The purpose of the in situ tests that will be performed at the potasse mines d'Alsace (France) is to measure the gas permeability of a rock salt bed previously submitted to a brine percolation.
This is of major importance for predicting the becoming of the gases present in the repository (gases coming from radiolysis, corrosion, bacteriological activity, trapped air ...).
The experiment is planed to include two tests in separate boreholes at Amelie underground facilities.
As the measured quantified will be very low, the choice of experimental parameters will allow to eliminate or minimize or quantify all the undesirable effects in order to facilitate the data interpretation.
A theoretical study of the gas permeability of a rock salt bed using an analytical model has been carried out to aid in interpretation of the borehole studies. The main hypotheses were: two geometries of cavity have been investigated (spherical and cylindrical with an infinite length), the rock salt has been considered as an isotropic, saturated porous medium with an elastic, viscous, plastic behaviour; the flows through the rock salt are assumed to be governed by Darcy's law.
Two cases have been investigated: constant pressure test; variable pressure test (pulse test). General disturbing phenomena have been evaluated: creeping of the cavity; thermal effects; solution crystallization of halite.
Recommendations regarding borehole tests have been made and feasability tests of the device inside the borehole have been carried out.
1. Pre-feasibility tests in order to validate the experimental device (tightness of the borehole system, accuracy of the measurements).
2. Pilot test in borehole No. 1, with brine, increasing the pressure up to fracture the rock salt.
3. Test in borehole No. 2 including:
- measurement of gas permeability;
- injection of brine and measurement of brine permeability;
- emptying of the borehole and measurement of gas permeability.
Funding SchemeCSC - Cost-sharing contracts