STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 685
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Figure 4-115. Schematic of toxicity test for drilling fluids [32A]. (Courtesy SPE.)
Results of these experiments usually give LC,,s ranging from 25,000 pprn to
greater than 1,000,000 pprn of the phase for a variety of muds.
The mysid shrimp, Mysidopsis bahiu, is the test organism for the liquid and
suspended particulate phases. This species has been shown to be exceptionally
sensitive to toxic substances and is considered to be a representative marine
organism for bioassay testing by EPA. An LC,, is determined the suspended
particulate phase (SPP) bioassay tests.
Low-Toxicity Drilling Fluids
The large number of existing oilfield facilities operation in or discharging
produced water into surface waters of the United States has prompted EPA to
issue general NPDES permits. The general permit allows discharge of low-toxicity
drilling fluid directly to the sea.
These "generic" muds were identified by reviewing the permit requests and
selecting the minimum number of mud systems that would cover all those named
by the prospective permittees. Eight different mud systems were identified
that encompass virtually all water-based muds used on the OCS (Table 4-53)
[32A]. Instead of naming a set concentration for each component in each mud
system, concentration ranges were specified to allow the operators sufficient
flexibility to drill safely.
There are several significant permit conditions. As with all other OCS permits,
the discharge of oil-based muds is prohibited. Similarly, the permit does not
unconditionally authorize the discharge of any of the eight generic muds. Their
discharge is subject to limitations on additives. To monitor the use of mud
additives, the permit requires the additive not to drop or to decrease the 96-hr
median lethal concentration (LC,,) test below 7,400 pprn on the basis of the
suspended particulate phase or 740 ppm for the whole mud. This parameter is
based on a test of Generic Mud 8, which is formulated with 5% mineral oil.
There is a mud-discharge-rate limitation of 1,000 bbl/hr, with reduced rates
near areas of biological concern. The discharge of mud containing diesel for
lubricity purposes is prohibited.