The second gas injection test of the GT experiment: what it tells us about the first test and self-sealing

In a geological disposal facility (GDF) for radioactive waste, hydrogen, carbon dioxide, nitrogen, and hydrogen sulphide will be produced through a range of processes including the corrosion of ferrous materials under anoxic conditions, radioactive decay of the waste, radiolysis of organic materials and water, and the microbial degradation of organic materials. Repositories for radioactive waste are designed and engineered to limit high hydrogen concentration and avoid explosive risk. In the Swiss repository concept, hydrogen is the main gas that will be produced from the corrosion of metals. The production of hydrogen is anticipated to span > 100,000 years post emplacement of the waste over which time the gas will move into the host rock through the combined processes of molecular diffusion and bulk advection. Understanding these processes and the long-term fate and impact of the gas is therefore important in the development of a repository for radioactive waste. In a clay-based GDF four primary phenomenological models can be defined to describe gas flow as summarised by Marschall et al. (2005): (i) gas movement by solution and/or diffusion governed by Henry’s and Fick’s Laws respectively within interstitial fluids along prevailing hydraulic gradients; (ii) gas flow in the original porosity of the fabric governed by a generalised form of Darcy’s Law, commonly referred to as visco-capillary (or two-phase) flow; (iii) gas flow along localised dilatant path-ways (micro-fissuring), which may or may not interact with the continuum stress field, the permeability of which is dependent on an interplay between local gas pressure and the effective stress state; and (iv) gas flow along macro fractures similar in form to those observed in hydrofracture activities during hydrocarbon reservoir stimulation, where fracture initiation occurs when the gas pressure exceeds the sum of the minor principal stress and tensile strength. The Gas Transport (GT) test was designed to gain detailed information on whether gas movement through the shaly facies of the Opalinus Clay was through visco-capillary or dilatant pathway flow. Diffusion is sufficiently understood in Opalinus Clay, and gas fracturing is not expected to occur. Thus, these two mechanisms were out of scope of the experiment. The field test has been running since November 2020.