Mercury Behaviour in Gas-Condensate-Monoethylene Glycol (MEG) Systems
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Gas and condensate production gets more challenging as the exploration takes place in deeper and in lower quality reservoirs. Under certain conditions, hydrate plugging is a major problem, causing damages to pipes and reduces the production rate. For hydrate prevention monoethylene glycol (MEG) is added at the wellhead. Another increasing problem is higher mercury content in the gas. This is a risk for equipment failure, product and catalyst poisoning and hazardous emissions. How mercury behaves and its influence in a gas and condensate system with MEG regeneration is not well known. The main objective of this master thesis was to acquire knowledge of mercury behaviour in gas-condensate-MEG systems. This included identification of what species and forms mercury exist in, how they will be distributed between the different phases and throughout a gas-condensate-MEG system, and what mercury reactions that could take place. The first two topics are known as mercury speciation and partitioning. Understanding of mercury behaviour is essential in order to determine the necessity of mercury reducing initiatives. A literature review was conducted on mercury speciation, partitioning and mercury reactions in gas and condensate systems. Despite little literature that was directly relevant, it was found from literature covering similar systems, that that the speciation of mercury in the different phases is dependent on the mercury partitioning and reactions. Elemental and ionic mercury are dominant in such systems. The solubility and volatility of the mercury species in the different phases determines which phase they will partition to and to what extent. The mercury concentrations are found to be highest in the condensates and somewhat lower in the hydrocarbon gas phase. The concentration in the aqueous phase in the system is minimal in comparison. The addition of MEG was found to not affect the mercury partitioning substantially. Mercury distributes linearly between the phases, but when the mercury concentration exceeds the mercury saturation in the phases it will precipitate as liquid mercury droplets. Some methylation and interconversion between mercury compounds are expected. It is also observed that mercury is oxidized and adsorbed at the steel pipe surface in the transport pipeline as HgS. This results in lower mercury concentration in the process stream reaching the process facility until the pipe surface is saturated. Simulations were performed for further study of mercury partitioning on a general level and the mercury distribution in a given gas-condensate-MEG system. For the simulations, Multiflash was utilised with the fluid property model Cubic Plus Association (CPA). The effect of gas composition, mercury concentration in the raw gas, operation conditions, and MEG concentration was investigated. The Multiflash mercury model using CPA was found to be valid for simulations of mercury in hydrocarbon systems with MEG. By simulations in Multiflash, the mercury partitioning trends found in the literature were confirmed. For the gas-condensate-MEG system, it was seen that most of the mercury follows the gas and condensate in the hydrocarbon separation stages. Less than 0.5% of the mercury that enters the system will remain in the rich MEG (50-60 wt% MEG, 40-50 wt% water) and enter the MEG regeneration system. When the mercury concentration in the simulations was below the saturation point of the process stream, changes in the mercury concentration in the raw gas did not have any significant effect on the mercury distribution in the system. The transport pipeline and the slug catcher were found to be most prone for mercury liquid dropout. If the mercury concentration exceeds the saturation point of the process stream in the high-pressure separator, less mercury will follow the gas and the hydrocarbon phases. This could increase the mercury content entering the MEG system considerably. High temperature and high pressure in this separator are therefore favourable, in order to minimize the mercury concentration into the MEG regeneration system. A richer gas yields lower mercury concentrations into the MEG regeneration system, and would also reduce the risk of dropout.