Capturing Early Evolution of Salt Openings

Abstract

In situ tests implemented in a research facility mined from salt deposits, if planned appropriately, provide an opportunity to characterize the host rock before, during, and after excavation of test rooms. Characterization of the test bed is essential to interpret structural deformation, creation and evolution of the disturbed rock zone, and measurement of first-order hydromechanical properties as the salt evolves from an impermeable undisturbed state to a more-transmissive damaged state. The strategy expounded upon in this paper describes recommended geophysical measurements to characterize the initial state of a potential test bed and its evolution over the course of a field test. Discussion includes what measurements could be made, why the measurements would be made, how they are made, and how accurately they need to be made. Quantifiable parameters will establish field-scale boundary conditions and data quality objectives to characterize the test bed in an underground salt research facility.

1. INTRODUCTION

Salt formations enjoy many favorable characteristics that combine to make them promising media for permanent radioactive waste disposal [1]. Salt formations are plentiful in the United States, providing ample areal extent and substantial thickness in aseismic geologic settings [2]. In addition to high thermal conductivity and plastic deformational response, undisturbed salt has extremely low permeability and porosity. Some of the favorable characteristics are modified during the excavation process and evolve during operations. Accounting for these fundamental structural and hydrologic changes is essential for establishing credible repository performance models needed to support waste disposal licensing activities. Before conducting experiments or operational demonstrations in a salt underground research facility, significant changes have already occurred in the salt, leading to a disturbed environment for hosting experiments. The disturbed rock zone (DRZ) near the excavation free surfaces contains inter-connected flow paths between pre-existing, but previously impermeable formation brine filled porosity, and becomes an anisotropic region of higher permeability and porosity. Depending upon experiment objectives, liberated brine, either in the vapor or liquid phase, and increased porosity can significantly influence evolution of the test bed. In addition, transient creep strain accumulates rapidly at the onset of excavation, but is not measurable once mining has already occurred. Using only observations from within the excavation overlooks potentially large strain accumulation in the salt formation. To model salt deformation completely, we must account for the transient creep contribution. Fortunately, evolutionary characteristics of salt are qualitatively known and straightforward engineering measurements can be made to quantify early evolution. This can allow the experimentalist to capture fully the early deformation for structural model development and validation.