Drilling Fluids Toxicity Testing
The accepted test for determination of relative mud toxicity, while rather complex, may be summarized as follows:
• A sample of the drilling fluid to be tested is transported under controlled conditions to a laboratory for analysis.
- A 200-cm3 aliquot of mud is mixed with 1000 cm3 of filtered seawater. The total volume is then brought to 2000 cm3 for a 1:9 mud-to-seawater ratio.
- The mud/seawater slurry is then stirred with a magnetic stirrer for five minutes and the pH is adjusted back to that of seawater by the addition of 6N HCl. The slurry is then allowed to settle for one hour, at which time it generally separates into two and sometimes three phases:
- Water Soluble Phase
- Suspended Particulate Phase
- Solid Phase
- At the end of the settling period, the suspended particulate phase (SPP) is decanted into a suitable container (see Figure 3).
- The SPP is then further diluted with seawater to obtain 5 or 6 concentrations (usually 1%, 5%, 10%, 25%, 50%, and 100%) plus a control of 0% SPP. The first "range-finder" test should be done following the guidelines outlined in this chapter.
- After a suitable range-finder number is established, a full 96-hour protocol test is run to obtain the LC50.
- At the end of 96 hours the live mysids are counted in each of the test concentrations. These results are then used to statistically determine an LC50 or lethal concentration at which 50% of the test organisms die. See Figure 4.
- This number is then reported as the 96-hour LC50.
The current LC50 minimum for offshore discharges, as designated by various EPA regions is 30,000 ppm of the suspended particulate phase (See Figure 5). The basis for this number was the toxicity study conducted by EPA's Gulf Breeze laboratory on the eight generic mud compositions (See Figure 6). For these muds, Generic Mud #1 was found to have a 96-hour LC50 of 27,000 ppm and 30,000 ppm when adjusted for control mortality. Therefore, the toxicity limitation of 30,000 ppm for the suspended particulate phase was based on the most toxic generic mud, which was Generic Mud #1. It is important to understand, however, that the bioassay test results do vary and are dependent on several factors. For this reason, field drilling fluids in offshore environments should not be run at LC50's below 150,000 ppm.
Although relative amounts of an additive may not appreciably lower LC50 values for a given mud, trends in bioassay test results must be considered. Each EPA Region and some state agencies have varying discharge regulations. Should an LC50 analysis yield results that are out of compliance with regulatory guidelines, a substantial penalty will be levied on a daily basis against the operator from the last date of compliance.
- Figure 3
Figure 4 Toxicity Testing Results
95% confktonc« interval (44,000 to 72,000 pprn)
Figure 4 Toxicity Testing Results
95% confktonc« interval (44,000 to 72,000 pprn)
Reported LCjp ; (56,000 pprn)
0 30000 60000 90000 120000 150000 180000 Drilling Fluid Concentration, ppm
Reported LCjp ; (56,000 pprn)
0 30000 60000 90000 120000 150000 180000 Drilling Fluid Concentration, ppm
Amoco recommends that the following guidelines for testing all water-based field muds and water-based mud additives. For Amoco's database, either the Amoco 96-hr range finder test or the full 96-hr LC50 protocol test is acceptable.
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