Gas Phase Measurements of Porosity, Diffusion Coefficient, and Permeability in Rock Samples from Olkiluoto Bedrock, Finland

摘要

The Finnish plan for final disposal of nuclear waste is an underground repository in crystalline bedrock in Olkiluoto. In the safety analysis of the repository, it is vital to know the transport properties of the bedrock. We set out to investigate these properties by measuring porosities of rock samples with Ar-gas pycnometry and diffusion coefficients and permeabilities of the same samples with a He-gas method. Diffusion of gases is about 10,000 times faster than diffusion of elements in water, and thus the gas methods are practical tools when determining the properties of relatively tight samples. The porosities were 0.44-0.63 % for pegmatitic granite (three samples) and 0.19-2.9% for veined gneiss (15 samples). The effective diffusion coefficients were 3.2 × 10~(-9)-8.2 × 10~(-9) m~2/s for pegmatitic granite and 5.0 × 10~(-10)-6.7 × 10~(-9) m~2/s for veined gneiss and the permeabilities 5.9 × 10~(-19)-6.4 × 10~(-18) m~2 and 1.4 × 10~(-20)-8.6 × 10~(-17) m~2 for the same rock types, respectively. We also investigated the dependences of said properties of one another, and found a correlation between effective diffusion coefficient and permeability.