated state through an annular cylinder (see for example, Smith et al., 1973). Two different schemes embodying the same concept are examined in the present investigation. These are illustrated in Figs. 1 and 2. In the first, cold water is pumped into the inner cylinder and extracted through the annular space. In the second, the situation is exactly reversed.

The problem, in both schemes, is to predict the outlet temperature of water under given conditions on the one hand; and on the other, to predict the temperature drop in the rock mass surrounding the well, otherwise known as energy depletion of the geothermal reservoir. (See Gringarten -- et al., (1975) for analytical solutions to a plane-crack situation embodying a similar concept.) Such prediction was undertaken in two stages.