Hydrology and hydraulic properties of a bedded evaporite formation

Abstract

The Permian Salado Formation in the Delaware Basin of New Mexico is an extensively studied evaporite deposit because it is the host formation for the Waste Isolation Pilot Plant (WIPP), a repository for transuranic wastes. Geologic and hydrologic studies of the Salado conducted since the mid-1970s have led to the development of a conceptual model of the hydrogeology of the formation that involves far-field (i.e. beyond the disturbance created by the repository) permeability in anhydrite layers and at least some impure halite layers. Pure halite layers and some impure halite layers may not possess an interconnected pore network adequate to provide permeability. Pore pressures are probably very close to lithostatic pressure. In the near field around an excavation, dilation, creep, and shear have created and/or enhanced permeability and decreased pore pressure. Whether flow occurs in the far field under natural gradients or only after some threshold gradient is reached is unknown. If far-field flow does occur, mean pore velocities are probably on the order of a meter per hundreds of thousands to tens of millions of years. Most hydraulic-test responses in the Salado do not appear to reflect radial flow, but instead imply subradial (e.g. intermediate between linear and radial) flow dimensions. We believe these subradial dimensions reflect channeling of flow through fracture networks, or portions of fractures, that occupy a diminishing proportion of the radially available space, or through percolation networks that are not ‘saturated’ (percolation terminology meaning fully interconnected). This is probably related to the directional nature of the permeability created or enhanced by excavation effects. Inferred values of permeability cannot be separated from their associated flow dimensions. Therefore, numerical models of flow and transport should include heterogeneity that is structured to provide the same flow dimensions as are observed in hydraulic tests. Modeling of the Salado Formation around the WIPP repository should also include coupling between hydraulic properties and the evolving stress field because hydraulic properties change as the stress field changes.