Several metals are present in most drilling muds (Table 1). Concentrations of individual metals may vary depending on the composition of the base ingredients and additives. The metals of greatest concern because of their abundance in drilling muds and cuttings and their potential toxicity to marine organisms include arsenic, barium, chromium, cadmium, copper, iron, lead, mercury, nickel, and zinc. Some of these metals are added intentionally to drilling muds as metal salts and organo-metallic compounds, if approved by local regulations. These metals are included in the added metals category. Most metals in drilling discharges are trace impurities in drill cuttings and major mud ingredients, particularly barite, ilmenite, and clay.
|Table 1. Concentrations of several metals in drilling muds, drilling mud barite, world sediments, and Norwegian continental shelf (NCS) reference (background) sediments. Ratio of metals concentrations in NCS barite to background sediments is included. Metals with a ratio greater than 1 are highlighted in gray.|
|World Sediments||NCS Sediments(b)||NCS
|Arsenic||1,8 − 2.3||2,2||NV||6,9 − 26||NV||≈ 0,33(c)|
|523 000||NV||1 − 2000||4,6 − 554||114 000(d)|
|Cadmium||0,16 − 54,4||0,03||0,7 − 1,7||0,1 − 0,6||0,003 − 0,13||18,9|
|11||9,8 −14,3||36 − 110||2,58 − 39,2||0,90|
|Copper||0,005 −||9,7||76,6 −||7 − 33||0,3 − 17,2||18,7|
|Lead||0,40 −||7,8||48,7 −||10 − 33||1,92 − 46,5||4,8|
|Mercury||0,02 − 10,4||0,12||0,31 −
|0,03 − 0,14||0,003 − 0,10||14,8|
|Nickel||3,8 − 19,9||NV||1,2 − 2,1||13 − 45||NV||≈ 0,09(d)|
|Vanadium||14 − 28||NV||NV||63 − 238||NV||0,44(e)|
|Zinc||0,06 −||8,6||42,9 −||27 − 88||0,42 − 83,7||2,1|
- a Data for 2001 – 2003;
- b Based on samples from 150 reference stations in North Sea;
- c North Sea background concentration not available, lowest value for world sediments used;
- d Concentration in NCS barite unknown, mean value for U.S. barite used;
- e Ratio of concentration in drilling muds to concentration in world sediments used. NV: no value available.
Weighting agents and clays are the main source of heavy metals in drilling discharges to the ocean on the NCS, because use of additives containing heavy metals is strictly regulated. In 2004, 20,26 kg of lead and copper in pipe dope and 3600 kg of As, Pb, Cd, Cu, Cr and Hg as impurities in weight materials were discharged to the NCS.
The metals most frequently present in drilling muds at concentrations substantially (>100-fold) greater than natural concentrations in soils and sediments are barium, chromium, lead, and zinc (Table 1). Mercury sometimes is present at elevated concentrations in US, Canadian, and North Sea drilling muds; it is derived from mercury contamination of drilling mud barite (Neff, 2002b). Impure grades of barite also may contain elevated (compared to natural sediments) concentrations of all the other metals listed in Table 1. Because of concern about possible adverse environmental impacts of metal contamination of drilling mud barite, many countries are encouraging or requiring use of cleaner grades of barite for drilling mud formulation, or are recommending use of alternative weighting agents, such as ilmenite, that contain lower concentrations of metals of highest concern. As a result,
mean concentrations of mercury and several other metals in ocean discharges of drilling muds and cuttings have decreased in the last decade.
The most abundant metal in most drilling muds is barium (actually an alkaline earth element like calcium and strontium). Nearly all the barium in drilling mud is from barite (BaSO4) added to the mud to increase its density. Barite in drilling muds and sediments has a low solubility in seawater, because
of the high natural concentration of sulfate in the ocean. Because it is insoluble in seawater, barite has a low bioavailability and toxicity to marine organisms. Although barite is a PLONOR chemical (not expected to cause harm to the marine environment), it should be included among chemicals used in risk calculations of drilling discharges, because of the large volumes of barite used in many drilling muds and the potential physical disturbance (burial and change in grain size).
Most of the other metals detected in drilling muds are present as trace impurities in barite, clay, or the drill cuttings from the geologic formations. In addition to barium, the most abundant metals in barite are lead, zinc, and iron; some barites also contain elevated concentrations of chromium. Iron usually is present in drilling muds and cuttings at high concentrations; however, because it is present in drilling wastes primarily as highly insoluble oxides or in the matrix of clay particles at concentrations similar to or lower than background concentrations in marine sediments, iron is regarded as of no concern. Five metals, other than barium, are present in a typical North Sea drilling mud barite at concentrations higher than the North Sea sediment background concentrations reported by Bjørgesæter (2006) (Table 1). These metals are cadmium, copper, lead, mercury, and zinc.
Frequently in the past, chromium was the only metal other than barium that was detected at elevated concentrations in sediments near drilling discharge locations (Neff et al., 1989). When present at elevated concentrations, compared to concentrations in clean sediment, drilling mud chromium was derived primarily from chrome- or ferrochrome-lignosulfonates or chromate salts added intentionally to the mud for viscosity control. Because of concern about the toxicity of chromium, most operators now use alternative, less toxic viscosifiers for WBM formulation. Chrome lignosulfonates are no longer used in drilling muds on the NCS; they are rarely used in the U.S. Gulf of Mexico. Their use in offshore drilling muds elsewhere in the world is unknown. Clays, the other major source of drilling mud chromium, have been replaced in many modern WBMs by organic polymers, mostly carbohydrates like carboxymethyl cellulose, decreasing the need for chemical viscosity-control chemicals. Thus, the mass of chromium discharged to the ocean in drilling wastes has declined substantially over the last decade. Considering the abundance of chromium in some cuttings piles, attributable to past discharges, the high toxicity of many chromate (CrVI) salts and the presence as a trace impurity in weight materials, chromium should be included among the metals used to model the ecological risks of drilling discharges in sediments.
Summary: Metals to Include in the Risk calculations for Drilling
Based on the criteria for selection of chemicals; relative concentration in drilling mud and cuttings, potential bioavailability and toxicity or potential for other non-toxic disturbances (burial, oxygen depletion etc.) to marine organisms (based on evidence from field monitoring studies of environmental
impacts of the chemicals), the following metals are recommended for inclusion in the EIF calculations for the sediment: cadmium, chromium, copper, lead, mercury, and zinc. Concentrations of nickel in drilling discharges usually are about 10 times below concentrations in natural sediments and are therefore regarded of no concern for sediments, but will be included for risk calculations in the water column. Barium is used as a tracer of drilling discharges in environmental monitoring, but is not considered toxic and, therefore the contribution to the risk for barium will not be included.
All the metals, except chromium, chosen for inclusion in the toxicity EIF for sediments, also were chosen for inclusion in the toxicity EIF for the water column. Nickel also was selected for inclusion in the toxicity EIF for the water column. As discussed above, slightly soluble chromium compounds are
no longer used in WBM discharged to the NCS, Gulf of Mexico, or most other offshore oil development areas. Concentrations of chromium in barite and clays usually are below those in natural marine sediments (Table 1). Therefore, chromium was not included in the toxicity EIF for the water column.