Laboratory Replication of Gas Mixture Transport in Fractured Porous Tuff Following a Subsurface Explosion
Authors: Li W. , KC B. , Meng M. , Frash L. P. , Neil C. W. , Stauffer P. H. 
Published: August 2024

ABSTRACT:
Detection of underground nuclear explosions (UNEs) require a sophisticated understanding of gas transport through fractured geological media. This is because surface detection of some signature gaseous radionuclides, such as xenon, is a strong indicator of a subsurface nuclear event. The complicated physics involved in gas transport in explosively generated cavities results in remaining questions about the accuracy of predictions of gaseous radionuclides transport through fractured geological media. For example, there is a limited understanding of breakthrough time of each gaseous species through porous rocks. Unclear also is the effect of temperature, pressure, and water saturation on the breakthrough times of the gaseous species. The objective of this work is to evaluate the breakthrough times of the gaseous species through tuff rocks by accounting for the porous rock matrix, fracture creation, and fracture displacements, using a laboratory testing protocol of SF6-Xe-Kr gas mixture transport in damaged rocks. The experiments included N2 gas permeability at a range of pore pressures and confining pressures, after which we conducted gas transport test to measure the breakthrough time of each gas species. We observed stress dependency of the gas permeability both for the intact and the fractured tuff. Interestingly, fracturing did not increase the gas permeability for the highly porous tuff. In the gas mixture transport tests, Kr is consistently the first arriving gas species, regardless of the confining pressures and the fracturing events. Most importantly, we observed that the gas breakthrough times become smaller when the permeability decreases at an increasing confining pressure, which is a subject for further study where a coupled gas sorption and transport modeling effort will be required.

1. INTRODUCTION
Detection of underground nuclear explosion (UNE) requires surface radionuclide monitoring for confirmation (Maceira et al., 2017). Gaseous radionuclides, including xenon and krypton radioisotopes, are often produced by nuclear fission (Carrigan et al., 1996; De Geer, 1996; Sun and Carrigan, 2014). These gaseous radionuclides tend to migrate from the explosively generated cavities to the surface. Therefore, surface detection of these signature gaseous radionuclides provides strong positive evidence of an underground nuclear explosion. Understanding the transport of noble gas radionuclides in the subsurface fractured rocks becomes important for UNE detection.
Prediction of gas migration in fractured rock masses requires sophisticated characterization of gas flow in rock matrix and fractures. Although fractures often account for a very small volume percent of the total void space, gas flow occurs primarily along fractures because fractures are typically much more permeable than rock matrix (Frash et al., 2016; Li et al., 2015 & 2021; Welch et al., 2021). Meanwhile, rock matrix interferes with gas flow in the neighboring fractures by, for example, sinking gas molecules into the pore space of rock matrix when gas concentration and pressure are greater in the fractures (Vairogs et al., 1971; Germanovich et al., 2001).