Passage A

Drilling fluids, including the various mixtures known as drilling muds, play essential roles in oil-well drilling. As they are circulated down through the drill pipe and back up the well itself, they lubricate the drill bit, bearings, and drill pipe; clean and cool the drill bit as it cuts into the rock; lift rock chips (cuttings) to the surface; provide information about what is happening downhole, allowing the drillers to monitor the behavior, flow rate, pressure, and composition of the drilling fluid; and maintain well pressure to control cave-ins.

Drilling muds are made of bentonite and other clays and polymers, mixed with a fluid to the desired viscosity. By far the largest ingredient of drilling muds, by weight, is barite, a very heavy mineral of density 4.3 to 4.6. It is also used as an inert filler in some foods and is more familiar in its medical use as the "barium meal" administered before X-raying the digestive tract.

Over the years individual drilling companies and their expert drillers have devised proprietary formulations, or mud "recipes," to deal with specific types of drilling jobs. One problem in studying the effects of drilling waste discharges is that the drilling fluids are made from a range of over 1,000, sometimes toxic, ingredients—many of them known, confusingly, by different trade names, generic descriptions, chemical formulae, and regional or industry slang words, and many of them kept secret by companies or individual formulators.

Passage B

Drilling mud, cuttings, and associated chemicals are normally released only during the drilling phase of a well's existence. These discharges are the main environmental concern in offshore oil production, and their use is tightly regulated. The discharges are closely monitored by the offshore operator, and releases are controlled as a condition of the operating permit.

One type of mud—water-based mud (WBM)—is a mixture of water, bentonite clay, and chemical additives, and is used to drill shallow parts of wells. It is not particularly toxic to marine organisms and disperses readily. Under current regulations, it can be dumped directly overboard. Companies typically recycle WBMs until their properties are no longer suitable and then, over a period of hours, dump the entire batch into the sea.

For drilling deeper wells, oil-based mud (OBM) is normally used. The typical difference from WBM is the high content of mineral oil (typically 30 percent). OBMs also contain greater concentrations of barite, a powdered heavy mineral, and a number of additives. OBMs have a greater potential for negative environmental impact, partly because they do not disperse as readily. Barite may impact some organisms, particularly scallops, and the mineral oil may have toxic effects. Currently only the residues of OBMs adhering to cuttings that remain after the cuttings are sieved from the drilling fluids may be discharged overboard, and then only mixtures up to a specified maximum oil content.