Regarding clay rock at the mesoscale (that is, millimeter to centimeter), the structure of the porous clay matrix treated as continuum controls transport properties. Upscaling these mesoscale properties to the macroscale requires that representative elementary volumes (REVs) exists. The objective of this paper is to quantify the mesoscale REVs of the continuous clay matrix. Here, the REV is defined as the minimum volume for which the variance of the determined clay matrix content for one measured sample is sufficiently small. The calculations reveal a strong dependence of the size of REV on the clay matrix content. Assuming a relative error of 10% on the true bulk clay matrix content, the size of the REV increases from 200 microns to about 15 centimeter when the clay matrix content decreases from 0.7 to 0.13. The dependency of the size of REV on the clay matrix content is related to the grain size distribution of sand grains (e.g. carbonates and quartz). Sand grains in clay rocks with high sand contents have a wider grain size spectrum, which leads to a higher geometric complexity. This increases the size of the REV.
Key words: Representative elementary volume, opalinus clay, shales.
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