Characterization of Gas Transport in Fractured Rocks for Underground Nuclear Explosion Detection

ABSTRACT Underground nuclear explosions produce noble gases that can migrate to the surface and become detectable by atmospheric monitoring tools. However, it is challenging to predict radionuclide gas migration in the complex engineered and natural subsurface systems due to several issues. These issues include generation of complex fracture networks near an engineered cavity, reactivation of natural fractures, coupled hydro-thermo-mechanical processes for transport of high-pressure gases in a fractured porous rock system. To improve our understanding of these technical challenges, we used a triaxial direct shear test scheme to characterize coupled hydro-mechanical gas transport in fractured porous rocks. In the experiments, we tested nitrogen (an analog noble gas) flow through two types of rocks with distinct petrophysical properties: porous Bandelier tuff and tight Climax stock granite. For each type, we measured the Biot effective stress coefficient, the rock matrix permeability, and the fracture permeability under various stress states, which allowed us to examine the stress-dependency of the Biot coefficient and investigate rock matrix-fracture interactions for transport of pressurized gas. Comparing the experimental results for the two distinct rock types, we observed some important findings for gas transport in fractured rocks. First, rock fracturing does not necessarily increase samples’ gas permeability when the rock matrix is highly porous. Furthermore, gas permeability of intact rocks can show strong exponential stress dependency even when the rock matrix is very tight. Additionally, Biot effective stress coefficients are not necessarily close to unity for highly porous rocks, especially when rocks are subjected to large effective stresses. In summary, the experimental results can be used to improve the physics in high-fidelity numerical modeling to elevate our understanding of radionuclide gas transport in fractured porous rocks after an underground nuclear explosion event. INTRODUCTION Surface radionuclide monitoring is the primary means of determining if an underground explosion is nuclear in nature (Maceira et al., 2017). Following an underground nuclear explosion (UNE), signature noble gas radionuclides, such as Xe-131m, Xe-133, Xe-135, and some krypton radioisotopes, will be produced by nuclear fission (Carrigan et al., 1996; De Geer, 1996; Sun and Carrigan, 2014). They are hard to contain and tend to seep from the underground explosion and migrate to the surface. Surface sampling and detection of these signature gaseous radionuclides, when above some background levels, is a strong indicator of the occurrence of an underground nuclear explosion. By comparison, seismic monitoring cannot definitively discriminate between chemical (for example, TNT) explosions and nuclear events. It is thus important to understand transport of noble gas radionuclides in the subsurface rock strata.