VERIFYING THE ORIGIN AND PREDICTING THE FATE OF PRODUCED WATER CONTAMINATION - SOME INSIGHTS FROM THE SOUTH LOUISIANA GULF COAST

Detailed study of the spatial distribution and composition of contaminated ground water at several oil and gas production and waste disposal facilities in south Louisiana provides insight into the physical and chemical behavior of produced waters released into shallow coastal plain sediments. Fine-grained transgressive sediments dominate the uppermost sedimentary section over much of south Louisiana. It was common practice in the past to store brines co-produced with oil and gas in unlined pits excavated into these near surface clays. However, these clays have proved to be leaky, in part as a result of fracturing produced by expansion and contraction during wet and dry seasons. At one site studied, saline waters have migrated downward from three brine storage pits at least 14 m into the subsurface, have spread out laterally through sandy units distances of up to 900 m, and have impacted over 2 million cubic meters of sediment.

Mixing of produced brine with ambient ground waters typically produces a suite of waters having a wide range of salinities and, hence, densities. Lateral migration of saline wastes at south Louisiana sites has been documented to occur at rates much higher and in directions far different than those predicted by conventional interpretation of water level measurements in monitoring wells. This is a consequence of the fact that hydraulic gradients cannot be determined from water levels in a variable-density fluid system. Independent determinations of fluid pressure gradients and fluid densities must be used instead to calculate hydraulic force. An alternative technique for determining rates and directions of shallow subsurface waste migration is to perform terrain conductivity surveys at specified intervals of time.

Partial chemical analyses made of contaminated ground waters at two Louisiana sites indicate they have significantly lower Na/TDS ratios than either produced waters collected at the wellhead or naturally occurring shallow brackish ground waters in south Louisiana. This is most likely due to ion exchange reactions on clay minerals, which favor removal of Na and release of Ca in saline environments. A sodium deficiency is thus a potential tool to distinguish recently contaminated waters from native brackish ground waters.