STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drilling Muds and Completion Fluids 683
originating with hydrogen sulfide in drilled formations. Because human safety
is at stake, there can be no compromising effectiveness, and substitutes for zinc
have not seemed to be effective. Fortunately, most drilling situations do not
require the addition of sulfide scavengers.
Indiscriminate storage/disposal practices using drilling mud reserve pits can
contribute toxicity to the spent drilling fluid as shown in Table 4-52 [31]. The
data in Table 4-52 is from the EPA survey of the most important toxicants in
spent drilling fluids. The survey included sampling active drilling mud (in
circulating system) and spent drilling mud (in the reserve pit). The data show
that the storage disposal practices became a source of the benzene, lead, arsenic,
and fluoride toxicities in the reserve pits because these components had not
been detected in the active mud systems.
The third source of toxicity in drilling discharges is drilled rocks. A recent
study [32] of 36 cores collected from three areas (Gulf of Mexico, California,
and Oklahoma) at various drilling depths (ranging from 300 to 18,000 ft)
revealed that the total concentration of cadmium in drilled rocks was over five
times greater than cadmium concentration in commercial barites. It was also
estimated, using a 10,000-ft model well discharge volumes, that 74.9% of all
cadmium in drilling waste may be contributed by cuttings while only 25.1%
originate from the barite and the pipe dope.
Mud Toxicity Test. Presently, the only toxicity test for drilling fluids having an
EPA approval is the Mysid shrimp bioassay. The test was developed in the mid-
1970s as a joint effort of the EPA and the oil industry.
A bioassay is a test designed to measure the effect of a chemical on a test
population of organisms. The effect may be a physiological or biochemical
parameter, such as growth rate, respiration, or enzyme activity. In the case of
drilling fluids, bioassays lethality is the measured effect.
To quantify the effect of a chemical on a population, groups of organisms
are exposed to different concentrations of the chemical for a predetermined
interval. The concentration at which 50% of the test population responds is
known as the EC,, (effective concentration 50%); when death is the measured
response, it is called the LC,, (lethal concentration 50%).
The LC,, concept is visualized in the dose-response curve presented in
Figure 4-114 [32A]. The dose or concentration is plotted on the abscissa, and
T0)aCANT
Bemene
Lsad
Batium
Arsenic
Fluaride
ACTM ~ECTIONRESERVEDETECTW
EmJo ME% PIT ME%
No - YES 39
No - YES loo
YES loo YES loo
No YES 52
No - YES loo