Using SF₆ and Xe to Monitor Gas Migration Through Explosion‐Generated Fracture Networks

Abstract

We describe a field study where tracer gas was injected into a subsurface cavity created by a small chemical explosion beneath the water table. The main objective of the study is to compare the migration of sulfur hexafluoride (SF6) and xenon (Xe) through an explosion‐generated fracture network and to study the influence of ground water on gas transport. A mixture of tracer gases (50% of SF6 and 50% of Xe) was injected on 31 October 2018 and gas sampling continued until 8 November 2018. We observe similar trends in SF6 and Xe concentrations at four ground surface sampling sites. The changes in the SF6/Xe ratios with time show that more SF6 than Xe is observed during the barometric pressure lows when the absolute measured concentrations are highest. Conversely, the ratio SF6/Xe is less than 1 during the high‐pressure intervals when absolute measured concentrations are low. The results of the experiment suggest that during barometric pressure lows the tracer is migrating to the surface primarily by advective gas phase transport, whereas during barometric pressure highs, advection is suppressed and near‐surface evaporation of interstitial pore fluid with tracer dissolved in it becomes more important. Thus, the results of the experiment show that the gas concentrations at the surface are controlled by the combined effects of the gas dissolution into pore water and the barometric pressure fluctuations.

Evaluation of subsurface transport processes of delayed gas signatures applicable to underground nuclear explosions

“…The phase-equilibrium partitioning of a gas component in adjoining gas and liquid phases is described as the ratio of its mole fractions in the gas phase, y , to its mole fraction in the liquid phase, x . A gas solubility model 17 – 24 is applied to the gas phase-equilibrium relations for xenon and argon 36 , 37 such that where S is the mole fraction (solubility) of gas component xenon and argon dissolved in the liquid phase measured at the reference gas pressure, , and is the gas-phase pressure. The solubility is determined using experimental data as a function of temperature 38 where A , B , C , and F are model parameters, and is the temperature in Kelvin degree divided by 100.…”

“…However, it is also a legitimate question to ask how well chemical tracers such as SF or Freon can be expected to serve as replacements for the radionuclides of interest. This question was partially addressed recently in a novel study by Stroujkova et al 17 . The authors conducted a field experiment involving injection of equal volumes of SF and stable Xe into an existing shallow cavity and fracture zone formed by a small detonation in granitic rock with a shallow water table.…”

“…While our simulations include the effects of partial saturation on reducing adsorption of xenon in the zeolitic containment zone, they would still appear to overestimate the adsorption of xenon compared to that observed by Neil et al 22 in their laboratory experiments. Solubilities of argon and xenon were determined as functions of temperature and pressure 17 – 24 using experimental data under saturated conditions. Experimental data on SF dissolution (between gas and liquid phases) are also available for several temperature ranges 25 – 28 .…”