Understanding of Mercury Corrosion attack on Stainless Steel Material at Gas Wells: Case Study

Abstract
The corrosion resistance of stainless steel is dependent on a hard, tough, chromium oxide film that is self-repairing under oxidizing conditions. This oxide film is not self-repairing in reducing or neutral conditions. If the film is disturbed, corrosion will result. We can postulate metallic mercury forming an amalgam with the chromium, iron, or nickel; with subsequent corrosion by liquid water or an aqueous acidic phase. The laboratory testing proved these phenomena. Based on these examinations, the corrosion of the wellhead downhole and top side facilities of X Gas field was believed due to the chromium-water reaction in the presence of mercury. The generalizations can be made about corrosion induced through mercury amalgamation and liquid water must be present and the metal involved must be above nickel in the electromotive series of metals for the reaction to proceed spontaneously at room temperature.

Introduction
Raw natural gas must be treated prior to export or its liquefaction for several reasons. These include removing compounds which interfere with the liquefaction process, with the separation and recovery of hydrocarbon liquids and with meeting the specifications set for the recovered products. For example, the gas must be dried to prevent ice formation during cryogenic operations. Hydrogen sulfide ordinarily must be removed because of its toxic nature. A large number of commercial processes are in use for treating and separating of raw wellhead gas. The steps used in these different processes are each well known to those skilled in the art. Some natural gas contains mercury (Hg) at levels as high as 200 to 300 micrograms (mgr) per cubic meter. For example, the mercury level of natural gas produced at one field in Indonesia is about 250 micrograms per cubic meter. Concentrations of mercury at this level creates safety hazards and air pollution problems. Mercury is not only troublesome, but also causes serious health and environmental consequences such as kidney and/or nerve-tissue damage if the human exposure to it. In small amounts, mercury and its compounds have an extremely harmful effect on human health. Liquid mercury is very volatile, vaporizing readily due to its low vapor pressure, and is highly toxic.

The X gas field owned and operated by the Petroleum Development of Oman includes two core deep gas accumulations. The gas condensate bearing sandstone reservoir was discovered around 10 years ago, the surface facilities gas plant as Figure 1 is a single train gas processing facility comprised of inlet separation and gas conditioning facilities. The X gas field is tight reservoir with high temperature (150oC downhole) and contains potential high condensate volume. As a part of field development, the next facilities project will be covering the reservoir pressure outside the phase envelope or as a minimum slow the pressure decline such that the volumes of condensate lost within the reservoir are reduced.

Meanwhile, the material selection and corrosion study has been carried out which is designed for 25 years life of operation, considering all gas and fluid impurities such as condensate, produced water that contains 70,000 mg/l Chlorides, 60 mg/l Bicarbonates, 3 mol % CO2 and 50 ppm H2S. During the life scenario, mercury is considered to be a possibility in the feed based on previous gas PVT study. The surface facilties design does not currently consider the impact of Mercury on the process facilities, although the Mercury Removal Unit (MRU) has been installed upstream of the export pipeline to deal with potential Hg levels in excess of the sales gas specification.