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

Society: SPE
Paper Number: 166220
Presentation Date: 2013
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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.

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