Hydrocarbon Engineering (August 2016)

Reprinted from August 2016HYDROCARBON ENGINEERING

M any in the oil and gas industry are now aware of the effects of produced mercury on hydrocarbon processing systems, and that produced mercury contaminates processing equipment and transportation systems from the riser platform to onshore

dehydration plants, and throughout the front end of natural gas liquids (NGL) and LNG plants. Downstream processing (refining and petrochemical) plants are also affected, and have process, environment, health and safety (EHS), and occupational exposure risks. Certain production regions around the globe are facing the same challenges at the same time, where assets are operating past their economic design, and are also contaminated with mercury (topside process equipment on production platforms, gathering platforms, floating production storage and offloading units [FPSOs], subsea pipelines and hydrocarbon processing plants). The continued operation, dismantling, removal and disposal of these systems present unique challenges and risks to personnel, the environment and marine/terrestrial 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) are not currently available. Decommissioning regulatory guidance updates are anticipated in Asia, but may not provide a specific mercury mass per surface area or concentration that can remain in process systems. Mercury is a trace metal in all

A DEEP CLEAN

Ron Radford and Tim Jenkins P.E., PEI Mercury & Chemical Services Group, USA, discuss the mercury chemical decontamination of hydrocarbon processing systems.


hydrocarbons, but is concentrated in specific oil and gas basins and during production, therefore international and regional decommissioning guidelines should be drafted and/or amended to address residual mercury mass and distribution within assets scheduled for decommissioning. Notably, decommissioning assets on the northwest shelf and onshore Western Australia (Figure 1) follow an objective-based approach under a legislative framework regulated by the Department of Mines and Petroleum (DMP) that ensures decommissioning is managed responsibly and sustainably for the benefit of all stakeholders (e.g., DMP is required to consider mass and distribution of THg contaminating each system during pre-decommissioning planning).

Mercury is a naturally occurring trace constituent of coal, crude oil, natural gas, and natural gas condensate. Virtually all geologic hydrocarbons contain measureable quantities of mercury. Coal bed methane (CBM) production can contain higher mercury concentrations than traditional or shale gas production. Trace metals, including mercury, are scavenged by carbon/stainless steel (adsorbed/chemisorbed) into interfacial surfaces and can complex into the scale/metal grain boundary, requiring special chemistry and chemical application methods for successful decontamination. The accurate measurement and quantification of mercury in operating and non-operating process systems is challenging,

and, even though this science and technology has advanced to its most accurate level in the last decade, many operators have not quantified mercury mass loading per surface area or flux throughout production systems scheduled for decommissioning. Understanding mercury mass per surface area, speciation, depth profile, and distribution within a system, is a critical first step to developing accurate decommissioning, chemical decontamination and spent chemistry processing plans. The development of these plans is imperative, and required to develop accurate cost estimates for each phase. The more assumptions that are made in this process, the less accurate the cost estimate to manage each phase will be. Costs are a critical factor in decommissioning programmes and, in this new energy market supply and demand paradigm, it is more important than ever for operators to understand what those costs will be.

Research and developmentPEI and its research partner International Specialty Chemicals and Technology (ISCT), based in Sydney, completed two important mercury mass loading, chemical reduction and materials processing research programmes in 2014/2015 that advanced PEI's chemical and processing technologies to state of the art. Three major objectives were achieved in these research programmes:

n Enhanced chemical formulations with modified surface reactive agents effective in cold water environments (i.e. subsea pipeline applications) tested 100% effective in temperatures as low as 10˚C.

n Increased chemical contact time on simulated vertical surfaces (i.e. FPSO cargo holds using existing infrastructure for chemical application) tested 97% and 99.7% effective in sequential applications.

n Processed simulated spent chemistries (chemistries spiked to 9000 ppm mercury and 1000 ppm hydrocarbons) treated to <1 ppb residual dissolved mercury in solution. These achievements were combined to develop a P80 cost estimate for full scale implementation.

What this means for the hydrocarbon processing industry is that the technology required for 'near total' and 'total' mercury mass removal from process systems has finally caught up to the need for such technology. Furthermore, the disposal and treatment challenges posed by spent chemistries can now be effectively managed though 'discharge ready' processing, eliminating an expensive waste stream that previously had few options for disposal.

Full scale applicationA summary of two mercury chemical decontamination projects performed for confidential producers with operations in Australasia is provided below: mercury chemical decontamination of process equipment scheduled for decommissioning, and mercury chemical decontamination of high value equipment for re-use. In each case, near total and total mercury mass removal was an objective, and spent chemistry processing goals were to be treated to discharge criteria.

In the first case, a section of 14 in. inlet piping was removed from a liquified petroleum gas (LPG) mercury removal unit (MRU) offshore, as part of ongoing maintenance

Figure 1. Australasian production.

Figure 2. Post-decontamination XRF data.


and inspection. The section of piping was scheduled for chemical decontamination in preparation for decommissioning (metals recycling). A critical step in any chemical cleaning programme is to determine the decontamination objectives and establish the performance criteria of the programme. The piping section was designated for metals recycling; therefore, to meet decommissioning criteria for safe recycling, approximately 99% mercury mass removal was required and accomplished using a sequential chemical cleaning approach. Initial mercury mass loading per surface area ranged from 10 - 60 g/m2. Chemical performance and maximum mercury uptake for the chemistries selected have been well established in ongoing research and development since 2014 (ISCT 400-66 and ISCT 400-69). However, during chemical circulation, a range of performance analyses were performed continuously to measure the performance of the chemistry (THg, PHg, DHg, Fe, pH, % oxidiser[s], NTU and TSS), as well as mercury mass removed over time. THg and compound uptake graphs were then generated for each chemical phase, depicting mercury uptake and trending over time. These graphs provided valuable information in real time to indicate when to stop a particular chemical phase, increase an oxidant or reduction agent, or increase the solubility of mercury in the chemical solution.

By calculating the difference between initial and final mercury mass loading measurements, the resulting residual mercury readings represented more than 99% mercury mass removal. This was a relatively small section of 14 in. piping, and was provided to Contract Resources (CR)/PEI to

demonstrate the technology and chemical efficacies for subsequent use during total asset decommissioning. Total soluble mercury removed during the three phase sequential chemical circulation was 23.5 g (21 g of which was removed in four hours during the first application of 400-66). The remaining 2.5 g of mercury was removed using two sequential applications of 400-69 and 400-66 over an additional eight hours of contact time. Final verification X-ray fluorescence (XRF) surface measurements are depicted in Figure 2.

In the second case study (decontamination of high value equipment for re-use), well flow test equipment and process systems on a semi-submersible drill ship were contaminated with mercury due to intermittent exposure to high mercury concentration hydrocarbon streams during a 2014 well flow test programme. The semi-submersible and associated well flow test equipment were scheduled for re-mobilisation to another production field with lower mercury concentrations in gas and condensate. Mercury chemical decontamination was required primarily to prevent mercury from desorbing from interior steel process system surfaces during well flow test operations and affecting mercury sampling and analysis data from the new production field. The project required performing a study to determine mercury mass loading, distribution and speciation of various systems prior to chemical decontamination offshore on the semi-submersible drill ship and onshore on well flow test process equipment. Based on this study, chemical decontamination plans, performance monitoring criteria and spent chemistry/materials processing plans were developed. A key objective was near total mercury mass removal with an emphasis on avoiding damage to primary 4130 and 4140 steel alloy components.

Most that have performed similar studies know that mass loading per surface area is generally not homogenous within systems, and varies with exposure, pressure, the temperature of the process, the chemical composition of the hydrocarbon streams, and many other factors. What was needed was versatility in the chemical’s maximum loading potential for mercury and hydrocarbons. ISCT 400-66 was selected, and the formula modified based on the estimated mercury mass per surface area of the various systems required for decontamination. Chemical performance and maximum mercury uptake for ISCT 400-66 is well established, and solubility of mercury can be increased to >100 000 ppm. Offshore, on the semi-submersible, this meant the chemistry had a certain mass loading potential, and was circulated in four piping runs of four hours each. The total calculated mass per surface area for offshore piping on the semi-submersible was approximately 4 g/m2, with a total mercury mass removal of 418 g.

Well flow test systems were re-connected onshore at a Contract Resources facility for chemical decontamination, performance monitoring and final verification. Systems were connected based on estimated mercury mass loading per surface area in four continuous circulation systems and circulated for four hours each. A combined total of 1.016 kg of mercury was removed based on performance and

Figure 3. Pre and post-XRF surface concentrations.

Figure 4. LNG carrier ship.


verification sampling/analysis of THg. A range of chemical analyses were performed continuously to measure the performance of the chemistry (THg, PHg, DHg, Fe, pH, % oxidiser[s], NTU and TSS), as well as mercury mass removed over time. THg and chemical compound uptake graphs were generated for each chemical phase, depicting mercury uptake and trending over time.

In both cases, XRF baseline and post verification surface screening was performed using a five point calibrated XRF in surface mode. PEI has been using XRF technology since the beginning of the 2014 research programmes to evaluate the effectiveness and limitations with the overall objective of deploying the technology as a semi-quantitative method to determine mercury mass loading per surface area. The limitations and accuracy need to be understood before attempting to use XRF technology in a chemical decontamination verification programme. Also, to further evaluate the effectiveness of the chemical efficacies and total mercury mass removal, sacrificial steel (2 - 6 in. high pressure flanges) from the most mercury contaminated equipment (three phase separator approximately 80 g/m2) was sent to a US lab for scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and acid digestions to show the overall correlation between XRF pre and post-screening versus acid digestions of representative test coupons. PEI has demonstrated, through its recent research and full scale project applications, a strong correlation between XRF readings and off-site acid digestion results, especially for post-verification samples. Total calculated mercury removal from the combined systems using the highest mercury mass loading per surface area equipment as a surrogate for the entire systems equated to approximately 90 - 95% mass reduction. Figure 3 depicts pre and post-XRF surface mercury concentrations, and the overlying grid that was used to cut representative test coupons for SEM, EDS and acid digestions.

Processing spent chemistry and materials minimisationIn both cases, spent chemistries were processed onsite using a combination of methods based on bench testing each batch (approximately 10 500 l per ISO to allow for volume expansion, sulfide deactivation and modified Fentons Reagent). There are few disposal options for high concentration mercury contaminated fluids (>5000 ppm) in many offshore/onshore environments. The best case is having an injection well available where limited processing is required (pH adjustment and %TSS). Processing can be as costly as the chemical decontamination, depending on what the discharge criteria are and what the final disposal options are for the supernatant and sediments generated from flocculants/coagulants. In general, the processing steps for both cases consisted of pH adjustment in the presence of H2S inhibitors (if not performed properly, this reaction can release hazardous concentrations of H2S). An intercooler/heat exchanger is required to control reaction temperatures, as well as direct chemical performance monitoring equipment, including an explosion-proof camera, to observe reactions in each batch. Processing analysis is required for a variety of parameters, similar to

those used to measure the performance of the chemical decontamination. After pH adjustment and sulfide/H2S deactivation, subsequent processing phases included flocculation, coagulation and settlement/decantation. Supernatant was transferred to a set of clean ISOs for final treatment and offsite disposal. A variety of chemical solutions were processed in batches, including chemistries used to clean process tanks, pumps and equipment that were exposed to the chemical decontamination and/or minimisation process (variable pH range and mercury/hydrocarbon concentrations). The final stage of materials processing consisted of dewatering the sediments, which, in both cases, was accomplished using a centrifuge to minimise the sludge volume. This resulted in a relatively small volume of highly concentrated mercury containing filter cake, and could be disposed under either the Basel Convention at a permitted facility, or sequestered for long term storage.

ConclusionBoth of these cases can be considered ‘proof of concept’ successes, and can be used as a model for similar projects, both offshore and onshore. An important lesson learned is that accurate mercury assessment, distribution, depth profile analysis and speciation are key to developing accurate chemical decontamination/processing plans, costs and schedules. As mentioned before, assumptions can lead to less accurate cost estimates and greater adverse effects to project schedules. The impacts of PEI's latest mass loading and chemical reduction research and full scale chemical decontamination/materials processing applications are a promising and positive development to the decommissioning industry. Applying these technologies and methods will reduce the costs of decommissioning by eliminating variables that, if not included in pre-planning, would negatively affect the schedule, compliance and, thus, costs.

PEI has developed a unique approach to understanding mercury mass flux, loading and distribution in hydrocarbon processing systems to develop mercury management processes, including chemical solutions for decontamination and materials processing. The focus is to share that technology and approach with the company's alliance partners (CR) and the hydrocarbon processing industry in the Australasian region. The regional alliance with CR builds on an existing alliance with CR Asia and extends PEI’s research and development, process stream sampling and analysis, and mercury chemical decontamination services throughout Australia, Papua New Guinea and New Zealand. Through the alliance with CR and the company's research partner ISCT, PEI is committed to improving and developing chemical technologies to support mercury management and chemical decontamination projects for operating assets, and those scheduled for decommissioning.

NotesThis paper was first presented at the Decommissioning and Mature Wells Management Conference on 2 - 4 December 2015 in Kuala Lumpur, Malaysia.

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Hydrocarbon Engineering (August 2016)

Technology

Transcript of Hydrocarbon Engineering (August 2016)