Cumulative Bioluminescence - A Potential Rapid Test of Drilling Fluid Toxicity: Development Study
- A.K. Wojtanowicz (Louisiana State U.) | B.S. Shane (Louisiana State U.) | P.N. Greenlaw (Louisiana State U.) | A.V. Stiffey (Naval Oceanographic and Atmospheric Research Laboratory)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1992
- Document Type
- Journal Paper
- 39 - 46
- 1992. Society of Petroleum Engineers
- 1.14.1 Casing Design, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.11 Drilling Fluids and Materials, 4.1.5 Processing Equipment, 1.8 Formation Damage, 1.6 Drilling Operations
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A new rapid test of drilling fluid toxicity is based on the spontaneous bioluminescence of Pyrocystis lunula, an easy-to-culture alga that vigorously responds to shear stress (mixing) by emitting a sharp burst of light. In contrast to other bioluminescence methods, a cumulative flux of light is measured with a photomultiplier that eliminates the effect of exposure time on test results. Light quenching, caused by the presence of a toxicant, results in the dose/response relationship (DSR) typical for the enzymatic reaction kinetics. The Michaelis-Menten (dissociation) constant is used as a direct measure of toxicity. 'The evaluation study involved multiple experiments with 60 samples of drilling fluids from the U.S. gulf coast, as well as such typical toxicants as diesel oil, mineral oil, and chrome lignosulfonate (CLS). The results of the test error analysis and comparisons with the Microtox and Mysid shrimp assays are reported.
When drilling fluids were first exposed to environmental scrutiny in the late 1970's, the main issue was whether the fluids were toxic. After several years of extensive research, the broadly accepted conclusion was ,that most drilling fluids were low-toxicity compounds (except for oil-based muds), but they might be drastically polluted with small amounts of agents conventionally used to control their properties. Examples of such agents are mud thinners (CLS and polycarboxylic acid salts), bactericides (isothiazolin mixtures and paraformaldehyde), lubricating compounds and spotting fluids (mineral and diesel oils), and H2S scavengers (zinc oxide). Elimination of these toxicants through substitution seemed to be the simplest solution. Many low-toxicity substitutes, however, proved to be less effective than their more toxic counterparts. During the 1980's, it was discovered that drilling fluids neutralized the, strength of several toxicants1-3 (i.e., at the same concentration of a toxicant, the resulting toxicity was higher in ambient water than in drilling fluid). This finding led to two conclusions: (I) synergistic effects of toxic components in drilling fluids are suppressed by adsorption onto clay particles and (2) useful (though toxic) agents can be tolerated in a drilling mud system at concentrations higher than those predicted with the principle of additive toxicity. This means that drilling fluids can be prepared and maintained with potentially toxic substances and still meet effluent discharge limitations only if the toxicity is measured and controlled.
Currently, the only toxicity test for drilling fluids approved by the U.S. Environmental Protection Agency (EPA) is the Mysid shrimp bioassay. The test is used as the criterion for offshore overboard discharge limitations, new mud formulation assessments, and approval of new mud additives. The EPA set the Mysid shrimp test LC50 value of 30,000 ppm as the maximum toxicity allowed to maintain the Natl. Pollutant Discharge Elimination System (NPDES) general permit for a drilling mud discharge into the waters of the U.S. Outer Continental Shelf (OCS) (EPA Regions II, IV, VI, IX, and X). The toxicity testing of a new mud must be conducted on the laboratory-formulated mud sample containing the highest concentration of additives sought for approval. Also, LC50=100,000 ppm was proposed as a minimum required value for approval of new additives of drilling fluids in EP A Region IX.4
The Mysid shrimp bioassay has been criticized for its imprecision and inconvenience in practical applications.5,6 Its 96-hour duration and the offsite location of a testing laboratory make the test's turnaround time as long as 2 to 3 weeks.5 Such a long time effectively eliminates, any onsite control of a mud system currently in use. The test is relatively complex and requires the expertise of an experienced microbiologist. The oil industry's recent performance evaluation of 12 toxicity laboratories revealed a high level of variability in the test results.6 This large variability (up to fivefold difference) was attributed to such factors as the source and the selection technique of shrimp, seawater quality, and inconsistent laboratory procedures. Control of mud toxicity requires precise and more frequent measurements, particularly when the toxicity nears the limiting value of 30,000 ppm, In such cases, the Mysid toxicity test is entirely ineffective because improving its variability through multiple measurements is too expensive. The interlaboratory variability of the Mysid test evaluated by the EPA is notable for the wide range of toxicity results (18,000, 25,000, 26,000, 31,000,38,000,61,000, and 74,000) from six commercial laboratories.7 The data indicate that the first three results are not within environmental compliance, while the last three are. Other research8 showed that the 95% confidence-interval sizes for muds with toxicity values near the critical value of 30,000 ppm ranged from 13,400 to 20,700 ppm, thus making the compliance decision unclear.
Recently, a considerable effort has been made to develop a simple, short-term toxicity test - a rapid bioassay.9-11 The three basic requirements for such a test include a few hours for completion, feasibility for use at wellsites, and correlation with the Mysid shrimp assay. The Microtox toxicity test is considered to be a promising alternative for rapid bioassay.8,9,11 The test is based on the bioluminescence of photobacteria that respond to various concentrations of drilling mud extracts. Unfortunately, the Microtox results correlated poorly with the Mysid shrimp test, particularly. for oil contamination, with Microtox values usually being lower than values from the Mysid shrimp test. The Microtox test also displayed a pronounced sensitivity to the color of the test substrate.
A novel application of the Microtox test is the use of its results in a compliance decision model.11 In this model, the NPDES compliance. decision can be made with both the Microtox test and the oil content measurements. The proposed model, however, has two drawbacks: (1) it gives a very conservative decision if a 100% correct decision rate is desired, and (2) it requires an initial calibration for each drilling fluid system in use.
The new method investigated in this research" in principle, also is based on bioluminescence. However, both the organisms and the measurement are different from those in the Microtox test. The organism is an alga, Pyrocystis lunula, and being a plant, is easy to grow in a flask. The stock solution contains about 2,000 cells/mL and can be readily used for testing during a 1-month period. The . cells are relatively large (0.1 mm long) and thus can be counted easily to control their concentration. When subjected to shear stress through vigorous stirring, the cells emit a strong burst of light that diminishes rapidly after about 1 minute.
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