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Gas Pressure Build-up in Radwaste Disposal: Hydraulic and Mechanical effects

Objective

The specific aim of this study is to evaluate the consequences of gas generation from a radioactive waste repository. The evaluation will be carried out in terms of gas migration in the backfill and the host rock and in terms of mechanical effects.
The phenomenology of the physical process involved in gas transport will be reviewed; then some selected numerical codes taking into account two phase flow will be presented. The gas pressure build up will be simulated for different designs of repositories, different host rock characteristics using numerical two phase flow models such as reservoir simulators used in the oil industry. A sensitivity study will then be performed. The mechanical modelling using new theoretical developments for fracture initiation and propagation through the bifurcation theories will be carried out with the cooperation of the French Institute of Petroleum (IFP): the scientific adviser is Prof. Dragon from ENSMA in Poitiers (France).
The specific aim of this study is to evaluate the consequence of gas generation from a radioactive waste repository. The evaluation will be carried out in terms of gas migration in the backfill and the host rock and in terms of mechanical effects.

The gas produced from corrosion and radiolysis is mainly hydrogen. The gas flow rate is a dimensioning parameter for the ensuing pressure build up. The information availiable in the literature related to gas generation has been reviewed. It is concluded that the volume liberated may be 2 or 3 orders of magnitude greater than the repository volume. The first phenomenon expected is the dissolution of the gas into the surrounding water, the quantity dissolved depends on the solubility, pressure and temperature. The molecular diffusion is a possible phenomenon for gas migration but the quantity of gas evacuated by diffusion is expected to be low beacuase of the low diffusion coefficients. A simple 3-dimensional model has been run in order to test the physical validity of the data set and get some order of magnitude of the pressure response for hypothetical gas flow rates. The geometry consists in a clay host rock 200 m thick, whose top is at 600 m depth, separated by 2 permeable sandstone layers 100 m thick. The rock properties are assumed to be isotropic and homogeneous. The preliminary results indicate that the order of magnitude of the pressure build up is 6 MPa, for a constant gas flow rate over a period of time of 5000 years. The horizontal distance concerned by the gas migration is dependent on the clay effective (or kinematic) porosity: the horizontal distance is 100 meters for a kinematic porosity of 15% and 150 m for kinematic porosity of 3%. The vertical migration is of the same order of magnitude.
Those results are indicative and based on conservative and maybe non realistic assumptions: simplified geometry, no backfill materials, nonrepresentative gas flow rates, isotropic and homogeneous conditions.
Work programme:

1. Phenomenology review: inventory of two phase flow phenomenology applied to gas migration from a radioactive waste repository.
2. Laboratory experiments: limited number of investigations (threshold pressure, capillary pressure, relative permeability, etc.) on some representative host rock core samples.
3. Two phase flow modelling: suitability of existing reservoir codes, simulation of gas pressure build up (isothermal flow, and in thermal conditions), in porous and fissured medium.
4. Mechanical developments and modelling: theoretical works for the elaboration of a new criteria for fracture initiation and propagation.
5. Conclusions.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

GEOSTOCK - SOCIETE FRANÇAISE DE STOCKAGE GEOLOGIQUES
Address
Rue Eugène Et Armand Peugeot 7
92563 Rueil Malmaison
France