Abstract

Crushed salt is being considered as a backfill material in the event of a salt repository for high level nuclear waste. The thermal-mechanical-hydrological properties of crushed salt as it reconsolidates in response to pressure and temperature changes are therefore important. An experimental system to measure gas flow through consolidating crushed salt at elevated temperature and pressure has been developed and tested. An experiment completed at 250°C, and hydrostatic pressures to 20 MPa, compacted a crushed salt sample from ~40 percent porosity to near zero porosity. For this consolidation history, apparent permeability decreased from greater than 10-12 m2 to ~10-22 m2.

1. INTRODUCTION

It is important to improve the understanding of the coupled thermal-mechanical-hydrologic response of granular (or crushed) salt used as a seal material for shafts, drifts, and boreholes in mined salt repositories. Granular salt consolidation is being investigated through an integrated program of laboratory measurements, observations, and constitutive model development and evaluation. In particular, we are focused on measuring the deformation and corresponding permeability reduction of granular salt as it consolidates to fractional densities greater than 0.90. Behavior at high fractional densities is of principal importance to repository applications because this is when the permeability of granular salt is expected to decrease to a condition comparable to intact salt, that is, it becomes nearly impermeable. An extension of an existing constitutive model is being developed using these data to allow prediction of changing properties as granular salt consolidates.

Rock salt has been considered as a potential disposal medium, because salt is essentially impermeable. The very existence of salt millions of years after deposition is proof that water has not flowed through the formation. The undisturbed formation permeability of salt is essentially too low to measure using traditional hydrologic and reservoir engineering methods. In undisturbed and healed salt, brine is not able to flow at rates that would lead to significant radionuclide mobilization and transport. In terms of disposal, one of the most important features of salt as an isolation medium is its ability to heal previously damaged areas. Damage recovery is often referred to as "healing" of fractures. The healing mechanisms include microfracture closure and bonding of fracture surfaces. Evidence for healing of fractures in salt has been obtained in laboratory experiments and through observations of natural analogs. Fracture healing can restore the low permeability of intact rock salt. The consolidation of granular rock salt is envisioned to occur under similar mechanisms to healing.

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