Leaching of rock fractures: Laboratory and field tests for borehole heat stores

Abstract

Abstract The aim of this research project was to increase the hydraulic conductivity of fractured rock by pumping a leaching fluid (NaOH) through rock fractures. A 16-week field test was carried out in a borehole heat store consisting of 19 vertical boreholes to a depth of 15 m in gneissic rock. The leaching process was studied simultaneously in a laboratory test where rock samples from core drillings of the test site were used. The hypothesis that NaOH-solution would leach and thereby widen the fractuers, was not fulfilled. On the contrary, the fractures were sealed as the leaching test went on. The explanation for this is that the leaching rate was higher than expected, the leaching fluid was saturated and the dissolved minerals precipitated. In principle, the minerals were dissolved and moved from one part of the fractures to precipitate at another part, causing clogging. The effect of the leaching field test has been simulated by means of a numerical model. The conclusion is that a leaching of rock as described in this paper should be combined with a deposition tank for the clogging material in order to avoid precipitation in the rock fractures. The results of this project have demonstrated a way of sealing rock fractures that has many more applications in engineering geology.