Source of emissions
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. 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.
Many wells, particularly in the southern gas basin of the North Sea, are drilled with water based mud (WBM). Although some of the WBM is discharged with cuttings it readily disperses and tends not to form cuttings piles. There is, however, the potential for these cuttings to contain oil from the reservoir section of an oil well.
Potential environmental impacts
As cuttings piles started accumulating at the feet of installations in the Northern and Central North Sea, oiled cuttings were identified as having the potential for long-term environmental impact on seabed fauna. As a result, UKOOA (now Oil & Gas UK) established a Joint Industry Project (JIP) to generate the detailed information required to develop a robust cuttings piles management strategy for the UK oil and gas industry. The initiative covered a wide range of topics with a significant proportion of the work being focused on the physcial and chemical characterisation of cuttings piles. Please access the JIP reports at the links below:
Most findings are in broad agreement suggesting that the major seabed effects associated with oiled cuttings discharges were smothering and subsequent development of anaerobic conditions due to the microbial degradation of the base oil within a close range of the installation. The impact is therefore expected to be localised and to gradually reduce over time thanks to biodegradation processes occurring in and around cutting piles. The conclusion of the Drill Cuttings Initiative suggests 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 at the time of decommissioning of the installation.
Currently, only water-based cuttings are discharged to sea. These materials are relatively environmentally inert therefore the main mechanism for seabed effects would be via the physical smothering of seabed organisms in areas of high deposition. The long term environmental toxicity and/or food chain effects associated with these materials are deemed to be negligible.
Key control and mitigation measures
Historically, oil based mud was formulated with diesel oil. The replacement of diesel with so called ‘low toxicity’ mineral oil in an attempt to address environmental concerns met with little success.
As the potential environmental implications of these discharge became apparent the industry, in collaboration with regulators, introduced a voluntary reduction programme which led ultimately to regulations which effectively banned the discharge of OBM contaminated cuttings in 1996.
Subsequently, synthetic, or non mineral, base oils were developed but issues with biodegradability were identified and these products were phased out between January 1997 and January 2001.
OSPAR Decision 2000/3 came into effect on 16 January 2001 and effectively eliminated the discharge of cuttings contaminated with oil based fluids (OBF) (includes OBM and SBM) greater than 1% by weight on dry cuttings. The Offshore Chemical Regulations 2002 implement this Decision and require a chemical permit for the use and discharge of chemicals including drilling muds. The Offshore Petroleum Activities (Oil Pollution Prevention and Control) Regulations 2005 also require a permit for the discharge or re-injection of cuttings containing hydrocarbons from the reservoir.
OSPAR agreed Recommendation 2006/5 on a Management Regime for Offshore Cuttings Piles which required all cuttings piles to be assessed against set criteria to determine if any were of immediate environmental concern. This assessment was completed in 2009, showing that existing piles were not of immediate concern and that appropriate management strategies for individual piles could be determined at the time of decommissioning of the installation.
Updated: November 2009