Coupled hydro-mechanical modeling of gas migration in compacted bentonite using a discrete fracture network approach
Abstract
Gas migration in clay-based barrier materials has been addressed as a main concern in the field of nuclear waste disposal. Gas generation from environmental factors in a geological disposal facility could pressurize the surrounding low-permeability materials to develop new porosity and dilatant pathways for gas movement, which will degrade the barrier performance. The mechanisms of gas movement through clay-based materials can be described by conceptual models accounting for (i) diffusion, (ii) two-phase (gas-fluid) flow, (iii) localized flow pathways, and (iv) hydro-gas fracturing of the rock. It is therefore necessary to consider all of these mechanisms for a better understanding the complexity of gas transport in low-permeability materials. However, the predictive capability of the models is thus far limited, and the basic gas transport mechanisms are not understood in sufficient detail to provide the ground for robust conceptual and quantitative models.
In this study, we have used the TOUGH2 code linked with a lattice-based model, Rigid-Body-Spring Network (RBSN), to model coupled hydro-mechanical (HM) processes of gas pressure-induced fracture and fracture-assisted gas flow. A discrete fracture network (DFN) approach is adopted to facilitate modeling of abrupt change of hydrological properties along discrete flow paths created by fracture formation. In order to address complex mechanisms in the strongly coupled HM processes, we integrated various multi-physics concepts, such as poro-elastic strain/porosity changes, moisture swelling/shrinkage effects, multiphase flow with gas entry threshold, localized damage and fracture propagation, and fracture aperture-dependent permeability change. We have performed simulations for linear and spherical gas transport in compacted bentonite samples. In the validation process, the simulation results demonstrate plausible descriptions of physical phenomena in the laboratory tests by matching key features such as evolutions of stresses, pore pressures, in- and out-flow rates, and injection pressure during gas entry and breakthrough within the bentonite samples. In addition, fracture patterns and internal pressure distributions are presented for qualitative insights into the complex mechanisms of gas migration in bentonite.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H41G1748K
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRY;
- 1805 Computational hydrology;
- HYDROLOGY;
- 1847 Modeling;
- HYDROLOGY;
- 3947 Surfaces and interfaces;
- MINERAL PHYSICS