Summary
The offshore oil and gas industry has moved toward the use of
synthetic-based drilling fluids (SBFs), changing potential exposure scenarios
for discharged cuttings when compared to those of water-based drilling fluids
(WBFs). Unlike WBFs, SBFs sorb predominately to particles in the cuttings and
are not dispersed extensively into the water column; therefore, a
sediment-toxicity test was required by the US Environmental Protection Agency
(EPA) in addition to the existing water-column test to define a
best-available-technology (BAT) limit. Inclusion of a sediment-toxicity test
for National Pollutant Discharge Elimination System (NPDES) compliance was
precedent-setting and unique. To fulfill the US EPA requirements, an
interindustry research group worked with the EPA to develop a suitable test
that met the technology-based discharge standard. Toxicity of discharged field
drilling fluid is compared to a reference SBF (C16–C18
internal olefin), and, for compliance, the ratio of the
reference-drilling-fluid median lethal concentration (LC50) to the
field-mud LC50 must be ≤1.0. Before the use of this test, there were
concerns that false positive results could lead to incorrectly identified
noncompliance events, limiting the use of SBF technology. Consequently, initial
application allowed the use of a variability factor (K-factor) in
determining the ratio. After use of the compliance test was initiated, research
was continued to reduce test variability and minimize false positives. This
research included (1) analysis of NPDES-compliance data (500+ tests), (2) two
interlaboratory testing programs, (3) analysis of reference-fluid data from one
commercial laboratory, and (4) refinements to test sediment type and effects on
animal health. The results of these efforts to date are reviewed in this paper
and are used to identify potential improvements in the application of the test
as a regulatory tool.
Introduction
The environmental safety of industrial activities in the US has evolved as a
self-monitoring system with permittees testing and reporting compliance results
to regulatory authorities. For offshore operations in the oil and gas
extraction industry, protection of marine resources is of paramount importance.
In the US, resources can be protected through the use of water-quality-based
and technology-based compliance limits for operations. Water-quality-based
limits follow risk-assessment principles, where estimated or measured exposures
in the environment may be measured against numerical standards such as
concentration of a chemical indicator (e.g., zinc, cadmium, mercury).
Technology-based limits reflect BAT for a specific purpose or industrial
sector. These limits are derived from the best possible performance of a given
technology for controlling possible environmental impacts. In contrast to
individual-chemical or single-parameter measurements, toxicity tests have been
used increasingly for both water-quality-based and technology-based compliance
in other industries and regulatory jurisdictions. These tests reflect a desire
to integrate the total environment into the assessment of the combined
potential effects of substances such as effluents and wastes. Laboratory
toxicity tests using an effluent or waste sample essentially integrate the
environmental exposures from all potential toxicants or environmental
conditions.
As with any assessment endpoint for such a test, the results exhibit
inherent variability, such that bright-line limitations (e.g.,
compliance is ³ 30,000 ppm) may result in false positive and negative results,
the former leading to violations and the latter to potential environmental
damage. A false positive is an incorrect judgment that the result is not in
compliance when it is; a false negative is judging the outcome to be in
compliance when in fact it is not. Both present challenges to efforts to
standardize testing methods to limit variation resulting from chance. The
introduction or development of such methods requires interlaboratory and
intralaboratory testing programs to calibrate variability, a goal that has been
accomplished already for water-based drilling fluids (WBFs) (Ray et al. 1989)
and effluents (DeGraeve et al. 1992).
The offshore oil and gas industry has controls in place for assessing
performance relative to both water-quality-based and technology-based limits
using toxicity tests. Water-quality-based toxicity limitations are used
routinely in the discharge of produced water, and technology-based limits call
for toxicity testing of WBFs. The discharge of WBF must pass an LC50
>30,000 ppm test of a suspended-particulate phase (SPP) of the
drilling-fluid sample. The industry has an exemplary record of regulatory
compliance in the Gulf of Mexico, and even when exceedences occur, they are
treated with serious attention and initiate an appropriate response to
understand and correct the incident.
Biological testing to a permit compliance limit has been used for WBFs in
the offshore oil and gas industry since 1986, when 96-hour LC50
testing was implemented for the SPP of WBFs. The water-column-type exposure
used the bottom- and water-column-dwelling, shrimp-like mysid, Mysidopsis
bahia, as the test organism. Early laboratory-testing difficulties were
observed and noted as with the introduction of any new test method. However,
biological-test variations seemed larger than those obtained with analytical
chemical testing (Ray et al. 1989; Engelhardt et al. 1989).
In the last 10 years, SBFs have been recognized as having advantages for
cost-effective drilling, including reducing drill time at a site and reducing
discharge volume, which also contributes to pollution prevention.
The behavior of WBFs and SBFs is different upon discharge because of
different water-solubility and dispersion behavior. SBFs sorb predominately to
particles in the cuttings, falling to the bottom with the cuttings, and are
therefore not dispersed extensively into the water column. The mysid
water-column test used for WBF discharge was therefore supplemented with a
sediment-toxicity test that would reflect probable field conditions. The US EPA
proposed using such a test for compliance with a BAT limitation for discharge
of SBFs. Although toxicity testing for regulatory compliance has been performed
since the early 1960s on effluents in the US through the NPDES process, the
proposal for a BAT assessment using a sediment test was without precedent. This
paper reflects on the development of the method and on efforts by industry to
understand and limit the inherent variability associated with this test. The
discussion uses key data and narratives to illustrate test development,
compliance issues, and ongoing work to date.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
18 January 2005
- Revised manuscript received:
30 November 2006
- Manuscript approved:
22 January 2007
- Version of record:
20 June 2007
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