How wet are shallow granites? An experimental investigation

The water content and porosity of a granite sample have been measured to test the assumption that the porosity in crystalline rocks hosting a waste repository will be water-saturated. Undersaturation would reduce the potential role for rock matrix diffusion and impact on the retention of contaminants. A core was drilled from the Grimsel Test Site, Switzerland, with minimal cooling water, and sealed with a resin coating to preserve its water content. The initial water content, porosity, water saturation and gas permeability were determined by multiple laboratory methods, including measurements under stress to recreate in situ conditions. The initial water content was c. 0.72 vol% while total porosity was distinctly higher (c. 1.3 vol%). Sub-samples separated from the core by chipping had anomalously high porosity. Gas porosity reduces with stress and was used to determine a loading factor. Correcting for 450 m of overlying granite gives a total in situ porosity of 1.03%, indicating the in situ porosity was <75% saturated. Our results show that the water content and porosity of low-porosity crystalline rocks can be measured separately under in situ conditions, although the saturation level of the cored region might have changed since construction of the test site.