Limitations in the Use of Conventional Ground Water Techniques in Characterizing the Fate of Produced Waters in On-Shore Gulf Coast Sediments : ABSTRACT

Abstract

ABSTRACT Produced saline waters and other possible oil field fluids have permeated over 2.2 106 m3 of shallow sediment at a production facility in Pointe Coupee Parish, south Louisiana. These fluids have radiated out laterally in all directions from an area of former brine storage pits distances of 150 to 900 m over a 45-year period. Contamination locally extends at least 14 m below land surface. This site provides a instructive example of the severe limitations of conventional ground water monitoring and sampling techniques in characterizing and in developing remediation strategies for sediments permeated by variable-density aqueous fluids, such as mixes of produced waters and native ground water. Dense saline wastes are preferentially migrating laterally along the base of a shallow silty-sand unit. Rapid downward vertical migration through the underlying clay into deeper sediments may reflect the presence of fractures in the clay, as have been noted at other south Louisiana sites. The wastes have migrated at rates much higher and in directions far different than those predicted on the basis of the conventional water level measurements which have been performed at the site. This is a consequence of the fact that in a variable-density fluid system, such as exists at this site, the measurements of water level and calculations of apparent hydraulic head gradient made cannot be used as reliable indicators of hydraulic force. Independent determinations of fluid pressure gradients and density must be used instead. Routine partial chemical analyses which have been made of the saline contaminants indicate that many have Na/TDS ratios far lower than those of normal produced waters. Either there has been a subsurface chemical modification of the composition of these fluids, perhaps involving ion exchange, or other types of saline wastes have been disposed of at this site, or both have occurred. Until complete chemical analyses are made it will not be possible to resolve this question.