Mercury is present in many north German gas reservoirs producing from Permian sandstone (Rotliegend formation). When the pressure and temperature of the produced gas are reduced in surface facilities, most of the accompanying mercury (700 to 4400 g/m3) is precipitated in separators. Still, a certain amount of mercury might remain in the hydrocarbon condensate and the produced brine. Total mercury concentrations in condensate and brine were determined with a view to safety of field workers and restrictions imposed by refineries processing the condensates. Furthermore, the individual concentrations of the highly toxic organic mercury compounds, of inorganic mercury compounds, and elemental mercury were analyzed, applying a new process for R-Hg-X compounds. In the process, mercury species are complexed with specially selected ligands and separated on an HPLC column. The complexes are destroyed by post-column oxidation with UV irradiation and analyzed by means of cold vapor atomic absorption spectrometry. Low detection limits were achieved by on-line preconcentration.
While the total mercury content in brine was less than 1 mg/l, total mercury in condensates amounted to 3 to 6 mg/l. Dialkyl-diphenyl mercury compounds were not detected. Out of the group of monoalkyl mercury compounds, only methyl mercury was detected in concentrations of between 0.005 and 0.1 mg/l, the same proportion as found e.g. in sea fish.
Mercury exists naturally in many gas reservoirs worldwide, for example in the United States, Canada, China, Japan, The Netherlands, and Germany. Gas produced from the Rotliegend sandstone in North Germany exhibits especially high amounts of mercury in raw gas. The "mercury source" is the underlying Volcanite, which - as its name says - is of volcanic origin. In gas recovery operations, mercury is mainly found in elemental form, but some authors reported the occurence of mercury compounds and alloys as well. Inorganic and organic Hg compounds have been identified, and the extremely toxic nature of organic mercury compounds makes even low concentrations dangerous to health, safety, and environment. Among the organic mercury compounds, monoalkyl- (or monoaryl-)mercury [R-Hg-X, X=halide, sulfate, or -S-R], and dialkyl mercury compounds are distinguished. The dialkyls, especially dimethy/mercury, are even more toxic than the monoalkyls and caused fatal poisonings at the time of their discovery. The Threshold Limit Value of organic mercury compounds in Germany is 0.01 mg/m3.
Dialkyl mercurials are fairly volatile (boiling point of (CH3)2Hg: 96 C) and soluble in organic solvents, while monoalkyl mercurials with sulfate counterions as well as inorganic mercury compounds are water soluble. On the other hand, monoalkyl mercurials with chloride or alkyl sulfide (-SR) counterions prefer organic solvents. Thus, organic mercury compounds might be found in the organic phase (gas condensate) and in the aqueous phase (formation brine).
Besides the toxicity of mercury and its compounds, mercury can lead to severe corrosion problems in gas treatment plants and to catalyst poisoning in refineries processing gas condensates. P. 509^