Direct tomographic observation of brine percolation into MgO-shotcrete drill cores

Heterogeneity and tightness of the barrier demand particular sensitive and lasting measuring methods, as well as resolution of small details. For this reason we developed a guard-ring surface-packer for process tomography of brine migration into barrier material with positron-emission-tomography (PET). Based on radiolabeling, this method is extremely sensitive, non-destructive and without retroaction.
As example for transport in the engineered barrier, we investigated injection of [²²Na]NaCl-sat – solution from a fluid reservoir with a diameter of 20 mm into the contact zone of adjacent MgO-shotcrete layers. The driving pressure was 2 bar, which caused intrusion of 1 mL of solution over a period of 260 d.
Overlay of the sequence of PET images and the structural CT image shows that deeper penetration (> 10 mm) occurred predominantly along one single pathway that was predetermined by the porous structure. Also, we observed a slowly propagating diffuse front that encompassed only a small portion of the injected fluid.
Although the permeability of the material is very low (1e-20 m²), the major portion of the brine propagates through a very confined channel and thus may reach a significant penetration depth, beyond the predictions based on assumed homogeneous material.