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The major challenge in formulating drilling fluids is to meet the increasingly demanding conditions of high temperature and pressure found in some deep wells, extended reach wells, and horizontal wells while avoiding harm to the environment. The components of drilling fluids should be selected so that any discharge of mud or cuttings has the least possible environmental impact. Environmental concerns are a major driving force behind current drilling fluids research and development. The health of rig workers also is important, and fluids are selected to minimize health risks.

Although fluids are essential for the successful drilling of an oil well, they can also be one of the messier aspects of a drilling operation. Cuttings that are brought up and out of the borehole have to be disposed of, as does any drilling fluid that remains attached to them. And while the environmental footprint at a wellsite is relatively small—mostly confined to the vicinity of the drilling operation—the environmental impact near the rig can be significant. The degree of impact that drilling fluids have on the environment depends on the type of mud used and the prevailing environmental conditions. Offshore, water-based muds are generally the least damaging, compared with oil-based ones. In contrast, discharges of drilling waste on land have different types of impact on the environment. For example, the salt content of mud may pose more of a problem than the hydrocarbon content of the mud.

With many pollutants the impact on the environment is influenced by the way the pollutant is discharged and subsequently dispersed throughout the environment. Oily cuttings, when discharged under water, do not disperse as much as water-based muds and may form piles of cuttings that blanket parts of the seabed. High concentrations of organic materials such as oil can have a profound effect on plants and animals living on the seabed. As the oil decomposes, oxygen is used up and toxic sulfides may be produced. Such conditions can result in the almost total elimination of bottom-dwelling organisms very close to the rig.

In the immediate area surrounding the rig, there is a recovery zone populated by plants and animals that are able to tolerate some degree of pollution. The less tolerant organisms, which live farther from the source of pollution, gradually reappear closer to the rig as the site recovers. Most of the disruption occurs within 500 m (1,600 ft) of the rig site, but some biological effects have been reported as much as 10 km (over 6 mi) away. When there is drilling offshore in regions having strong water currents, the discarded cuttings tend to spread out, leaving a thinner covering of the seabed near the discharge site. This makes them more susceptible to the action of microorganisms that act to degrade the drilling fluid that is carried along with the currents, speeding up recovery of the area.

Why Use Synthetic-Based Drilling Fluids?

The environmental impact of cuttings contaminated with oil-based muds has resulted in severe restrictions on their use in many parts of the world. This has led to the development of more environmentally friendly, synthetic-based drilling fluids that not only perform well but are less toxic. In most cases, these synthetic muds are also more biodegradable.

How Are Drilling Fluids Tested and Regulated?

Regulation of drilling fluids varies according to geographic location and local legislation. Testing is performed to determine the toxicity of various chemicals. Additional tests are performed to gather data about biodegradation and bioaccumulation.

Toxicity Tests

Toxicity tests predict the impact of a pollutant on the receiving environment. The results of these experiments are used to estimate the maximum amount of material that can be discharged without having a direct, toxic effect on the environment. The exact type of test performed depends on the laws of the country in which the drilling site is located, and the likely fate of the contaminant. For example, in some areas oil-based muds are tested on bottom-dwelling creatures known as sediment re-workers. These animals obtain nutrition by eating sediment and are likely to be affected by oily cuttings that accumulate on the seabed. Water-based muds, on the other hand, are tested on fish that are more likely to be exposed to water-soluble substances.

Reducing Environmental Impact

Biodegradation is a key factor in reducing the long-term environmental impact of drilling fluids. Another consideration in drilling-fluid design is reducing toxicity to fish, sediment re-workers, algae, and zooplankton. But it is equally important to reduce the amount of waste generated in the first place. This is achieved by recycling drilling fluids as much as possible and by designing them in such a way as to make this easier to achieve. For example, on shale shakers—the vibrating sieves that are used to remove cuttings from drilling mud on oil rigs—low-viscosity fluids separate more readily from the cuttings. This improves the recovery of drilling fluid and reduces the amount of organic material discharged into the sea.

Drilling fluids began as mud—just clay and water. Now little is the same but the name. Modern muds are designed for a wide range of drilling conditions. Many factors must be carefully weighed and balanced, not the least of which is environmental safety.