Environmental Report 1999 Index

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Drill Cuttings Drill cuttings are waste rock produced when a well is drilled, just as swarf or sawdust are produced by drilling metal or wood. However in an oil well, the cuttings are produced hundreds or thousands of feet underground and have to be carried up to the surface. This task is accomplished by 'drilling mud', which is pumped down to the drill bit and then returns to the surface carrying the cuttings. Mud also cools and lubricates the drill bit, and stops the bore hole from collapsing. In addition, it plays a crucial role in keeping the well under control: if the drill penetrates a reservoir, the weight of the mud stops oil or gas from entering the well and causing a 'kick' or blow out. When the mixture of drilling mud and rock cuttings arrive at the surface, the two must be separated so that clean mud can be pumped back down to the drill bit. The characteristics that make drilling mud an effective agent for carrying rock cuttings inevitably mean that separation is a difficult and complex operation. As a result, some mud always adheres to the cuttings. The name 'drilling mud' is a simple name which does not do justice to the sophistication of these fluids, which are very carefully formulated to achieve specific performance characteristics. While drilling mud is frequently mixed with water to make water-based mud (WBM), very often the required properties can best be achieved if it is blended with a significant quantity of mineral oil. In this case it is called oil-based mud (OBM), and the residual mud adhering to the cuttings will contain some of this oil. Over the last 15 years or so, there have been progressive changes in the limits on the type and amount of residual oil that can be discharged to sea on drill cuttings. These changes have influenced research into the effects of drill cuttings in the marine environment, and also research into improvements in the design and treatment of the drilling muds. The research and resulting technological developments have influenced the direction of subsequent changes in regulations. Technological developments have occurred in a number of areas. Improved techniques for separating mud from cuttings allow more of the mud to be recycled and equipment has been developed to enable cuttings to be re-injected into subsea formations. In addition, treatment and disposal of cuttings at a shore base is becoming more practical as opportunities for recycling and re-using drill cuttings are explored. The most recent regulations affecting drilling mud, which came into force on 1 January, 1997, specify that the amount of oil discharged on drill cuttings must not exceed 10 grammes per kilo of cuttings. However, it is not technically possible to achieve this level of residual oil and, as a result, cuttings coated with OBMs are no longer discharged to sea. Recognising environmental concerns about OBMs, the industry had invested considerable time and effort in developing 'synthetic' mud systems based on specially developed fluids. Whilst the synthetic mud systems bring some improvements these have not been as great as we had thought in all cases and tests carried out by the industry confirmed the biodegradation of some synthetic mud systems is little better than on based muds. In the light of this information the industry has not waited for legislation but instead has anticipated the need for further reductions in synthetic drilling mud discharges. UKOOA member companies have started a voluntary reduction programme that will result in the elimination of virtually all non water-based mud discharges by the end of the year 2000. The use of oil based mud and synthetic mud systems will continue beyond this but the mud will either be injected into an underground reservoir offshore or brought back to shore for treatment and reuse or disposal on land. Cuttings Piles The northern North Sea has an historical legacy of cuttings piles, many of them deposited before 1997 when oil-based mud was commonly used. The piles are very small at single well sites but can reach more than 15m high at large multi-well developments. It is estimated by the Scottish Executive that only one tenth of 1% of the seabed is covered by cuttings piles. Although oil-based muds are no longer discharged in the North Sea, many of the installations that will be decommissioned over the next 20 or so years have OBMs in their cuttings pile. The environmental impacts of cuttings piles remain unclear and the industry has embarked on a programme of research to identify whether cuttings piles give rise to significant environmental issues and, if so, how these should be addressed. In June 1998 UKOOA, supported by the International Association of Oil and Gas Producers (formerly the E&P Forum) and in co-operation with the Norwegian Oil Association, launched an initiative to tackle the legacy of accumulated drill cuttings. The project is being reported in public and has the active participation of Government departments, NGOs and other concerned organisations. Workshops have been held for contractors and research institutions, and an independently chaired Scientific Review Group is being established to review progress on a regular basis. Further research is essential because, to date, there are no proven remediation techniques nor is there clear detail of the consequences of trying to disturb the cuttings piles. The UKOOA Drill Cuttings Initiative aims to fill the gaps and establish solutions that are practicable and acceptable to all stakeholders.