Abnormal fluid pressures in sedimentary basins

Jiao J.J. and C. Zheng, 1998, Abnormal fluid pressures caused by erosion and subsidence of sedimentary basins, Journal of Hydrology, vol. 204, 12-137.

ABSTRACT  Gravitational loading or unloading due to deposition or erosion in sedimentary basins is one of the most common mechanisms responsible for abnormal fluid pressures in geologic formations. This paper examines the integrated response of fluid pressures to both deposition and erosion using vertical one-dimensional numerical models. The modeling results show that vertical loading and unloading of sedimentary basins containing thick interbeds of low permeability can give rise to abnormal fluid pressure-depth trends that closely resemble those observed in the field. On a geologic time scale, an underpressured formation may become normally pressured or overpressured and vice versa due to transient flow induced by vertical geologic movements. A case study in the Guymon area of Texas County, Oklahoma, shows that there are two underpressured reservoirs in this area. The burial curve constructed from geologic information indicates that this area has undergone multiple subsidence and uplift events. Based on stratigraphic information, a vertical geologic section of this area is represented by a numerical model consisting of two reservoir formations and two shale (confining) units. The numerical model is able to reproduce the observed pressure abnormalities based on the drill-stem test (DST) data. The hydraulic conductivities of the two shale formations are estimated to be 4.210-14 and 5.110-14 m/s from model calibration.

Jiao, J.J. and Zheng, C., Hennet, R. J.-C. 1997, Analysis of underpressured geological formations for disposal of hazardous wastes, Hydrogeology Journal, vol. 5, no. 3, p. 19-31.

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 under-pressurization 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.

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Last modified: June 14, 2001