In situ diffusion experiment in granite: phase I

Area of Science:

• Geosciences
• Hydrogeology
• Rock Mechanics

Background:

• Understanding solute transport in sparsely fractured rock (SFR) is crucial for geological disposal and resource management.
• Previous studies lacked in situ data on diffusion processes within intact crystalline rock under realistic stress conditions.

Purpose of the Study:

• To develop and validate a methodology for estimating diffusion parameters in SFR.
• To investigate diffusion processes in highly stressed crystalline rock at a depth of 450 m.

Main Methods:

• Conducted four in situ diffusion experiments using an iodide tracer over a two-year period.
• Employed laboratory studies to complement in situ findings.
• Utilized the MOTIF code for estimating diffusion coefficients and permeability.

Main Results:

• Estimated rock permeability at 2 x 10(-21) m(2) and effective diffusion coefficients ranging from 2.1 x 10(-14) to 1.9 x 10(-13) m(2)/s.
• Observed a characteristic "dog leg" diffusion profile near the borehole, suggesting stress-induced changes.
• Documented alterations in physical properties of rock samples when removed from high-stress environments.

Conclusions:

• In situ diffusion behavior in SFR is influenced by stress redistribution and borehole effects.
• Laboratory measurements may not fully represent in situ rock properties under high stress.
• Lessons learned from Phase I informed the initiation of a Phase II program for enhanced understanding of diffusion in SFR.