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 Environment Index
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Drill Cuttings
What are drill cuttings?
In the same way that sawdust is produced when a hole is drilled in wood by a
domestic drill, small pieces of rock – called cuttings – are created when a
well is drilled through rock to reach an oil or gas reservoir. These
cuttings vary in size and texture, ranging from fine sand to gravel,
depending on the type of rock being drilled and the type of drill being
used. To prevent the well being clogged, the cuttings are carried back to
the surface by a special fluid which is pumped down the well to keep it
clean and also to lubricate the drill bit and to control pressure within the
well. This fluid is known as ‘mud’ because of its appearance and
consistency.
Fig 1 - Drill Cuttings
On the drilling rig the cuttings are separated from the mud; the mud is
recycled to be used again and the cuttings are either; discharged to the
seabed, re-injected into a well or taken ashore for treatment and disposal.
The choice of disposal route depends on the type of drilling mud being used
and the location of the well.
One of the most effective drilling fluids is oil based mud (OBM). Although
most of the OBM is removed from cuttings on the rig, some adheres to the
cuttings and is discharged. OBM coated cuttings are ‘sticky’ and do not
disperse readily when discharged, resulting in accumulations of cuttings and
OBM beneath installations. When the environmental implications of this
discharge became apparent the industry, in collaboration with regulators,
introduced a voluntary reduction programme. This led ultimately to
regulations which effectively banned the discharge of OBM in 1996.
Many wells, particularly in the southern gas basin of the North Sea, were
drilled with water based mud (WBM). Although some of the WBM is discharged
with cuttings it readily disperses and has not been shown to have a
significant environmental impact.
Where do drill cuttings accumulations occur?
In the southern sector of the North Sea, the vigorous currents and wave
action rapidly disperses the cuttings and drilling mud, so that there are no
significant accumulations in this sector.
In the northern and central areas, cuttings accumulations remain relatively
undisturbed by wave action and currents because of the water depth. In these
sectors there are significant accumulations – usually referred to as
‘cuttings piles’ – beneath most installations. The lack of disturbance,
together with the physical nature of the cuttings piles, slows down the
release and degradation of OBM within the piles. However, some erosion does
occur and together with the effect of bio-turbation, the surface of the pile
is continually changing.
How to manage these ‘legacy’ piles and the oil within them is the big
question that the industry has been working on for some years.
What do we know about the effects of cuttings accumulations on the marine
environment?
Between 1999 and 2005, UKOOA (now Oil &Gas UK) undertook a three phase Drill
Cuttings Initiative to understand better the physical characteristics of
cuttings piles, their environmental impact and the options for long term
management.
The reports from each phase of the initiative can be accessed at
http://www.oilandgasuk.co.uk/issues/drillcuttings/reports.cfm
What action is being taken on this issue?
The conclusion of the Drill Cuttings Initiative was that cuttings piles on
the UKCS do not pose an environmental threat which requires immediate
remedial action. A range of potential management options for cuttings piles,
ranging from removal to leaving in place were identified and that the best
option would be decided on a case by case basis following detailed
assessment. Because there is no immediate environmental threat the
management options can be included as part of the final installation
decommissioning programme.
These findings were incorporated in OSPAR Recommendation 2006/5 which
requires all cuttings piles to be screened to ensure that they do not exceed
the thresholds which would indicate a need to take immediate remedial
action. Oil & Gas UK is working closely with DBERR and member companies to
gather the appropriate data. |
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Environment Index
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