Changes in Permeability during Flow of Water through Granite Subjectedto a Temperature Gradient

Abstract

The useful lifetime of a geothermal reservoir as an energy source can depend strongly on the technique used to extract heat. In both naturally occurring geothermal fields and artificial geothermal reservoirs, such as the Los Alamos hot dry rock experiment, continuous extraction of heat requires maintenance of a channel of permeable material to and away from the heat source. Since such a system must be recharged to maintain production over a useful period of time, water must be heated and thus transported through rock in the presence of large temperature gradients. In both naturally

We have designed experiments to study permeability changes in rock due to fluid flow along a temperature gradient to further understanding of how heat production in geothermal reservoir s may depend on these effects. samples of Westerly Granite, 8.9 cm long, 7.6 cm in diameter, and containing a 0.56 cm diameter borehole, were used. A resistance heater was placed in the borehole as a heat source, giving a radially symmetric temperature gradient. Confining pressure and pore pressure were applied to the sample; a pore Cylindrical -50- pressure gradient was applied to induce radial flow either toward or away from the borehole were used. the experiments, an apparent permeability, averaged over the whole sample, was calculated from Darcy's Law by measuring the pore fluid flow rate.