Mercury is a natural occurring element and could be present in various stages of oil and natural gas exploration, production and processing. Mercury is not only hazardous to human health and the environment but could also attack process equipment components that have mercury reactive materials, leading to potential catastrophic failure to the plant. The mercury associated with petroleum and natural gas production and processing enters the environment primarily via wastewater, solid waste streams, and air emissions. Wastewater originates in production and oil refining operations in the form of produced water and wastewater respectively. The solid wastes are also generated in production (e.g. drilling muds), transportation (e.g. sludge), and refining operations (e.g. spent mercury adsorbent. The primary opportunities for mercury emission to atmosphere include fuel combustion for process utilities and fugitive emissions from process equipment. Several approaches have been used to reduce mercury emissions from oil and gas production and processing which include mercury waste treatment, recycling and disposal.

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Mercury is considered as a serious toxic heavy metal to both humans and the ecosystem because of its high toxicity to the central nervous system and the tendency to bio-accumulate in a human body [1]. Mercury is a natural occurring element and could be presenting varying concentrations and of various species in oil and gas fields [2]. It is freely distributed throughout production, processing, transportation and consumption systems. Table 1 shows the range of variations of the mercury content in oil and gas [3].
Wilhelm and Bloom [4] reported that the concentration of mercury in crude oil and natural gas varies between 0.01 ppb and 10 ppm depending on the geologic location, which may exist in three different forms, namely elemental (Hgo), inorganic (HgCl2), organic ((CH3)2Hg), and organo-ionic (ClHgCH3) mercury compounds. Several mercury species shown in Table 2 were detected in natural gas, gas condensate and crude oil. The properties of mercury species are difference in terms of mobility, reactivity, toxicity and bioavaibility [5].
Table 1 Range of Mercury Content in Oil and Gas Fields in the World [3].
Component
Mercury Concentration
Oil
0.003 – 21 mg/kg
Condensate
Gas
0.01 . 10-6 – 14 000 . 10-6 g/m3
The existing mercury in oil and gas can cause problems during oil and gas exploration, processing and transportation. This mercury needs to be removed from oil and gas streams especially from natural gas, in order to get pure product as well as to protect the process equipment and catalyst used in the downstream processes. For instance, it may cause mechanical failure and gas leakage of cryogenic aluminium heat exchangers. The mercury in the natural gas can degrade the aluminium coldbox materials by three basic mechanisms [Wilhelm, 1994]: amalgamation with various metals (primarily Al, Au, Ag and Zn), amalgam corrosion, and also liquid metal embrittlement (LME) [Coade and Coldham, 2006; Wilhem, 1994]. Besides, reported by Phannenstiel [7], mercury is pointed as a caused of corrosion in gas-gathering system at Groningen field in Holland [8].
Table 2. Approximate Natural Abundance Mercury Compound in Natural Gas and Gas Condensate [2].
Mercury Element
Natural Gas
Gas Condensate
Crude Oil
Hg0
>50 of total mercury
>50 of total mercury
>50 of total mercury
(CH3)2Hg
HgCl2
Rarely detected
(10-50) %
(10-50) %
HgS
Rarely detected
suspended
suspended
HgO
Rarely detected
Rarely detected
Rarely detected
CH3HgCl
Rarely detected
Mining activities such as exploration and processing could also generate mercury waste generate mercury waste in the form of produced water, refinery wastewater, drilling waste, and associated wastes. The mercury wastes need to be treated and disposed due to the environmental and safety considerations. The failure to monitor and control the existing mercury in oil and gas can caused contamination on process facilities and mercury emission to water, soil and atmosphere [U.S. EPA, 2001]
Mercury Removal Process From Natural Gas
Mercury removal systems are most often located at gas processing facilities that produce the feedstock materials for downstream chemical manufacturing plants. It is properly designed and operated, to make sure the removal systems can scavenge mercury from the feed gas and reduce the impact of mercury on downstream processes [2]. Table 3 summarizes several methods used for mercury removal in natural gas processing. All these methods have limitations that detract from their applicability to natural gas processing [El Ela et al., 2006; El Ela et al., 2008].
Table 3 – Mercury Removal Systems for Natural Gas [Bingham, 1990; El Ela et al., 2008 ].
Method
Comments
Chemisorption on sulfur impregnated activated carbon
Most used, cheap; disposal problems
Adsorption on activated carbon
Low saturation loading
Chemisorption on iodine impregnated activated carbon
Good for high mercury concentrations
Adsorption by amalgamation with a metal: Silver impregnated alumina, silver zeolites, metal sulfides and metal oxides
High investment costs, high removal capacity
Acid absorption of mercury – chromic acid and acidic permanganate
Increased corrosivity, through system contamination, low saturations
Oxidizing solutions – permanganate, sodium hypochlorite, and sodium vanadates
Regeneration problems, system contamination
Chemical reaction with H2S
Increased corrosivity, limited H2S access
Condensation and separation
Poor removal efficiency, liquid contamination
Stripping through liquid hydrocarbons
Poor removal efficiency, liquid contamination
The basic requirements for successful mercury removal are economics of the process and the removal medium needs to be capable to reducing mercury concentrations to extremely low and acceptable levels. The medium must have a high capacity for an active bonding to mercury so that they can retain the mercury in a form that can be disposed. The examples of commercial mercury removal systems are shown in Figure 1 (c,d), where the Salam mercury removal system is the most efficient removal method ever reported [9]. It is loaded with 19 tons of catalysts “PURASPEC Absorbent 1156” (pre-activated sulfide) [El Ela et al., 2008]. Figure 1 (a,b) shows the mercury removal unit located in Malaysia which is the successful systems used to remove mercury from raw condensate [Sainal et al., 2007].
Figure 1. Joint Delivery System (JDS) mercury removal system situated in Kerteh, Terengganu, Malaysia (a), Resak Delivery System (RDS) mercury removal system situated in Kerteh, Terengganu, Malaysia (b), Mercury removal unit at Salam gas processing plant (c), Vessel loading diagram of Salam mercury removal unit (d) [El Ela et al., 2008; Sainal et al., 2007
]
Mercury Waste Generation
A wide variety of waste streams contains of mercury generates from oil and gas processing. The mercury in produced hydrocarbons may escape to the environment by several avenues which are generally categorized as wastewater, solid waste streams and air emissions. During exploration and processing, wastewater originates from the produced water, refinery wastewater, drilling waste, and associated wastes (Ahmadun et al., 2009; Frankiewicz et al., 2000; U.S. EPA 2001). Solid wastes are also generated in production (e.g. drilling muds), transportation, refining operations (Juvkam-Wold, 1976; U.S. EPA 2001). Air emissions originate from fuel combustion for process utilities and fugitive emissions in the process equipment (Wilhelm, 1999; U.S. EPA 2001. These are the possible avenues of mercury to be transferred from produced hydrocarbons to the environment (Mussig and Rothmann, 1997; US EPA, 2001). Table 4 shows a wide variety of waste streams that contain mercury generated in conjunction with petroleum production and processing (Wilhelm, 1999).
Table 4 – Petroleum Processing Mercury Waste Streams [Wilhelm, 1999].
Type
Matrix
Mercury
Sludge Hg++
Hydrocarbon
Hgo, XHgX, HgS
Sludge
Water
HgS, Hg++
Cleaning Solutions
Water
Hg++ inorganic compounds
Cleaning Solvents
Hydrocarbon Solvent
Hgo, XHgX
Hg Sorbent
Carbon, Sulfur
HgS, S, Organic compounds.
Hg Sorbent
Metal Sulfide, Alumina
Hgo, XHgX, HgS, Cu, Al2O3
Hg Sorbent
Zeolite, Ag
AgHg
Hg Sorbent
Carbon, KI
HgI2
Dehydration Fluid
Triethylene glycol
Hgo, XHgX, Hg++
CO2 Removal
Water
Hgo, XHgX, Hg++ KCO3
Sour Gas Treatment
DEA, TEA
Hgo, XHgX, Hg++ amines
Catalysts
Metal
Hg amalgam
Filtration Material Clays, Fibers
Hgo, XHgX, Hg++
Debris
Hgo, XHgX
Produced Water
Generally, in oil and gas production operation one of the upstream activities involves a primary separation of water, gas and oil followed by treatment of the produced water for discharge or re-use. Produced water is the largest waste stream generated in association with oil and gas production operations which contains various organic and inorganic components. It originates from water that is trapped in permeable sedimentary rocks within the wellbore. The separated water is either disharged (to an ocean, lake or stream) or re-injecting back into rock formations from whence it originated [Hayward Gordon Ltd.; Gallup and Strong]. However, some of the produced water is fairly fresh and is readily re-used for specific purposes such as agricultural, industrial, or treated water use [Sullivan et al., 2004].
There are some major components containing in produced water such as hydrocarbons and organic compounds (e.g. oil, grease, benzene, dissolved organic compounds such as napthalene, toluene, phenanthrene, and pentacholorophenol), salts (e.g. chlorides and sulfides of Ca, Mg, and Na), metals (e.g. lead, chromium, nickel, barium, manganese, iron, strontium, zinc, silver, cadmium, lithium, copper, mercury, arsenic, selenium, and boron), radionuclides and production chemicals [Frankiewicz and Gerlach, 2000; Sumi, 2005]. Moreover, produced waters are typically more saline and have total solid dissolved concentrations from less 100 mg/l to over 300 000 mg/l compared to sea water [Stewart, 2008; U.S. EPA, 2001]. Research by United States Geological survey reported that conventional oil and gas wells produce produced water that increases over time [Sumi, 2005].
In some states in United States, surface discharges of produced water is allowed and is used for stock and watering an agriculture. However, it can be problematic to the environment due to its highly saline nature and contaminated with mercury. Neff [18] reported that, about 8 pounds per year of total mercury are discharged with produced water. The dominant forms of mercury available in produced water are suspended mercury sulfide (HgS) and elemental mercury (Hgo) [2,13]. Similar forms of mercury are also found in produced water associated with gas production in the Gulf of Thailand [Frankiewicz and Tussaneyakul, 1997]. A treatment process to remove mercury from produced water prior to overboard discharge has been developed consisting of a three-phase separator to remove the gas and also condensate [Gallup and Strong, ; Sullivan et al., 2004].
Refinery Wastewater
Refinery wastewaters streams typically contain vary widely chemical compositions having a large volume of water (per barrel of oil processed). Moreover, it also contains many regulated organic and inorganic contaminants (soluble or insoluble form) that can restrict its use and disposal thereof. In refineries, wastewater that entering the water treatment system is a composite of water discharge from processing units with different types and functions. The water streams from process units, cooling water and leakage in the system may contain some contaminated hydrocarbons [Veenstra et al., 2008].
The amount of mercury in refinery wastewater cannot be stated in certainty because of very little information is availably reported. The refinery biological water treatment generates several mercury compounds with a wide range of concentrations. The amount less than 5 percent of the total mercury concentration exists as a monomethylmercury, less than 0.01 percent as a dialkylmercury, and less than 0.1 percent as elemental mercury. Moreover, in a range of 10 to 30 percent exits as suspended particulate mercury with less than 10 percent as a labile Hg2+ and between 60 and 90 as an organochelated Hg2+. Reported by Bloom and Falke [21] that the concentration of total mercury in effluents from sewage treatment facilities is in the range of 5 to 20 ng/L.
Drilling Wastes
The exploration for oil and gas produces the drilling wastes which primarily consist of extracted cuttings and drilling muds. During drilling process, the drilling muds (also termed fluids) which inject into the well bore are identified as the sources of toxic materials that may discharges into aquatic environment surrounding offshore drilling operations [2,22,23]. Drilling muds is generally a viscous, heavy fluid designed to perform a variety of functions such as to transport rock chips (cuttings) from the bottom of the well up and out of the well bore, where the cuttings are screened and removed, and the separated mud is re-used. In addition, the drilling muds act to cool the drill bit, to stabilize the wells walls during drilling process and to control formation fluids that may flow into the well [23]. Besides, it also functions to minimize reservoir damage and limits corrosion.
Most drilling muds are engineered slurries made up from drilling fluids, liquid-based mud (such as water-based muds, oil-based muds and synthetic-based muds), barite (optional additives), low gravity solids and treatment chemicals (bentonite clay) [23,24]. The drilled formation cuttings and barite contribute to the existing mercury as a trace mineral in drilling discharges. The concentrations of mercury in barite ores can vary widely from as little as 0.05 ppm to as much as 31 ppm. Moreover, mercury in drilling discharges is completely made of inorganic mercury which is extremely insoluble in water and not readily converted into organic methylmercury. The methylation process, which bioconverted mercury to methylmercury is dependent on several variables including an anoxic environment, low pH, presence of organic materials, low salinity, and warm temperatures.
Associated Wastes
The process of producing, treating, storing and transporting of oil and gas generates low volumes of a variety of wastes such as sludges, cleaning fluids, process treatment fluids, spent catalysts, spent removal system sorbents, debris and soils [12]. These wastes commonly known as associated wastes which are produced less than 1% of total volume of waste generated by oil and gas exploration and production [2].
Sludge is a semi-solid material tends to aggregate with mixture of one and more liquids and suspended solids. In hydrocarbon processing facilities, sludges are removed from tanks and vessels during maintenance and inspection. Mercury in hydrocarbon sludge is usually higher than the process fluid in the process stream. Since mercury has higher solubility in higher molecular weight organic compounds, it tends to accumulate in the sludges and turn out to be contaminant [12].
Solid wastes such as spent materials (i.e. spent catalyst and spent adsorbents) contain a significant quantity of mercury. The catalysts used in oil refining and chemical manufacturing accumulate mercury during their operations. The spent adsorbent obtained from the separation system, which is designed to remove mercury from gas, liquid and condensate has very high mercury content. Mercury also exists and contaminates the solvent used to liquefy sludge deposit in the process equipment and in process treatment of fluids for dehydration and sweetening processes [12].
Mercury Emissions to Atmosphere
Air pollution has been linked to a number of significant problems such as ozone depletion, global climate change, acid rain, environmental degradation, and health effects in humans, plants, and animals. It is created by a number of different of sources and exits in a number of different forms. The point sources in industrial activities include chemical plants, oil refineries, power plants, hazardous waste incinerators and, oil and gas processing. It was estimated that the annual mercury emissions from oil and gas industry to atmophere in US is about 6,300 kg/year [2].
In natural gas industry, the emissions of mercury to the atmosphere could be through a glycol-overhead-gas. In the dehydration process, absorption liquids like glycols are used to absorb water and also mercury. On the elevated temperature, the glycol is regenerated and mercury is evaporated together with water. Therefore, the glycol-overheads containing mercury are released directly to the atmosphere [11].
In oil refinery, volatile and particulate mercury emissions to the atmosphere are claimed to generate mostly from the fuel combustion that are used to fire refinery process heaters and some amount from fugitive emissions. The fuels also include mostly gas and coke [25]. In refinery processing, catalyst is regenerated by using gas and some of the catalyst collect mercury and released it when regenerated. The gas from regeneration process which may contain higher amount of mercury than other typical gas flares is sent to flare [2].
Mercury Contaminated Facilities
Mercury enters into gas gathering and processing facilities through the feed gas, from instruments used to measure gas properties and in used-catalyst. Several problems occur due to the existing mercury during the restoration. Mercury contaminated on steel surfaces and creeps by adsorbs into the micro-crevices and pore, although the temperature and pressure are at below condensation and without forming amalgams [11]. The pigging operations of mercury contaminated pipelines can generate sludge and debris that have high in mercury content [11,12]. Furthermore, during maintenance and cleaning activities, spillage of mercury from instruments contaminates buildings and soil [11,12].
Mercury Waste Treatment, Recycling, And Disposal
Due to the extreme threat of mercury pollution, several remediation technologies have been developed, aim to remove mercury from wastewaters, although some works have targeted for mercury removal in gas phase. Mercury removal systems that are now prevalent for many processes which employ to protect equipment and catalysts for such systems depend on their chemical properties and process location [10]. Moreover, mercury waste can be treated and disposed by recovery; physical and chemical treatments, incineration and thermal process.
Physical, Chemical and Thermal Treatments
Physical separation methods depend on elemental mercury’s high density and surface tension. This could be achieved by allowing segregation process to taking place. In contrast, mercury compounds differ from elemental since it cannot be physically separated. One of the examples of physical treatment is by using filtration equipment which employs to remove solid mercury from the waste streams. Removal and segregate waste mercury through equipment decontamination, soil remediation, fluid decontamination or disposal sludge processing are accomplished by using chemical treatments, precipitation treatment for filtration and aqueous extraction treatment. In addition, the thermal process (refer to distillation process) is used to remove mercury from the most mercury-contaminated area of oil and gas industry [11,12,26].
Several treatment processes have been developed to remove contaminants (i.e. mercury) from produced water prior to overboard discharge. The treatment processes involved re-injecting produced water back into rock formations from whence it originated. Figure 2 shows the original water treatment process based upon a sequence of stages according to the influent oil and solid content [27]. The first stage is characterized by a de-oiling unit, where water leaves the bottom of the separator and passes through desanding and de-oiling hydrocyclones. The water then enters the chemical treatment process followed by addition of an oxidant (NaOCl), ferric ions and a flocculant sequentially to form a floatable sludge consisting of ferric hydroxide, chemisorbed mercury, ferri-arsenate, and hydrocarbons (known as flotation units). The oxidation-reduction potential of the water is controlled by oxidant addition to allow Hg in elemental form [15,16].
There are several commercial processes available to prevent the mercury contamination at processing facilities. Table 5 summarizes the mercury removal systems for hydrocarbons and water which involve adsorption, chemical precipitation, ion exchange, iron cementation, membranes separation, and activated carbon adsorption [12].
Figure 2 – Typical Produced Water Treatment [27]
Recycling and Restoration
The recovery process known as recycling or reuse method which involve a common process such as gravity separation, filtration, distillation, solvent, and chemical regeneration [28]. Physical methods could be neutralization, precipitation or separation and detoxification (chemical). Equipment decontamination is accomplished using chemical cleaning solutions that selectively oxidize complex elemental mercury deposits. These cleaning solutions consist of aqueous base solution having iodine as a complexing agent and organic solution (alcohol). In the case of incineration, the mercury contaminated waste is burnt at medium or high temperatures. For soil, sludge and debris must be thermally processed to remove mercury. The thermal process uses a vacuum, inert gas, or air as a carrier medium. However, if air is used, sulfur existed in matrix is converted to SO2 and hydrocarbons are oxidized to CO2 and H2O. Anaerobic thermal systems employ selective condensation and/or adsorption to separate sulfur and hydrocarbons from mercury. Spent adsorbent materials are also thermally processed using strictly anaerobic conditions to avoid exothermal reaction involving carbon [12,28].
As described above, mercury contamination can be preventing by using appropriate treatment processes. However, at the end of life cycle of gas installations, it may remain abandoned and need to be restored. For steel factories, scrap materials (i.e. tubings, flowlines, and facilities contaminated mercury) were cleaned before scrapping and added to steel production. Therefore, in European steel factories, the steel has to be cleaned or re-melting if the mercury contamination exceeds 2 to 10 mg per kg steel. In other cases, there are some processes available to clean mercury contaminated in pipelines and equipment such as sand blasting, high pressure water jetting, chemical process, milling techniques and also thermal treatment. Besides, the restoration or intermediate remediation of soil contaminated area is available with several soil cleaning methods [11].
Table 5 – Mercury Removal Systems for Hydrocarbons and Water [12].
Method
Process
Comments
Adsorption
(Activated carbon, Sulfur, iodine impregnated carbon )
Mercury (Hgo) physically adsorbs and reacts to form non-volatile mercuric sulfide.
Low saturation loading. Most used cheap, disposal problems.
Prevalent wet collection
To bubble gas (contain Hg) through permanganate solution. All Hg species convert to mercuric ion.
Accurate, reasonably sensitive, increased corrosivity.
Sulfide precipitation
Sulfide reacts with ionic forms of Hg to form the insoluble mercuric sulfide and separated by filtration
Increased corrosivity
Ion exchange
To remove ionic Hg from some waste streams.
Regeneration problems, system contamination.
Reverse osmosis treatment (semi-permeable membranes)
Produce a clear permeate and a concentrate containing mercury
Effective in treating specialized water streams
Iron cementation (metal replacement process)
Dissolved mercury cemented in a active metal (Zn or iron)
Carried out in acid solution
Disposal and Storage
Waste materials that contain mercury need to be identified and characterized. They must be treated prior to disposal to avoid the long-term liabilities of burial or storage. Practically, all the mercury and contaminated materials should be accounted and collected because of the potential impacts of mercury into the environments. Removal of mercury from complex mixtures can be accomplished by combination of physical, chemical, immobilization, thermal, electrolytic and in-situ vitrification treatment methods. Sludge is one of the more difficult waste materials to process for treatment and disposal due to the existing of hydrocarbon in the matrix of sludge [12,28].
In the case of drilling fluid, it is often disposed of when a well is completed, and fresh fluid is used for any adjacent wells. Filtration processes have allowed drilling fluid to be reconditioned, so that it can be used for multiple wells before being discarded. Other possible uses for used drilling fluids are to plug-in the productive wells or to spud in new wells. Reuse of oil-based and synthetic-based drilling fluids to drill additional wells is common because of the high cost of the base fluids [29].
Mercury Emission Regulations
Mercury is released through emissions from manufacturing, use or disposal activities. Environmental laws and regulations have been introduced by various regulatory bodies in order to protect the environment. Several specific laws such as Mercury Export Ban Act of 2008 and Mercury-Containing and Rechargable Battery Management Act of 1996 have been subjected related to mercury. In the case of environmental statutes such as Clean Air Act, Clean Water Act, Resource Conservation and Recovery Act, and Safe Drinking Water Act, EPA has the responsibility to develop regulations to control some mercury emissions to air, water, or from wastes and products.
The stringent regulations were recommended to set an upper-bound limit on the amount of mercury for any facilities. Besides, it is require for every power plant in the country to adopt the maximum available control technologies (MACT). The possible approaches of these MACT are to achieve the reduction in mercury emissions by setting the uniform emissions limits for existing facilities and more restrictive limits for new ones; and the mandatory emissions reductions with an emissions credit trading system.
Concluding Remarks
Mercury is often present in oil and gas with various concentration and species. Mercury is toxic to both human health and the environment. It also leads to the potential of plant failure. Mercury emissions from oil and gas exploration, production, and processing into the environment could be via wastewater (produced water, refinery wastewater), solid waste (drilling waste, refinery waste) and contaminated facilities. The mercury emissions to atmosphere originate from gas processing plant, flared gas refineries, and fuel burning for process utilities. To minimize the amount of mercury emissions, several techniques for monitoring and removal of mercury have been developed. These include segregation, treatment, recovery, and disposal of mercury waste in the process. In treatment process, it involves physical, thermal, and chemical processes to remove the contaminated mercury. The waste materials that contain mercury are usually treated to remove mercury prior to the disposal and storage.
Concluding Remarks
The financial support from the MOSTI under the e-Science Research Program (Project No. 03-01-06-SF0464) is gratefully acknowledged.
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