Fracture vs. matrix reactivity in a tight crystalline rock. Modeling of a fractured-gneiss core infiltration experiment.

Two infiltration experiments using a fractured gneiss core were performed to address the reactivity of this crystalline rock (host rock for the Finnish geological repository for spent nuclear fuel). The core was 5 cm in diameter and 6.2 cm in length, with fracture opening values up to 1.1 mm. Mineralogy and fracture volume were characterized by X-ray diffraction and X-ray computed microtomography, respectively. Groundwater from the site (dominated by Cl-Na-Ca, pH 7.26, ionic strength 0.22 molal) was injected in the first experiment, while milli-Q water (pH 6.05) was used in the second one. Both solutions were at equilibrium with the atmosphere, and the experiments were performed at room temperature. Flow rates were about 0.005 mL/min.

The results (evolution of outlet solution chemistry) were interpreted by 1D and 2D reactive transport modeling using the CrunchFlow code. The 1D model included flow, solute transport and reaction only along the fracture. Very large mineral surface areas, much larger than the exposed areas on the fracture surfaces, were needed to reproduce the experimental results. To address this issue a 2D model was developed, which also included diffusive transport and reactions in the rock matrix. The 2D model did not need the large surface areas in the fracture to match the experimental results. These results show the important role that rock matrix plays in the overall reactivity of the fractured rock, despite the small porosities (of the order of 1%) and effective diffusion coefficients (of the order of 10^-13 m²/s). However, the 1D approach could still prove useful for large repository-scale calculations, given appropriate calibration.