Three-dimensional coupled hydro-mechanical modeling of gas injection

Abstract

This study investigates gas injection processes using a three-dimensional (3D) hydro-mechanical (HM) coupled model to enhance understanding of gas transport in low-permeability geological formations. Conducted as part of the DECOVALEX 2023 project, the research focuses on in-situ gas injection tests in Callovo-Oxfordian (COx) claystone, a crucial material for nuclear waste storage within the French concept for high-level radioactive waste disposal. Numerical simulations have traditionally relied on two-dimensional (2D) models for their computational efficiency, but these are inadequate for capturing the full complexity of gas migration. Indeed, conclusions drawn from 2D models can vary significantly depending on the chosen cross-section, underscoring their limitations. This study advances the field by conducting a more thorough investigation of gas injection in 3D models under HM coupled conditions with three different simulation strategies: (1) incorporating an ad-hoc excavation damage zone (EDZ) around the borehole within an elastic framework, (2) introducing perfectly plastic behavior for the claystone material, and (3) integrating softening effects to represent progressive material weakening. Sensitivity analyses have been conducted on key parameters, including EDZ permeability, test interval volume, and embedded fracture spacing, to evaluate their influence on gas migration. Additionally, prolonged gas injection simulations have been performed to further investigate the underlying mechanisms governing gas “breakthrough” threshold pressure. The findings underscore the critical role of 3D modeling in predicting gas transport, with significant implications for the design and long-term safety of nuclear waste repositories, ensuring the effective containment and isolation of both engineered and natural barriers.