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

HANOR, J. S.

Produced saline waters and possible other oil field fluids have permeated over 2.2 x 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 up 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 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 and downward through underlying clays. 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 because in a variable-density fluid system, such as exists at this site, measurements of water level and calculations made of the apparent hydraulic head gradient 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/Cl 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.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana