Composition, environmental fates, and biological effects of water based drilling muds and cuttings discharged to the marine environment: a synthesis and annotated bibliography.

Abstract

From the Abstract: “Water based drilling muds (WBM) consist of fresh or salt water containing a weighting agent (usually barite: BaSO4), clay or organic polymers, and various inorganic salts, inert solids, and organic additives to modify the physical properties of the mud so that it functions optimally. Drill cuttings are particles of crushed rock produced by the grinding action of the drill bit as it penetrates the earth.

The U.S. Environmental Protection Agency, or a state environmental agency designated by EPA, regulates discharges of drilling muds and cuttings to state and federal waters of the U.S. Current National Pollutant Discharge Elimination System (NPDES) permits allow discharge of WBM and cuttings to federal, but not state, waters if they meet restrictions in the Effluent Limitation Guidelines (ELG).

WBM are pumped from the mud tanks on the platform down the hollow drill pipe and exit the drill string through holes in the drill bit. They sweep cuttings from the drill bit up the space between the drill string and the wall of the well (the annulus) to the platform deck. The mud/cuttings mixture is passed through separation equipment that separates the cuttings from the drilling mud, which is returned to the mud tanks for recirculation down-hole. If permitted by local regulations, the cuttings may be discharged to the ocean. Periodically, some of the drilling mud may be discharged as well. The total mass of WBM and cuttings discharged per exploratory well is about 2000 metric tons/well, and somewhat less for most development wells.

When WBM and cuttings are discharged to the ocean, the larger particles and flocculated solids, representing about 90 % of the mass of the mud solids, form a plume that settles quickly to the bottom. The remaining 10 % of the mass of the mud solids consisting of fine-grained unflocculated clay-sized particles and a portion of the soluble components of the mud form another plume in the upper water column that drifts with prevailing currents away from the platform and is diluted rapidly in the receiving waters. In well-mixed ocean waters, drilling muds and cuttings are diluted by 100-fold within 10 m of the discharge and by 1000-fold after a transport time of about 10 minutes at a distance of about 100 m from the platform. Because of the rapid dilution of the drilling mud and cuttings plume in the water column, harm to communities of water column plants and animals is unlikely and has never been demonstrated.

WBM and cuttings solids settle to and accumulate on the sea floor. If discharged at or near the sea surface, the mud and cuttings disperse in the water column over a wide area and settle as a thin layer of a large area of the sea floor. If mud and cuttings are shunted to and discharged just above the sea floor in order to protect nearby sensitive marine habitats, the drilling solids may accumulate in a large, deep pile near the discharge pipe.

The accumulation of mud and cuttings on the bottom, the cuttings pile, may contain higher concentrations of several metals, particularly barium (from drilling mud barite), and sometimes petroleum hydrocarbons than nearby uncontaminated sediments. Chromium, lead, and zinc are the metals, in addition to barium, that are most often enriched in cuttings pile sediments. Until EPA placed limits on the concentrations of cadmium and mercury concentration in drilling mud barite in 1993, some of the barite used in drilling muds contained elevated concentrations (compared to concentrations in natural marine sediments) of several metals. Cuttings piles containing these muds often contained elevated concentrations of several metals.

Barite has a very low solubility in sea water. The metals associated with drilling mud barite are present as insoluble sulfide minerals. Several laboratory and field studies have shown that the metals associated with drilling mud barite or cuttings piles have a low bioavailability to marine animals; they do not accumulate in the tissues of bottom-living animals.

Small amounts of petroleum products may be added to WBM for lubrication or they may be associated with the cuttings drilled during penetration of some geologic formations. These hydrocarbons may accumulate in the cuttings pile. Although WBM containing petroleum hydrocarbons are more toxic than those without any hydrocarbons, WBM cuttings piles usually do not contain sufficient petroleum hydrocarbons to harm bottom-living communities. Petroleum hydrocarbon concentrations greater than about 50 to 60 mg/kg in cuttings piles often are associated with altered benthic community diversity. These concentrations usually are not observed unless synthetic base mud cuttings or oil based mud cuttings were discharged.

WBM are non-toxic or practically non-toxic to marine animals, unless they contain elevated concentrations of petroleum hydrocarbons, particularly diesel fuel. Most drilling mud ingredients are non-toxic or used in such small amounts in WBM that they do not contribute to its toxicity. Chrome and ferrochrome lignosulfonates are the most toxic of the major WBM ingredients. Although used frequently in the past in the Gulf of Mexico, these deflocculants are being replaced in most WBM by non-toxic alternatives to reduce the ecological risk of drilling discharges.

Many field monitoring studies, mostly in the U.S. Gulf of Mexico and the North Sea, have been performed since the 1970s to determine short- and long-term impacts of drilling discharges on the marine environment. As a general rule, effects of WBM and cuttings discharges on the bottom environment are related to the total mass of drilling solids discharged and the relative energy of the water column and benthic boundary layer at the discharge site. In high energy environments, little drilling waste accumulates on the sea floor and adverse effects of the discharges can not be detected. In low-energy environments or where mud and cuttings are shunted to near the sea floor, large amounts of mud and cuttings solids may accumulate on the sea floor and adversely affect bottom communities within a few hundred m of the discharge.

Effects of WBM cuttings piles on bottom living biological communities are caused mainly by burial and low sediment oxygen concentrations caused by organic enrichment. Toxic effects, when they occur, probably are caused by sulfide and ammonia byproducts of organic enrichment. Recovery of benthic communities from burial and organic enrichment occurs by recruitment of new colonists from planktonic larvae and immigration from adjacent undisturbed sediments. Ecological recovery usually begins shortly after completion of drilling and often is well advanced within a year. Full recovery may be delayed until concentrations of biodegradable organic matter decrease through microbial biodegradation to the point where surface layers of sediment are oxygenated.”