Chemical Monitoring of North Sea Oil Installations

Abstract

Despite the extensive network of oil-producing installations in the North Sea the development of only one field - Beatrice in the Moray Firth - has been formally required to include an environmental monitoring program. However, in November 1984, the UK Prevention of Oil Pollution Act (1971) was amended and now requires that a program of defined sea bed monitoring is undertaken for production and development drilling with oil-based drilling fluids. Chemical methods form the basis of a number of approaches used in these programs. Many of the survey methods are still in the trial and development stages, whilst a few, notably sea bed surveys, have become widely accepted by the oil industry. The basis of these studies is that there is a net flow of materials, including pollutants, to the sea bed by sedimentation. Recent information from offshore operations shows that some development drilling practices, e.g., the use of drilling muds, particularly oil-based drilling muds, give rise to major, acute environmental concern and that steadily increasing chronic effects may come from production discharges, notably of produced water. Although drilling muds contain many heavy metals, several of the major cations present - Ca, Fe, K, Mg and Na - are common to sea water and terrestrial environments and are not considered hazardous to marine life. However, others, either present in the mud for specific functions, e.g., Ba, Cr, Zn or as impurities in "natural mud additives" such as clay minerals and barite, e.g., As, Cu, Ni and Pb, not only give rise to a deterioration in environmental quality but are also potentially toxic. Diesel oils were used in the initial OBM formulations but they were found to be toxic to a wide range of organisms. Due to the higher concentrations of potentially toxic materials in diesel, a large number of highly-refined, white mineral oils subsequently became available, with a much smaller percentage of aromatics than diesel. Production water consists of water occurring naturally in the producing reservoir (formation water) and, in many cases, breakthrough sea water from water injection. Formation and injection water composition are highly variable depending, respectively, on the natural geology of the oil reservoir and on the sea water matrix. On the basis of constituent concentrations toxic levels of Cd, Cu, Ni, Pb and Zn can occur adjacent to the installation but rapid dilution should restrict these levels to the immediate vicinity of the mixing zone. Petrogenic hydrocarbons are present in concentrations of 20-40 mg/L, of which 10-20 mg/L is derived from residual chemical additives, mostly corrosion inhibitors and biocides. Oil-based drilling muds and production water are considered to be the two major anthropogenic sources associated with the operation of offshore oil production platforms. The major deleterious effect of the discharge of oil-based mud is currently thought to be confined to within a 500-1000 m zone around the platform. Toxic concentrations of certain heavy metals in production water are rapidly diluted after discharge and are not considered to be of significant environmental concern.