A Comparison of Laboratory and Field Techniques for Characterizing Diffusion in Sparsely Fractured Granite

Abstract

The in-situ diffusion experiment intended to improve the understanding of diffusive solute transport in SFR was conducted at AECL's Underground Research Laboratory (URL) using a comparative series of laboratory and in-situ field experiments. The work was intended to explore issues surrounding the influence of stress relaxation in rock samples, stress redistribution about underground openings, rock texture, porosity, pore geometry, and an isotropy on derived effective diffusion coefficients (De). The in-situ diffusion experiments were conducted within 10 m long sub-horizontal NQ boreholes situated to intersect a range of rock stresses and textures on three levels of the URL. Following efforts to minimize the effects of well bore pressure histories and hydraulic gradients, tracers (I, Br, Li, Rb, uranine, lissamine) were injected into two test intervals within each borehole. After 15 months, one experiment from each URL level was over-cored to determine the extent of tracer diffusion into the rock. In-situ diffusivities were estimated from these tracer profiles using the finite-element code MOTIF. In-situ permeabilities were estimated from the analysis of shut-in hydraulic tests using the code nSIGHTS. The laboratory efforts were principally focused on the completion of 44 steady-state diffusion cell experiments using replicate granodiorite, granite and pegmatite rock coupons with a length of 0.03-m. Tracers included 3H, I, Li, Rb, lissamine and uranine. Proof of concept laboratory work included radial diffusion experiments with 0.20 m diameter cores and steady-state experiments to investigate diffusion across fracture surfaces and at elevated temperatures (50oC). Estimates of rock permeability at different orientations were determined by use of a high-pressure permeameter. Porosity estimates were determined by water immersion and diffusion experiments. Experimental results show that rock samples removed from high stress conditions are altered as a result of the combination of in-situ stress relaxation and stresses created by drilling, resulting in laboratory-estimated De and permeability values that were higher than those determined in situ. However, laboratory and in-situ De values determined for the 240-m Level were similar implying that samples removed from stress condition less than 30 MPa are subject to only minor alteration. In-situ experiments in SFR are also subject to uncertainty created by stress redistribution around borehole openings that could be responsible for the presence of a radially symmetric interface or resistance effect. This resistance may influence in-situ measurements of diffusion and permeability. Despite the agreement between laboratory and in-situ De values at the 240 m Level, laboratory and in-situ experiments still possess uncertainties associated with sample alteration, scaling, in-situ stress redistribution effects, and hydrogeology.