North Sea operators plan measures to capture mercury in production

Corrosion, environmental issues raised

• Summary of formation geology and mercury presence [47,319 bytes].

Oil and gas production in the North Sea is still rising. The same may also applys to an unwanted by-product. Mercury co-exists with hydrocarbons in certain zones around the world, and the UK is one of the offshore sectors most at risk.

Co-production of mercury in its liquid metal form or via organic or inorganic salts can affect integrity of plant and equipment on offshore installations. More seriously, it may threaten human well-being if produced unchecked.

Emissions to the atmosphere or ocean are another matter. Ongoing research by corrosion specialists Capcis of Manchester, UK has found no evidence of a causal link between North Sea production operations and mercury levels in the environment. But with pressure groups stepping up attacks on toxic waste disposal in the North Sea, the industry cannot afford to take the mercury issue lightly. A major oil and gas company has already responded by fitting mercury stripping equipment at its Forties pipeline reception terminal in eastern Scotland.

Problem claim

In 1986, Dieter Schmidt of the German Oceanographic Institute became the first researcher to detect "reactive mercury" associated with oil and gas activities off eastern Scotland. His findings were passed onto Capcis as evidence of a general mercury "problem" in the North Sea. Capcis decided to investigate the claim, approaching BP for support through the Petroleum Engineers Association (PEA). The PEA-30 Mercury Programme was then initiated in 1992, backed also by British Gas, Kerr-McGee, Norsk Hydro, and Statoil.

Liquid mercury has been recovered from some operators' gas chiller units. Removal involves potentially hazardous maintenance prodcedures. When inhaled, ingested or contacting skin, mercury and its compounds can generate loss of coordination, visual impairment, renal damage, and psychological disturbance.

In 1995, Capcis performed a survey of fields in several UK offshore sectors. Lithographies were predominantly sandstone. Values of 50-150 micron g/cu m were found in gas near the German/Dutch North Sea median lines. Much lower readings were taken (10 micron g/kg in oil and 5 micron g/cu meter in gas) in the Central UK North Sea. Levels fell to 1 micron g/cu meter in gas and 1 micron/kg in oil, condensate and water samples taken from fields in the UK Northern North Sea and Irish Sea. No Hg was found in any hydrogen sulfide-producing fluids.

But despite the high readings in the southern North Sea, Hg concentrations in inshore waters around the production facilities here were found to be well within the range occurring "naturally" in the ocean (up to 0.005 micron g/liter of water). Theoretically, there are four potential escape routes for mercury or toxic compounds produced offshore:

• Via gas flared to the atmosphere
• Produced water discharged to the sea
• Oil/gas piped to land
• Decommissioning of redundant structures.

Regulatory requirements

The Netherlands requires mercury-contaminated plants to be thermally processed before being returned to service, and scrapped pipelines to be thermally stripped prior to re-smelting. This is a new lead that other European Union countries will likely follow, which would add to the offshore industry's already high decommissioning costs.

Over the years, Capcis has developed a mercury sampling and analysis service for offshore oil and gas producers. The basic requirement for authentic data is that the samples collected are representative and that the conentration of analyte does not alter during sampling or storage. To minimize mercury loss, samples are analyzed as soon as possible following collection. Preservative solutions are designed to trap mercury in a non-volatile and un-reactive form. Sampling takes place at line temperature and pressure to minimize phase equilibrium changes - particularly with gas samples.

Normally on platforms, samples are taken from around the primary separator. Inlet crude, drop-out water and gas samples are taken at the separator and condensate at the first knock-out containment vessel. Samples are also extracted downhole using customized tools such as piston bombs.

For gas sampling, Capcis has also developed portable equipment to allow mercury trapping to occur with minimal disturbance of the gas phase equilibrium. The trapping agent is an iodine/silica gel which maintains efficient mercury trapping even in the presence of liquids or hydrogen sulfide in the gas stream.

Capcis' mercury in oil analysis method is based on organic phase bromination of mercury followed by extraction into hydrobromic acid. Mercury in the acid is treated to remove excess bromine, then liberated in elemental form through the addition of alkaline stannous chloride for subsequent analysis by atomic fluorescence spectrometry. This method provides a detection limit of 1 micron Hg/kg of oil.

Currently, Capcis is seeking sponsors for a new program to examine partitioning of mercury in the oil, gas and water production phases. One aim is to develop a real-time monitoring technique. A seminar is also planned this year in Aberdeen, outlining the measures adopted by the Forties trunkline users.

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