In situ diffusion experiment in granite: Phase Ⅰ

摘要

A program of in situ experiments, supported by laboratory studies, was initiated to study diffusion in sparsely fractured rock (SFR), with a goal of developing an understanding of diffusion processes within intact crystalline rock. Phase Ⅰ of the in situ diffusion experiment was started in 1996, with the purpose of developing a methodology for estimating diffusion parameter values. Four in situ diffusion experiments, using a conservative iodide tracer, were performed in highly stressed SFR at a depth of 450m in the Underground Research Laboratory (URL). The experiments, performed over a 2 year period, yielded rock permeability estimates of 2 x 10~(-21)m~2 and effective diffusion coefficients varying from 2.1 x 10~(-14) to 1.9 x 10~(-13)m~2/s, which were estimated using the MOTIF code. The in situ diffusion profiles reveal a characteristic "dog leg" pattern, with iodide concentrations decreasing rapidly within a centimeter of the open borehole wall. It is hypothesized that this is an artifact of local stress redistribution and creation of a zone of increased constrictivity close to the borehole wall. A comparison of estimated in situ and laboratory diffusivities and permeabilities provides evidence that the physical properties of rock samples removed from high-stress regimes change. As a result of the lessons learnt during Phase Ⅰ, a Phase Ⅱ in situ program has been initiated to improve our general understanding of diffusion in SFR.