Analysis of underpressured reservoirs for waste disposal

Abstract

Underpressured reservoirs are known to exist in sedimentary basins throughout the world and are common in North America. Deep-well injection of hazardous liquid wastes into underpressured reservoirs has been mentioned as a safe means for waste disposal because of their tendency to contain fluids for long periods of time. In this study, a numerical model based on the geological setting of the Hugoton field in the southwestern United States is used to analyze the potential of underpressured reservoirs for safe disposal of liquid wastes. The factors controlling the pressure buildup and disposal volume are evaluated by studying the sensitivities of the numerical model to various flow and reservoir parameters. The safe disposal volume on a per-well basis is estimated under the restrictive conditions that underpressurization persists during and after injection, and that the migration of the waste is restricted to prevent contamination of the overlying aquifers for an operationally permanent time period. This study demonstrates that the presence of the ultra-low permeability formation surrounding an underpressured reservoir makes it possible for the waste to be contained safely and permanently. Even if the pressure in the reservoir and the overlying formations rises to hydrostatic, possible upward migration of the contaminants is likely to be dominated by molecular diffusion across the ultra-low permeability formation and thus would be too slow to be of significance at human time scale.