During drilling, a fluid known as drilling mud is used to lubricate the drill bit and carry the drill cuttings to the surface, where they are separated from the mud and cleaned. The mud is reused where possible and the drill cuttings are either discharged or taken ashore for further treatment and disposal. When most of the wells at Brent were developed, it was standard practice to discharge the drill cuttings onto the seabed.
Drill cuttings at Brent
Drilling was carried out on the Brent Field between 1976 and 2004. Drill cuttings were discharged to sea from all four of the Brent platforms having removed the bulk of the drilling fluids. The subsequent prohibition of the discharge to the marine environment of drill cuttings contaminated with more than 1% oil by weight of oil based fluids on dry cuttings (OSPAR Decision 2000/3 on the Use of Organic Phase Drilling Fluids [OPF] and the Discharge of OPF-Contaminated Cuttings) came into force on 16 January 2001.
Notwithstanding this Decision, we had already stopped discharging drill cuttings contaminated with OPF to sea in 1998. Following this, the arrival of new processing equipment on the market which could clean contaminated drill cuttings to the required standards of OSPAR Decision 2000/3 caused a revision of the internal Shell requirements and we began to discharge (under permit) drill cuttings contaminated with OPF in line with the OSPAR Decision.
As well as on the seabed, drill cuttings have settled on top of, and between, some of the oil storage cells that form part of the three gravity base structures (GBS).
The drill cuttings have been surveyed and sampled regularly since the late 1990s, most recently in 2015. Using our drilling records and the data from these surveys, we estimate that there are around 18,752 cubic metres (m3) of drill cuttings on the seafloor at Brent, and about 13,412m3 on the cell tops.
What do the regulations say?
Regulations governing the disposal of oil-based drill cuttings in the North Sea came into effect as a result of the OSPAR Decision 2000/3 (as mentioned above), the regulations prohibit the discharge of oil-based drill cuttings containing more than 1% oil by weight of dry cuttings.
In 2006, a second regulation came into force due to OSPAR Recommendation 2006/5, which requires drill cutting piles to be assessed to confirm that the impacts of pollution by oil and/or other substances from cuttings piles are reduced to a level that is not significant. OSPAR 2006/5 also recommends that for cuttings piles which fall below the two thresholds, no further action is necessary. http://www.ospar.org
What is our decommissioning recommendation?
Over the decades, our sampling and modelling has consistently shown that all the seabed drill cuttings piles at Brent and the cell top drill cuttings at Brent Delta and Bravo are below the OSPAR thresholds. Our long-term fate modelling analysis predicts that this will not change over the next 1,000 years. As none of these drill cuttings piles exceed the OSPAR thresholds, our decommissioning recommendation is to follow the advice of OSPAR and leave them in place to degrade naturally.
Brent Charlie cell top cuttings
Analysis of more recent samples from the Brent Charlie cell top cuttings pile, and the revised long-term fate modelling using new data from these samples, indicated an oil leach rate which slightly exceeds the OSPAR threshold.
In line with the OSPAR Recommendation for cuttings that exceed the threshold, we carried out a stage 2 Comparative Assessment to compare a number of options to handle these, including leaving them in place. As a result of this Comparative Assessment, our decommissioning recommendation is leave these drill cuttings in place to degrade naturally to avoid disturbing the drill cuttings in the water column.
What if the drill cuttings need to be disturbed?
It is possible that the drill cuttings pile under the steel jacket of Brent Alpha may be disturbed when the jacket is decommissioned. Similarly, some of the drill cuttings piles on the cell tops of the three GBS may be disturbed if we are required to gain access to the top of the cells or during other required decommissioning activities, for example small quantities on Brent Delta have been water jetted under permit to enable attic oil operations to be carried out.
We have carried out comparative assessments to identify recommended decommissioning options for the drill cuttings piles at Brent Alpha and on the cell tops if they need to be disturbed as part of the decommissioning process.
Our recommended option for Brent Alpha seabed drill cuttings piles is to leave them in place to degrade naturally alongside the footings of the steel jacket. For the Bravo and Delta cell tops our recommended option is either to water-jet them onto the seabed if only small quantities need to be disturbed, or remove them and treat them onshore if larger quantities need to be disturbed. For Brent Charlie, the recommended option is to remove the drill cuttings and treat them onshore if the cell-top pile needs to be disturbed.
Monitoring and Liability
After decommissioning we will discuss and agree with the Department for Business, Energy and Industrial Strategy (BEIS), a long term monitoring programme for the drill cuttings.
Shell and Esso will remain liable for any remaining facilities, pipelines and drill cuttings after decommissioning the Brent Field, in accordance with applicable legislation.