Mercury is found in raw and untreated oil and gas produced from in-situ reservoir and must be removed for stable production and response to environmental issues. For optimization of Mercury Removal Unit (MRU) in production facilities, it is important to know accurately mercury concentration and chemical form of mercury species in production fluids. In design phase of production facilities, specification of MRU may be determined based on mercury concentration in production fluids collected during Drill Stem Test (DST). However, mercury concentration and chemical form of mercury species in DST may be unstable and different from those in stable production phase, because brine and drilling fluids containing dissolved oxygen flow in wells during DST. In this paper, the sampling and mercury analysis on production fluids after acid washing in a natural gas well was conducted to investigate influences of fresh brine containing dissolved oxygen on mercury concentration and chemical form of mercury species in flowback fluids in unsteady production phase like DST and just after well workover. As the result of the sampling and analysis, particulate mercury was detected in filtered solids collected from condensate and flowback water. In addition, mercury sulfide (β-HgS: metacinnabar and γ-HgS: hypercinnabar) was identified in the filtered solids by X-ray diffractometer. These particulate mercury concentration reached a peak (condensate: 1693 µg Hg/l, flowback water: 7751 µg Hg/l) around 40 hours elapsed from the beginning of clean up flow, and then went down with the passage of time afterward. After around 2700 hours elapsed, they decreased to 72.1 µg Hg/l (condensate) and 131 µg Hg/l (flowback water). At the same time, pH and total iron concentration in flowback water at that time were close to those in stable production phase. Hence, these results indicate that particulate mercury in production fluids was generated by the reaction of dissolved oxygen in workover fluids with hydrogen sulfide and elemental mercury in natural gas. Therefore, the occurrence of particulate mercury in flowback fluids after the well workover is considered to be temporary, and not to be produced in stable production phase. Although further investigations will be needed for the interpretation of mercury analysis results in the sampling and analysis, these results will be an important knowledge for interpretation of mercury analysis at DST.

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