Mercury in oil and gas must be removed for stable production and environmental concerns. Especially in design phase of production facilities, it is important for optimization of Mercury Removal Unit (MRU) to assess accurately concentration and chemical form of mercury in production fluids based on mercury analysis during Drill Stem Test (DST). However, mercury behavior during DST may be different from that in steady production phase, because of dissolved oxygen (DO) in brine. If an incorrect assessment on mercury analysis during DST is made, modification of MRU after start of production is difficult in offshore facilities which have limited space for equipment, and forces unscheduled production downtimes. To investigate influences of fresh brine containing DO on mercury behavior in unsteady production phase like DST, the mercury sampling test was conducted during clean up flow at a natural gas well after acid washing. As a result, particulate mercury concentration in filtered solids collected from condensate and flowback water reached a peak at the early phase of clean up flow, and then decreased to around two orders of magnitude lower at the late phase when pH and total iron concentration in flowback water were close to those in steady production phase. Furthermore, mercury sulfide was identified in some filtered solids. Hence, the results of the sampling test indicate that mercury sulfide may be produced by the reaction of DO in workover fluid with hydrogen sulfide and elemental mercury in natural gas, and reveal that different chemical form of mercury is produced in unsteady production phase like just after well workover. As there are hardly any reports such as this study, although further investigations will be needed for the interpretation of these results, they will be an important knowledge for interpretation of mercury analysis during DST.
