Page 130 - Environmental Control in Petroleum Engineering

The Impact of Drilling and Production Operations 117

contain chrome lignosulfonate and oil may fail the 30,000 ppm require-
ment for offshore discharge. Bioassay on many commercial drilling
fluid additives have also been conducted (Offshore, 199la and 199Ib).
Oil-based muds using diesel are more toxic than those using mineral
oils. Studies have shown that the toxicity of mineral oils can be 5 to
14 times lower than diesel (Wojtanowicz, 1991), The mechanisms of
toxicity reduction has been attributed to a reduced content of aromatic
hydrocarbons in mineral oils and a low water solubility of the toxic
components that are present, Diesel oil typically has between 30% and
60% aromatic compounds, while some mineral oils have virtually no
aromatic compounds.
Conklin and Rao (1984) reported that the toxicity of whole drilling
fluid on grass shrimp varies significantly with its formulation. The
addition of diesel oil to the drilling fluids at a level of 0,9% increased
the toxicity to grass shrimp by a factor of about 200, while the addition
of mineral oil at the same concentration increased toxicity by a factor
of about 50.
One of the difficulties with conducting bioassays on drilling muds
is that new additives and formulations are continually being developed.
The high cost of bioassays makes it difficult to justify bioassays on
all conceivable combinations of additives and formulations. One
approach that has been suggested to minimize the number of bioassays
conducted is to measure the toxicity of the individual additives
and then use an appropriate mathematical model to estimate the
toxicity of their combinations. One mathematical model that has been
proposed is to add the mass weighted reciprocals of the LC 50 values
of all constituents.

x
! = v i
LC,, " ^LC (3-5)

where x. is the mass fraction of the i component. Toxicity measure-
ments on additives and their combinations have shown that this model
results in calculated LC 50 values for mixtures that are significantly
lower than those measured, i.e., the mixture is less toxic than predicted
by this formula (Parrish et al., 1989).
Drilling fluids can have significant sublethal effects on marine
organisms. Parrish and Duke (1990) have summarized the work of a