A study of thermal pressurization and potential for hydro-fracturing associated with nuclear waste disposal in argillaceous claystone

Hao Xu,Jonny Rutqvist,Jens Birkholzer

doi:10.1016/j.ijrmms.2020.104536

Abstract

We present coupled Thermo–Hydro–Mechanical (THM) modeling of geologic nuclear waste disposal in argillaceous claystone, focusing on thermally-induced pressure changes and the potential for such pressure changes to induce hydro-fracturing. To investigate this possibility, we first conduct a three-dimensional repository scale model, with host rock properties, repository design, and nuclear waste decay heat functions derived from the French concept of geologic disposal in argillaceous claystone. The model simulations show that the highest potential for hydro-fracturing occurs between emplacement micro-tunnels (cells) at the center rather than at the edge of the repository. We further investigate the use of a two-dimensional single cell model as a simpler surrogate for a full three-dimensional model. Our results reveal that such geometric simplification is reasonably accurate for modeling coupled THM processes at the center of the repository, though it overestimates the likelihood for hydro-fracturing and substantially overpredicts ground surface uplift. A parameter study shows the importance of adjacent higher permeability geological layers that play a significant role in dissipating overpressure in the host rock layer. Finally, the study shows the importance of the spacing between emplacement tunnels, which if too short results in a higher fluid pressure and a strongly increased potential for hydro-fracturing. Overall, the study suggests, the limiting factor in the thermal management and design of a repository is not necessarily the maximum temperature in the engineered barrier system near the waste packages, but rather the more modest host rock temperature between emplacement tunnels, due to the potential for thermal damage.

Highlights

Argillaceous claystone formations have been studied as a potential host rock for nuclear waste disposal
Our results demonstrate the strong impact of thermal pressurization on the repository stress evolution in low permeability argillaceous claystone.[9,28]
The host rock properties, repository design and nuclear waste decay heat evolution were derived from the French concept of geologic disposal in COx claystone

Summary

Introduction

Argillaceous claystone formations have been studied as a potential host rock for nuclear waste disposal This includes Callovo-Oxfordian (COx) claystone in France,[1,2] Boom clay in Belgium,[3,4] and Opalinus clay in Switzerland,[5,6] while argillaceous clay or shale is considered as an option in the U.S and Canada.[7,8] Argillaceous claystone has several favorable properties for nuclear waste isolation, including low permeability, good sorption capacity, limited natural fracturing and capacity for self-sealing.[7] the low thermal conductivity may lead to higher temperatures and thermal gradients in response to the emplacement of heat-producing nuclear waste. This together with a low rock permeability tends to induce strongly coupled Thermo-Hydro-Mechanical (THM) processes that are important to investigate for performance assessment.[2,9] Thermal pressurization is a prominent coupled process that occurs in low permeability claystone as a result of the difference between the thermal expansion coefficients of the fluid and the claystone.[9,10,11] If exceeding the minimal principal stress, the increase in the pore pressure could induce hydro-fracturing or might induce shear on existing fractures.[11,12]

Results

Discussion

Conclusion