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 Today
the major challenge in formulating drilling fluids is to
meet the increasingly demanding conditions of high temperature
and pressure found in some deep wells and extended reach
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 minimum possible
environmental impact. Environmental concerns are a major
driving force behind current drilling fluids research and
development. Health of rig workers also is an important
influence in the use of drilling fluids, and products 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 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, being confined
to the vicinity of the drilling operation, the environmental
impact near the rig can be significant. The degree of impact
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 to oil-based. (In contrast, discharges of drilling
waste on land have different types of impact and the salt
content of mud may pose more of a problem than the hydrocarbon
content.)
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Biodegradation
Biodegradation is the breakdown of an organic
substance, such as oil, by the action of living organisms,
usually microorganisms and especially bacteria. Some
substances biodegrade more rapidly and more completely
than others. Ultimate biodegradation results
in a compound being converted to water and carbon
dioxide. Some substances may degrade to smaller, intermediate
molecules. This is called primary degradation. These
molecules are usually intermediates in the process
of ultimate biodegradation, but can in some instances
be more persistent or more toxic than the original
pollutant.
Biodegradation may occur under both
aerobic (with oxygen) and anaerobic (without oxygen)
conditions. If the contaminant is well dispersed in
water there is usually more oxygen available for aerobic
biodegradation. Water-based mud is more readily dispersed
because it is water-soluble. Oily cuttings do not
disperse as well and tend to settle in a small area
of the seabed near the rig. This high concentration
of organic material can result in the generation of
anaerobic conditions when rapid bacterial activity
uses up the available oxygen in an area
Bioaccumulation
The accumulation of chemicals in an organism's
cells is called bioaccumulation. This amount of bioaccumulation
depends on the balance between the rate at which the
substance enters the organism's cells and how quickly
it is broken down or excreted. If an organism ingests
a small amount of a pollutant it may be able to eliminate
it without significant accumulation, however if the
organism is unable to eliminate the contaminant from
its body then bioaccumulation will result. Alternatively,
when an environment is heavily contaminated an organism
may absorb more of the substance than it can excrete
in the same amount of time. Bioaccumulation will result
unless the concentration of the contaminant is reduced.
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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 cuttings piles which blanket parts of
the sea bed. High concentrations of organic material such
as oil can have a profound effect on plants and animals
living on the seabed. As the organic matter 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.
Surrounding the immediate area of the rig there is a recovery
zone where there are plants and animals that are able to
tolerate some degree of pollution. The less tolerant organisms,
which live further from the source of pollution, gradually
reappear closer to the rig as the site recovers. Most of
the disruption occurs within 500 meters (about 1600 feet)
of the rig site, but some biological effects have been reported
as much as 10 km (over 6 miles) away. When drilling offshore
in regions where there are 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 entrained drilling fluid, speeding up recovery
of the sea bed.
The environmental impact of cuttings contaminated with
oil-based muds has resulted in severe restrictions of their
use in many parts of the world and has led to the development
of more environmentally friendly, synthetic-based drilling
fluids that not only perform well but are less toxic and,
in most cases, more biodegradable.
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 are also used to 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 upon local legislation 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 who are more likely to be exposed to water-
soluble substances.
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 shaker screens
low-viscosity fluids separate more readily from the cuttings.
This improves the recovery of drilling fluid and reduces
the amount of organic material discharged to 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.
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