Page 133 - Environmental Control in Petroleum Engineering

120 Environmental Control in Petroleum Engineering

materials by modeling the deposition of the cuttings. Models for
sediment deposition following discharge from offshore platforms are
available (MacFarlane and Nguyen, 1991).
In one study, the heaviest accumulations of hydrocarbons and
heavy metals were found to be within about 100 meters of the plat-
forms, with lower accumulations at farther distances (American Petrol-
eum Institute, 1989c). The impact of these accumulations on the
benthic community was uncertain. Seasonal variations in the organic
matter content from nearby river runoff was greater than the concen-
trations from the platform. Seasonal variations in the benthic com-
munity were also greater than those observed at varying distances from
the platform.
The greatest impact of offshore discharge of drilling fluids is when
oil-based muds are used. Elevated hydrocarbon levels in the sediments
and impacts on the benthic community have been measured at dis-
tances of several kilometers from platforms (Bakke et al., 1990;
Peresich et al., 1991). The hydrocarbon concentration in the sediments,
however, decreased significantly over a period of several years follow-
ing discharge. The distance away from a platform that elevated levels
of hydrocarbons can be detected may also depend on whether the
cuttings were washed prior to discharge (De Jong et al., 199la). The
threshold level of hydrocarbons in subsea sediments below which no
effects were observed on the mortality of the heart urchin (Echino-
cardium ordatum) was determined to be on the order of 10–100 mg
oil/kg sediment (De Jong et al., 1991b). Because of these effects, the
discharge of oil-based muds and their associated cuttings is prohibited
in many areas around the world.

3,7 PRODUCED WATER

The potential for environmental impact following the discharge of
produced water arises primarily from its high salt content, its heavy
metals content, its dissolved or suspended hydrocarbons, and its
oxygen deficiency.
The acute toxicity of a selection of produced waters to mysid shrimp
(96-hr LC 50) was found to range between 1.3% to 9.3% by volume of
produced water in seawater (U.S. Environmental Protection Agency,
1989). Sublethal effects were observed for produced water concentra-
tions as low as 0.5% after 19 days of exposure. The toxicity of the