Environment Report 2018

3.4. Drill Cuttings Drill cuttings are rock fragments generated during well drilling. They are brought to the surface by drilling fluids which surround the wellbore and are either water- or oil-based, depending on geological, safety and environmental factors. The cuttings, which are coated in the chosen drilling fluid, are disposed of according to the fluid type. Water-based fluid drill cuttings pose a lower environmental hazard and are generally permitted for discharge to sea. Oil-based fluid cuttings cannot be discharged to sea unless they are treated to reduce the oil-on-cuttings content to below 1 per cent of the total mass. Whether oil- or water-based, as part of the permitting process, operators must conduct stringent environmental assessments to determine the risks posed by cuttings discharged. As with drilling chemicals, the mass of cuttings discharged to sea is closely related to drilling activity. At 47,200 tonnes, 2017 saw an increase in drill cuttings discharged in comparison with the previous two years. With 320 kilometres drilled on the UKCS in 2017, this represents 147 tonnes of cuttings discharged per kilometre drilled. The peak in 2013 (shown in Figure 8 overleaf) is due to more complex wells being drilled. Of the 32,400 tonnes of cuttings coated with water-based fluids, less than 1 per cent were returned to shore for treatment and disposal, with the rest discharged to sea as permitted. Of the 39,100 tonnes of oil-based fluid cuttings, 54 per cent (21,000 tonnes) were returned to shore for treatment, down from 66 per cent in 2016. Around 15,000 tonnes were thermally treated offshore to reduce their oil content to below 1 per cent and discharged to sea; the remainder were injected into the reservoirs.

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