In-Situ Diffusion Experiment In Sparsely Fractured Granite

Abstract

The in-situ diffusion experiment was conducted at AECL's Underground Research Laboratory (URL) to improve the understanding of diffusive solute transport in sparsely fractured or intact granitic rock (SFR). The experimental program used a comparative series of laboratory and in-situ field experiments to evaluate the ability of laboratory measurements to estimate in-situ rock properties and to explore issues surrounding the influence of stress relaxation, rock texture, porosity, pore geometry, and anisotropy on derived effective diffusion coefficients (D_e). In-situ experiments yielded iodide D_ebetween 1.4 × 10⁻¹³ and 1.1 × 10⁻¹² m²/s. Unlike laboratory results, the in-situ D_e estimates did not exhibit correlation with sample depth or varied stress regime. Laboratory-derived measurements of D_e, porosity and permeability were found to systematically increase for samples removed from greater depths and higher stress regimes. Laboratory-derived iodide D_e values consistently trended higher than in-situ values by a factor of 1 to 15, except on the shallowest 240-m Level (σ₁ ≍ 30 MPa) where differences were negligible. Laboratory-derived estimates of permeability were consistently higher than in-situ derived values by a factor of 2 to 100. This experimental program provides evidence that laboratory steady-state diffusion experiments are most likely to yield conservative values of D_e for simulation of diffusive mass transport in SFR.