Throughout the oil and gas industry, the impact of mercury in produced hydrocarbons is an emergent issue. This is not only the case for unconventional resource plays but also for conventional plays as they near the end of their economic production and as process systems require decommissioning. Produced mercury ultimately contaminates hydrocarbon processing equipment and transportation systems like those found on offshore production platforms, offshore gathering platforms or subsea pipelines.

The dismantling, removal and disposal of these systems present unique challenges and risks to decommissioning personnel and to marine ecosystems. Global conventions provide guidance for decommissioning of oil and gas facilities in international waters, but specific regulations with regard to residual mercury concentrations in production systems (either as scale or complexed in the grain-boundary of metals) is not currently available.

Mercury adsorbs and chemisorbs to carbon steel surfaces, primarily through amalgamation and diffusion into the scale, making carbon and stainless steel excellent mercury scavengers. Fortunately this process is somewhat reversible depending on many factors.

However, mercury complexed and incorporated into steel surfaces is not easily affected by typical hydrocarbon chemical decontamination chemistries and methods. The research group at PEI has concentrated efforts on understanding sorption dynamics of mercury in steel pipe and in the development of effective chemical decontamination solutions. Pilot studies based on steel coupon samples from impacted pipelines are a key component of evaluating mercury decontamination methods and identifying the most cost-effective technology for application to mercury-impacted pipelines.

Oil and gas companies across the globe are faced with significant decommissioning challenges, including operations in the Gulf of Thailand and Gulf of Mexico (GoM). Particular attention should be considered in decommissioning strategies for process systems previously or currently exposed to mercury-contaminated hydrocarbon streams. Two key considerations required for development of safe decommissioning strategies (including decontamination) are: 1) an understanding of the nature and distribution of mercury along with depth profiles in pipelines and process equipment and 2) consideration of mercury decontamination goals.

Mercury assessment

Decommissioning preplanning should consist of attempts to determine the extent and type of mercury contamination present in process systems and pipelines. It is recommended that operators perform routine mercury sampling and analysis of process streams that include periodic mass flux and loading studies so that a substantial and meaningful body of data exists by the time decommissioning is considered. In situations where production has been shut in, assessment and characterization may consist only of establishing the amount of mercury in steel and remaining process fluids. Assessing the nature and distribution of mercury throughout process systems in this event becomes more challenging and can be based on many assumptions. The probability of negative effects to costs and schedules is directly correlated to the number of assumptions that must be relied upon. Predicting deposition of mercury in process systems can be accomplished by computational modeling; however, models should be verified with process stream mercury concentration data and representative steel test coupons. The appropriate selection of hydrocarbon/mercury removal chemistries, residence times and application methods depends on accurate mass flux, loading and distribution data. Likewise, the integration of data from mercury assessments, mass flux models and mass loading studies provides planners the insight needed to develop appropriate decommissioning plans.

Chemical decontamination

A critical step in decommissioning and chemical decontamination planning is determining objectives and establishing measurement performance criteria. Criteria that might be applied to pipeline decontamination are not strictly established or based on existing regulatory requirements. Some may be based on regulations such as decontamination for disposal or recommendations and conclusions from an environmental impact assessment. Most criteria applied to decontamination efforts are based on those established by companies for safe decommissioning and thus related to safety and environmental impact.

Since decommissioning can include temporary measures for systems that may go back into service after some period of time, it is important to note there are some differences in objectives and chemical selection. Chemical decontamination methods and chemistry used to prepare process systems for reuse—i.e. temporary decommissioning—differ substantially from those used for permanent decommissioning of process systems (subsea pipelines that will be abandoned post-decontamination or topside process equipment planned for metals recycling, for example). This is in part because most chemistry that is used to oxidize or otherwise remove mercury from the scale oxide layer and into the steel profile is not suitable for use in equipment going back into service. If the objective is preparation of equipment for reuse, the target may be to remove hydrocarbon-soluble mercury and particulate mercury or to convert mercury to a nonvolatile species rather than removal. Chemistries used to meet decommissioning objectives can consist of strong oxidizers and inhibited acids designed to penetrate the oxide layer into the metal substrate. Total mercury removal is possible for systems scheduled for permanent decommissioning, but as the chemicals used for this purpose are aggressive to mercury, they typically are aggressive to other metals as well.

Recent studies

Two recent projects are briefly described below. An offshore study included a mercury mass loading and chemical reduction component to support decommissioning of a 16-km (10-mile) section of 24-in. subsea pipeline located in the GoM.

A key goal was to develop a set of chemistries that could be used in a chemical pig-train to remove up to 75% of the mass of mercury from the pipeline. Bench scale chemical reduction tests were conducted with test spool coupons from the production platform, gathering platform and dehydration plant to evaluate the efficacy of mercury removal technologies.

Data (mercury mass per area) from test spool coupons were integrated with results from measurement and monitoring of mercury in process and waste streams to develop a robust mercury flux model (production platform to dehydration plant). A combination of thermal desorption experiments and sequential acid digestions can be used to quantify mass and depth in metallic coupons. Six chemistries (a combination of mineral acids, surfactants and chelants) were tested and evaluated on coupons from the production platform with an average mercury mass loading of 13 g per sq m (per 10.8 sq ft). Coupons were inserted into a Silconert stainless steel chemical reaction chamber and subjected to each test-case chemistry for predetermined residence times while process and chemical parameters were continually monitored.

A second recent study involved an onshore processing facility where experiments were performed on carbon steel test spools collected from process piping subjected to 20-plus years of process conditions. Results indicated the mass loading potential of steel pipe exceeds estimates reported in previously published studies. Research performed on a test spool from a butane process stream measured mercury mass loading rates greater than 70 g per sq m. This is important to note as recent thermal desorption experiments were conducted by increasing temperature over time from 25 C to 400 C (77 F to 752 F) in a quartz-tube furnace. Results indicated field steam-out temperatures are ineffective in removing mercury from steel but may still be effective in volatilizing hydrocarbon-soluble mercury and volatile mercury species.

Functional and molecular speciation of mercury is key. One of the main objectives of this study was the development of a chemical solution and process that would remove hydrocarbon, iron oxide/sulfide scale and mercury in one chemical step.

This study was successful in meeting objectives, including the development of a chemical solution that was 99% effective in removing mercury down to 1 mm over four hours of residence time at 60 C (140 F).

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