Estimation of Permeability of Disturbed Halite using Inverse Modeling

The numerical modeling tool ITOUGH2 was examined to determine its effectiveness in parameter estimation for the small scale seals performance tests (SSSPT) at the Waste Isolation Pilot Plant (WIPP) near Carlsbad, New Mexico. Gas flow injection tests were performed at various distances inward from the wall of Room D to estimate the effects of creeping of the disturbed rock zone (DRZ) since excavation of Room D and boreholes, and to estimate the amount of healing of the DRZ that has taken place due to an emplaced concrete seal. Salt creeping into an excavation is expected to show increased porosity and absolute permeability values compared to undisturbed halite. Healed halite is expected to show porosity and absolute permeability values comparable to that of undisturbed halite. Single-phase flow estimates of absolute permeability were made using ITOUGH2. Estimates of DRZ absolute permeability ranged from 10−11 near Room D to 10−21 m2 at the seal. Simultaneous estimates of absolute permeability and porosity were calculated for the disturbed rock zone in halite, and the effects of salt creep and healing were seen in the estimates of porosity and absolute permeability, as evidenced by the differences in absolute permeability and porosity with distance inward from the rib of Room D. Estimate of porosity were 0.04 near Room D, and the locations near the seal showed porosities of around 0.03. ITOUGH2 proved effective in interpreting test data seen in the SSSPT, especially test data with constant injection pressure.