Quantification of Low Levels of Mercury in Gas Reservoirs Using Advanced Sampling and Analysis Techniques

Abstract

In addition to health, safety, and environmental issues, the presence of mercury can potentially have a huge impact on the economics of a gas development project. A mercury removal unit (MRU) must be designed in order to reduce mercury concentrations to below detectable levels to prevent such issues as liquid metal embrittlement (LME) which could lead to catastrophic events. It is therefore imperative to accurately quantify levels of mercury in a gas reservoir to design gas processing and mercury removal equipment accordingly.Without consistent and precise procedures, accurate quantification is challenging with current technologies used in laboratory apparatus, especially when concentrations of mercury are very low. The conventional method of mercury content quantification is by analysis of samples captured either with wireline formation testers or while drillstem testing (DST) a well. However, these measurements are rarely in line with what is observed once a field is put on production. Loss of mercury due to adsorption by a DST string or by the metal surfaces of sample chambers and sampling tools is significant, especially when the sampling point is thousands of feet away from the producing reservoir. In addition to reaction with metals, test results have also shown how mercury can go undetected if reservoir fluid samples are compromised by small amounts of drilling fluids or mud filtrate. There have been a number of catastrophic failures in gas processing plants attributed to liquid metal embrittlement, such as the explosion at the Skikda LNG plant in Algeria in 1973, or the more recent 2004 New Year's Day Moomba gas plant fire which was confirmed to be due to LME in an aluminum vessel.Workflows detailing procedures for capturing, storing, if required, and analyzing representative gas samples for the quantification of low levels of mercury have been developed, tested, and proven. Results of recent experiments conducted simulating downhole sampling conditions reveal the reasons why there have been numerous cases in which false negative results were obtained from laboratory analyses. Advanced focused sampling methods together with accurate downhole fluid analysis with wireline formation testers have been applied in the field to provide representative reservoir fluid samples for quantification of mercury levels.

Methods for Total and Speciation Analysis of Mercury in the Petroleum Industry

“…However, significant losses of Hg are observed for H 2 S-containing streams, which probably indicates a reaction involving both H 2 S and Hg. Two studies reported the following reactions: , Cinnabar (HgS) is a solid mineral and can therefore accumulate in pipes. These two studies did not show this reaction but cited an earlier study to support this concept.…”

“…Although sampling is made easier with such a system, its reliability is of great concern. Hg is known to adsorb to most surface materials, including the inner walls of most cylinders. ,, Measuring the concentration of Hg would therefore underestimate the true concentration of natural gas. Solutions have been developed to overcome this problem.…”

“…Direct in situ analysis via direct AAS analysis can monitor the concentration of Hg in the gas over time, even during the equilibration time. This can provide additional information on the adsorption process of Hg, especially in natural gases containing H 2 S. , …”

CeO2-Decorated α-MnO2 Nanotubes: A Highly Efficient and Regenerable Sorbent for Elemental Mercury Removal from Natural Gas

“…In natural gas, elemental mercury (Hg 0 ), due to its high volatility and low solubility, 9−11 is the dominant form. 12,13 In the past, catastrophic industrial accidents 14 have been attributed to mercury-induced corrosion of aluminum heat exchangers that are broadly utilized in cryogenic gas plants. 15,16 Moreover, mercury present in acid gas removal and dehydration processes may be partially removed and released to the environment, exiting either in the water vapor from the glycol unit 17−22 or/and in the acid gas from the amine unit.…”

“…Decisive actions need to be undertaken to reduce anthropogenic mercury pollution , that poses serious threats to human health and wildlife. , The concentration of mercury in hydrocarbon reservoirs may considerably vary from below 0.01 μg·m –3 to well above 1000 μg·m –3 , , depending on the geological conditions, making detection systems essential for effective mitigation planning. In natural gas, elemental mercury (Hg 0 ), due to its high volatility and low solubility, − is the dominant form. , In the past, catastrophic industrial accidents have been attributed to mercury-induced corrosion of aluminum heat exchangers that are broadly utilized in cryogenic gas plants. , Moreover, mercury present in acid gas removal and dehydration processes may be partially removed and released to the environment, exiting either in the water vapor from the glycol unit − or/and in the acid gas from the amine unit.…”