aromatic produced water

Report No. 1.20/324January 2002

Aromatics in produced water: occurrence, fate & effects, and treatment

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Aromatics in produced water:occurrence, fate and effects, and treatments

Report No: 1.20/324

January 2002

ii

International Association of Oil & Gas Producers

© 2002 OGP

Table of contents

Consolidated summary........................................................................................................................................ 1Defi nition and occurrence .............................................................................................................................. 1Fate................................................................................................................................................................. 1Effects ............................................................................................................................................................ 2Treatment ....................................................................................................................................................... 2

Introduction ........................................................................................................................................................... 3

Definition of aromatics........................................................................................................................................ 5Produced water composition........................................................................................................................... 5

General ...........................................................................................................................................................................................5

Detailed aromatic composition ....................................................................................................................... 7

Fate and effectsof aromatic hydrocarbons in produced water.................................................................. 9Environmental fate of aromatic compounds in produced water ........................................................................Bioavailability of aromatic compounds in produced water ............................................................................ 11Effects of BTEX, NPD and PAH in the marine environment ...................................................................... 11Environmental impact of aromatic hydrocarbons ......................................................................................... 15

Best available techniques (BAT) for the treatment of produced water (in particular aromatics) .. 16Treatment technology................................................................................................................................... 16Dispersed and aromatic species ..................................................................................................................... 16Limits of performance for dispersed oil removal ........................................................................................... 17Options & limitations for removal of aromatic species.................................................................................. 17

Absorbents .................................................................................................................................................................................... 18C-Tour .......................................................................................................................................................................................... 19Membranes ...................................................................................................................................................................................20Steam stripping .............................................................................................................................................................................20Biodegradation.............................................................................................................................................................................. 21Produced water re-injection .......................................................................................................................................................... 21

Best management practices for produced water treatment............................................................................. 22Summary...................................................................................................................................................... 22

References ............................................................................................................................................................ 23

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Aromatics in produced water: occurence, fate and effects, and treatment

© 2002 OGP

Consolidated summary

This report is the latest in a series of contributions by OGP to the work of the OSPAR Commission on a range of aspects of produced water. The report focuses on the topic of aromatic substances in produced water, covering occurrence of individual substances, fate and potential effects in the marine environment and the techniques available for treating produced water that will generally and specifi cally reduce their concentration in produced water discharged to the marine environment.

Definition and occurrence

The term ‘aromatic substances’ encompasses a diverse group of unsaturated cyclic com-pounds principally of carbon and hydrogen (some substances will contain a hetero-atom such as nitrogen, oxygen or sulphur). Aromatic substances possess a range of physical, chemi-cal and biological properties and it is neither feasible nor practicable to address their range of occurrence and potential impacts on the basis of a single categorisation. Many of the practi-cal groupings of substances are determined by appropriate analytical methodologies that are not the subject of this report. For the purpose of this report, aromatics are separated into three groups:

• BTEX: benzene, toluene, ethylbenzene, and xylene (ortho, meta and para isomers). These are monocyclic aromatic compounds.

• NPD: naphthalene and phenanthrene and dibenzothiophene, including their C1-C3 alkyl homologues These are 2-3 ring aromatic compounds.

• PAH: Polycyclic aromatic compounds, represented by the 16 EPA PAH† (except naph-thalene and phenanthrene, which are included in the NPD-group). These are 3-6 ring aromatic compounds.

The range of concentration of individual aromatic substances in produced water depends on the nature of the reservoir (whether oil, gas or gas condensate) with the highest concen-trations of individual substances in the BTEX and NPD groups; these being more soluble in water than other hydrocarbons. The bulk composition of aromatic hydrocarbons in pro-duced water does not vary signifi cantly over the life of a fi eld and there appears to be little relationship between ‘total oil content’ and the concentration of aromatic compounds.

Fate

Concentrations of aromatic hydrocarbons in treated produced water are attenuated rapidly in the sea by dilution and also, in the case of BTEX and NPD, by evaporation. For the latter groups, dilution factors of 50,000 to 150,000 have been found within 20 to 50 metres of the discharge point. For the heavier PAHs dilution rates of 1,000 to 5,000 have been measured. Moreover, BTEX and the lighter NPD compounds (naphthalenes) also degrade rapidly in the marine environment. The compounds will pass into marine organisms, but, while they may accumulate at lower trophic levels, vertebrates including fi sh have detoxifi cation mecha-nisms that break aromatic compounds down.

† The 16 EPA PAH compounds are:naphthalene, phenanthrene, acenaphthylene, acenaphthene, fl uorene, anthracene, fl uoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fl uoranthene, benzo(k)fl uoranthene, benzo(a)pyrene, indeno(1,2,3-c,d)pyrene, dibenz(a,h)anthracene, benzo(g,h,i)perylene

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Effects

The environmental effects of aromatic substances have been extensively researched and a number of toxicity mechanisms has been identifi ed. Some PAHs have been identifi ed as human carcinogens, but evidence of carcinogenicity, mutagenicity or teratogenicity attribut-able to PAHs in the marine environment is scarce. Furthermore, there is limited information on the ability of PAHs to act as endocrine disrupters. Combining information on toxicity mechanisms with modelling and monitoring studies has shown that threshold concentra-tions for toxic effects for aromatic substances are reached within, at most, a few hundred metres of the discharge point. As a consequence, taking into account the short exposure times experienced by marine organisms, the overall risk posed by aromatic substances is very low.

Treatment

The options available for the removal of hydrocarbons, whether aromatic or non-aromatic, from produced water are determined principally by whether they are dispersed or dissolved in the aqueous phase. The distinction between dispersed and dissolved hydrocarbons is not pre-cise. However, it can generally be accepted that BTEX and NPD partition to a greater degree into the dissolved phase, whereas the PAHs (excluding naphthalene and phenanthrene) are associated predominantly with oil dispersed in the aqueous phase.

We have reviewed and compared a range of types of treatment technology in terms of their ability to remove BTEX, NPD and PAHs, taking into account the source of that water (oil or gas production) and the volumes of water to be treated.

Technologies are available that can reduce or remove dissolved aromatic hydrocarbons, although most are still under development, and yet to be fi eld proven for this application. However, the techniques are heavily dependent on reservoir conditions and the quantity of produced water to be treated. In addition, all of the options reviewed must be considered as a tertiary ‘polishing’ step following on from the more ‘traditional’ removal of dispersed oil.

The application of treatment technologies to remove aromatics from the produced water presents a higher degree of complexity, risk and cost to the operations. The re-injection of produced water, as a management option, can obviate the need for introduction of aromatics to the marine environment.

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Introduction

Produced water is a by-product of the production of oil and gas hydrocarbons from under-ground reservoirs. Water is naturally present in these reservoirs and, despite all efforts to produce the hydrocarbons selectively, some water is produced, admixed as a liquid with the oil or as vapour in the hydrocarbon gas. When the hydrocarbons are produced, the water component is separated from the oil and gas in the fi rst stages of processing, and for offshore operations separated produced water has generally been discharged to the sea.

Oil and gas reservoirs contain a mixture of oil, gas and water at equilibrium; a small propor-tion of the hydrocarbons will be dissolved in water as a result of their inherent solubility. Different hydrocarbon compounds have different solubilities in water, and aromatic hydro-carbons are relatively more soluble than aliphatic hydrocarbons. There will, therefore, be a small dissolved hydrocarbon component in the produced water consisting of light aromatic hydrocarbons, in addition to suspended oil droplets.

The treatment processes for separation of oil and water before discharge of the produced water have until now been based on the use of gravitational force, utilising the difference in specifi c gravity between oil droplets and water. The oil droplets will generally fl oat to the top of the water phase where they can be removed. Gravity treatment methods are not able to remove dissolved hydrocarbon components. At wastewater treatment plants at refi neries or other facilities dealing with signifi cant quantities of hydrocarbons, biological treatment (breakdown by micro-organisms) is the best means of breaking down and removing the dis-solved hydrocarbons. This option is not available at offshore oil and gas installations.

The discharge of produced water from offshore installations has been a major topic of discus-sion in the framework of the OSPAR Commission and the preceding Paris Commission. As early as 1978, at the fi rst meeting of the Paris Commission, the Commission adopted the provisional target standard for discharges from offshore installations and recommended to national authorities that they should set limits on the total amount of waste water permitted to be discharged.

Since then, the Commissions have revisited questions related to the management of pro-duced water on a regular basis, addressing sampling and analytical problems, treatment tech-nologies (including produced water re-injection) and the types of hydrocarbons contained in produced water. Most of the ‘measures’ adopted by the Commission have focussed on the dispersed fraction; however, there has also been clear recognition of the discharges of hydrocarbon constituents in the dissolved phase, most notably the aromatic hydrocarbons. Throughout this period, OGP (until 1999 known as the E&P Forum) has made regular submissions to the debate on the technical aspects of produced water treatment and on the fate and effects of its hydrocarbon constituents after discharge.

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In 2001, at the culmination of an extensive review of Best Available Techniques for produced water management, the OSPAR Commission adopted Recommendation 2000/1 on ‘Pro-duced Water Management’. This measure addresses the management of both the dispersed and dissolved hydrocarbon components in produced water. Specifi cally in relation to aro-matic hydrocarbons, the Recommendation states (in paragraph 4.2.5):

“By the end of 2001 Contracting Parties should exchange information on methods of analysis of aromatic hydrocarbons on the basis of work in hand. Contracting Parties should collect data on aromatic hydrocarbons in produced water in particular with regard to:

a. concentrations of different groups of aromatic hydrocarbons;

b. methods of sampling and analysis for aromatic hydrocarbons;

c. BAT and BEP for the reduction of the concentrations of these substances in pro-duced water.

On the basis of this information, the Offshore Industry Committee should prepare for the Commission in 2003 a proposal for one or more performance standards, including appropriate reference analytical methods, and a timetable for the dates by which any such performance standards should be met.”

As a contribution to this work, OGP has prepared this report, drawing together the experi-ence of the industry in this area in respect of measurements of aromatic hydrocarbons in produced water, their fate and effects in the marine environment and treatment technolo-gies.

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Aromatic compounds have one or more ring structures and are held together in part by par-ticularly stable bonds that contain delocalised clouds of so-called π-electrons. Benzene is the simplest of this class of compounds, consisting of 6 carbon and 6 hydrogen atoms (monoaro-matic compound).

Polycyclic Aromatic Hydrocarbon compounds (PAH, occasionally also termed Polynuclear Aromatic Hydrocarbons) refer to hydrocarbons containing two or more fused benzene rings. The term ‘hydrocarbons’ classically refers exclusively to compounds containing only hydro-gen and carbon atoms. There are, however, many aromatic compounds which also contain atoms of other elements, such as nitrogen, sulphur, and oxygen.

In the context of the OSPAR Recommendation 2001/1 on Management of Produced Water from Offshore Installations, and according to the outcome of the OIC workshop in Voor-burg, NL (October 2001), aromatics are hereby defi ned as mono- and polycyclic aromatic compounds containing only hydrogen and carbon atoms. As an exception, it also includes dibenzothiophene, a sulphur-containing compound. Due to the wide range of concentra-tions of these compounds in produced water from different oil and gas fi elds, and also differ-ences in potential for causing environmental effects, the compounds are divided into three groups:

• BTEX: benzene, toluene, ethylbenzene, and xylene (ortho, meta and para isomers). These are monocyclic aromatic compounds.

• NPD: naphthalene and phenanthrene and dibenzothiophene, including their C1-C3 alkyl homologues. These are 2-3 ring aromatic compounds.

• PAH: Polycyclic aromatic compounds, represented by the 16 EPA PAH† (except naph-thalene and phenanthrene, which are included in the NPD-group). These are 3-6 ring aromatic compounds.

Produced water composition

GeneralProduced water contains naturally occurring dispersed oil and dissolved organic compounds, including aromatic hydrocarbons, organic acids, phenols, inorganic compounds as well as traces of chemicals added in the production/separation line. Its chemical composition varies over a wide range and depends mainly on attributes of the reservoir’s geology. The composi-tion of produced water may also change slightly through the production lifetime of the res-ervoir.

Produced water from oil production fi elds differs from that from gas production fi elds. Water from gas production fi elds generally has a higher content of low molecular weight aromatic hydrocarbons, such as BTEX (benzene, toluene, ethylbenzene and xylene), than water from oil production platforms. However, the total amount of water produced from gas fi elds is much smaller than from oil production fi elds. While many gas fi elds discharge less than 10m3 of produced water per day, most oil fi elds discharge hundreds or even thousands of m3 of produced water per day.

The composition of produced water changes during the lifetime of a fi eld. At fi rst, wells produce very low quantities of water. This is water that has condensed in the well, due to

Defi nition of aromatics

† The 16 EPA PAH compounds are:naphthalene, phenanthrene, acenaphthylene, acenaphthene, fl uorene, anthracene, fl uoranthene, pyrene, benz(a)anthracene, chrysene, benzo(b)fl uoranthene, benzo(k)fl uoranthene, benzo(a)pyrene, indeno(1,2,3-c,d)pyrene, dibenz(a,h)anthracene, benzo(g,h,i)perylene

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pressure drop between downhole and surface. After a couple of years, wells start to produce increasing quantities of water from the reservoir. The composition of this water is rather stable (as shown in table 3, with regards to aromatic compounds). However, if water is injected for pressure maintenance, or for any other reason, injected water may reach the pro-duction well (water breakthrough). Then the composition of produced water will change, due to dilution of the reservoir water by the injection water. This would displace the water composition away from equilibrium and create the potential for additional dissolution of aromatics from the oil phase.

The chemical composition of produced water has been described extensively by several authors (Brendehaug et al, 1992; Cofi no et al, 1993; Jacobs et al, 1992; Sørstrøm et al, 1993; Flynn et al, 1996; Utvik, 1999). OGP has conducted similar reviews in its Fate & Effects report (E&P Forum, 1993) and a subsequent update (E&P Forum, 1997). Many of the stud-ies listed, however, present results from analysis of aggregate groups of compounds rather than as individual compounds in produced water.

In 1998, the Norwegian Oil Industry Association (OLF) prepared guidelines for chemical characterisation of produced water for the Norwegian sector of the North Sea. In the Dutch, UK and Norwegian sectors, operators are now reporting on the composition and volumes of their discharges (composition and volumes). Measurements are done at least annually. Guidelines specifi cally for determination of concentrations of aromatic compounds are being developed and they were one of the main topics discussed at a workshop held in Voorburg in October 2001.

Data on aromatic compounds of produced water in all sectors of the North Sea are sum-marised in Table 1, according to the three groups of compounds defi ned above.

Table 1: Concentration range (mgl-1) of aromatic compounds in PW from oil & gas fi elds in the North Sea (1999-2001)

Sector N N UK UK NL NL DK DKProduction Oil Gas Oil Gas Oil Gas Oil Gas

BTEX1 0.7-24.1 1.9-36 <0.5-34 0.5-2244 0.042-4.8 0.01-1164 8.7-14 N/A

NPD2 0.8-10.4 0.24-0.8 0.007-0.74 0.001-0.74 N/A N/A 0.22-0.436 N/A

PAH3 0.001-0.13 0.003-0.05 0.002-0.12 0.0004-0.23 0.0026-0.1545 0.002-4.125 0.12-0.285 N/A1. BTEX: benzene, toluene, ethylbenzene, xylene (ortho, meta and para isomers)2. NPD: naphthalene, phenanthrene, dibenzothiophene, including their C1-C3 alkyl homologues3. PAH: polycyclic aromatic hydrocarbons represented by 16 EPA PAH, except naphthalene and phenanthrene4. Only naphthalene and phenanthrene (parent compounds) included5. Naphthalene and phenanthrene included6. Only naphthalene (parent compound) includedN/A not available

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Detailed aromatic composition

Results from detailed characterisation with regard to aromatic compounds (BTEX, NPD, PAH) from 18 production fi elds operated by Norsk Hydro and Statoil in the Norwegian sector of the North Sea are shown in Table 2. The data are mean concentrations based on analysis of three replicate samples from each of the 18 platforms. All data are from 2000.

Table 2: Concentration of aromatic compounds in produced water from 18 oil production fi elds operated by Norsk Hydro and Statoil in the Norwegian Sector of the North Sea

Compound Min. concentration Max. concentration (µg l-1) (µg l-1)

Sum BTEX 730 24070

Benzene 32 14966

Toluene 58 5855

Ethylbenzene 86 565

m-Xylene 258 1289

p-Xylene 74 331

o-Xylene 221 1064

Sum NPD 766 10439

Naphthalene 194 841

C1-naphthalenes 309 2901



Phenanthrene 9 111

C1-phenanthrenes 17 323



Dibenzothiophene 1 23

C1-dibenzothiophenes 6 103



Sum 16 EPA PAH1 5.8 129.2

Acenaphthylene 0.1 6.1

Acenaphthene 0.3 15.3

Fluorene 4.1 66.7

Anthracene 0.1 2.6

Fluoranthene 0.1 3.6

Pyrene 0.2 7.7

Benz(a)anthracene 0.1 2.8

Chrysene 0.6 15.2

Benzo(b)fl uoranthene 0.1 3.4

Benzo(k)fl uoranthene 0.0 0.6

Benzo(a)pyrene 0.0 1.1

Indeno(1,2,3-c,d)pyrene 0.0 0.4

Dibenz(a,h)anthracene 0.0 1.2

Benzo(g,h,i)perylene 0.0 2.71. These concentrations include contributions from naphthalene

and phenanthrene

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The aromatic fraction in produced water is dominated by BTEX and NPD. These com-pounds are the most water-soluble and, for most fi elds, their concentration in water will mainly be a result of the phase equilibrium in the reservoir (rather than the separation step on the installation). Nevertheless, there are substantial BTEX and NPD concentrations in the oil phase. High molecular weight PAHs are less water soluble. They will be present mainly in, or associated with, the dispersed oil.

During the removal of dispersed oil, BTEX and NPD concentrations will be less infl uenced by the effi ciency of the separation process than the high molecular weight PAHs. There is relatively weak correlation between the total concentration of aromatic compounds (domi-nated by BTEX and NPD) and the oil-in-water content. For high molecular weight PAHs, however, much stronger correlation exists, since a higher proportion of these compounds is found in the oil phase.

Composition and standard deviation, in mg l-1

Year 1998 1999 2000

BTEX1 5.8 ± 0.5 5.6 ± 0.3 6.6 ± 0.3

NPD2 1.60 ± 0.08 1.10 ± 0.03 1.30 ± 0.04

PAH3 0.027 ± 0.003 0.016 ± 0.001 0.020 ± 0.001BTEX: benzene, toluene, ethylbenzene, xylene (ortho, meta and para isomers)NPD: naphthalene, phenanthrene, dibenzothiophene, including their C1-C3

alkyl homologuesPAH: polycyclic aromatic hydrocarbons represented by 16 EPA PAH, except

naphthalene and phenanthreneNote: Different laboratories were used in each year. Data are means of analysis

of three replicate samples each year.

Studies of chemical composition of aromatic compounds from one specifi c fi eld over a period of one week show that there is little variation in composition during that time span (Utvik, 1999). Results from annual environmental reports and data from the OLF Produced Water database show that the composition of produced water is also relatively constant from one year to another at individual fi elds.

Table 3: Concentration of aromatic compounds (mg l-1) in produced water from the oil production fi eld Oseberg C from 1998 to 2000:

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Environmental fate of aromatic compounds in produced water

In the fi rst phase of production from an oil or gas reservoir, the main source of water is water that has condensed in the production tubing. At that stage the water will be relatively fresh. As production continues and water is recovered from the reservoir, the fl uid becomes increas-ingly more saline.

Produced water will disperse rapidly on discharge to an environment such as the North Sea, since this discharge would be into well-mixed waters. Dispersion modelling studies and fi eld measurements of the fate of produced water in the North Sea show a rapid initial dilution of the discharges by 1000-fold within the fi rst 50-100 metres downstream from the discharge point (Furuholt, 1996; Riksheim and Johnsen 1994).

The most abundant aromatic compounds in produced water, the BTEX compounds, are volatile and will evaporate rapidly from produced water discharged close to the sea surface or from produced water discharge plumes reaching the surface due to density gradients. These losses, allied to dispersive mixing, result in 50 000 to 150 000-fold reduction of the benzene concentration in seawater, 20 metres away from the produced water discharge point (Brooks et al, 1980; Rabalais et al, 1991; Terrens and Tait, 1996).

The NPD compounds are less volatile, but will also evaporate to some degree. This is par-ticularly important for high-temperature produced water discharges, or for produced water with a gas/air injection before discharge.

The less water-soluble fraction of aromatic compounds, the PAH compounds, is expected to be associated with particulates and oil droplets in the produced water. As the discharge plume for most fi elds will rise towards the surface after discharge (principally due to its tem-perature), these compounds will follow the plume, or be retained at certain depths of the water column depending upon the buoyancy of the supporting particulate matter.

Aromatic hydrocarbons span the whole range from readily to poorly biodegradable, depend-ing on the nature of the actual compound. Biodegradation half-lives ranging from less than a day up to several months are described in the literature (Johnsen et al, 2000), with the lower molecular weight (and more abundant) compounds being more degradable.

In 1999, the Norwegian pollution control authority (SFT) introduced water column moni-toring in their guideline for environmental monitoring of petroleum industry activity on the Norwegian continental shelf. Water column monitoring includes measurements of aromatic hydrocarbons (naphthalenes and selected PAH) on a regional scale. Several sampling and analytical techniques have been employed for this purpose, and the monitoring programme has resulted in a signifi cantly improved understanding of the environmental fate of produced water originating aromatic hydrocarbons (Johnsen et al 1998, Utvik et al 1999, Durell et al 2000).

Typical water column concentrations of naphthalenes and PAHs at 10 metres depth and at different distances from an oil production platform are shown in Table 4.

Table 4: Water column concentrations (in µg l-1) of naphthalenes and PAHs at different distances from an oil production platform in the Norwegian Sector of the North Sea

Fate and effects of aromatic hydrocarbons in produced water

Concentration (in µg l-1) of napthalnes and PAHs: in PW at 500m at 2000m at 10000m

Naphthalenes 1200 0.040 0.013 0.007

PAHs1 33 0.004 0.001 0.00041. In this case, ‘PAHs’ means here the summed concentration of the16 EPA

PAH, except naphthalene.Note: the concentrations are calculated from mussel residue measurements as

described in Utvik et al (1999).

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Comparison of predicted and measured concentrations of naphthalenes and PAHs show a good correlation in studies performed in the Tampen (Utvik et al, 1999) and Ekofi sk areas (OLF, 2000). Figure 1 shows the comparison of C1-naphthalene concentration predicted by the DREAM model versus water equivalent concentrations calculated from semi-permeable membrane devices (SPMD) and blue mussel residues for all sampling sites in the Ekofi sk region (OLF, 2000). The levels predicted by the model are generally in good agreement with the observed concentration levels. This therefore supports the use of the model for regional evaluation of produced water fate and dilution.

Figure 1. Comparison of C1-naphthalene concentration predicted by the DREAM model vs. water equivalent concentrations calculated from SPMD and mussel residues.

0

2

4

6

8

10Mussel

SPMD

Model

500 2000 5000 10000 Referencesite

Co

nce

ntr

atio

n (

ng

/l)

Distance from discharge point (m)

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Bioavailability of aromatic compounds in produced water

Bioavailability can be defi ned as the concentration of a compound in water that can be absorbed by an organism. It is one of the most important factors that infl uence the uptake and bioaccumulation of compounds in aquatic organisms (Gobas, 1992). For many com-pounds, transport through biological membranes requires that the compound is dissolved in the surrounding water. In this case, the bioavailable fraction is then equal to the water-soluble fraction of the compound. Factors that infl uence bioavailability are: adsorption to suspended solids, sediments, or macromolecules (eg humic acids), formation of colloidal sus-pensions, chelation, complexation and ionisation (Spacie et al, 1995).

There are several methods for estimating the bioavailable fraction of organic compounds. Membrane-based passive samplers, including the semi-permeable membrane devices (SPMDs), consisting of ‘layfl at’ tubing of polyethylene fi lled with neutral lipids (Huckins, 1990; Huckins et al, 1993), represents one approach. Blue mussels (mytilus edulis) and other bivalve molluscs have been used as indicator organisms due to their ability to accumulate trace levels of certain pollutants from the water column. Mussels have been used for monitor-ing hydrocarbon levels by placing cultured mussels of same age, size and history at sampling sites in cages, often in combination with membrane-based techniques (Herve, 1991; 1995; Prest et al, 1995; Peven et al, 1996; Hofelt and Shea, 1997). This method is also the basis for the Norwegian water column environmental monitoring programme mentioned above. It must be borne in mind, however, that the potential exposure of mussels in their natural habitats will be smaller than the exposure of mussels intentionally placed within produced water plumes.

Caged fi sh have also been used in environmental monitoring programmes to investigate bio-availability of compounds from the water and their effects, though the exposure received by caged fi sh will be substantially greater than that experienced by fi sh in the wild. The ability of fi sh to transform or metabolise PAHs rapidly through activation of a detoxifi cation system (mixed function oxidase enzymes) severely limits possible correlation of PAH concentrations in fi sh tissue with exposure concentrations in water. It is, therefore, diffi cult to predict bio-availability of a compound by this technique. The method has been included in two fi eld surveys in the Norwegian sector of the North Sea, in 1996 and 2001. While the 2001 study is still in progress, the 1996 results showed that no signifi cant biological effects were found in the fi sh placed 500 metres downstream one of the major discharge points in the Norwegian sector (the Troll fi eld).

Effects of BTEX, NPD and PAH in the marine environment

In general, the mechanisms of toxic action of aromatic hydrocarbons, such as PAH, are still subject to considerable discussion (van Brummelen et al, 1996), but it has been clearly demonstrated in the literature that PAHs do not have one single type of toxic action. Dif-ferent toxicity mechanisms play a role depending on the compound, the exposure (acute or chronic), the organism and the environmental compartment. A number of toxicity mecha-nisms have been linked to PAH, including non-polar narcosis, phototoxicity, biochemical activation that, in turn, may result in mutagenicity, carcinogenicity and teratogenicity. Some PAHs may also have infl uence on hormone regulation, also referred to as endocrine disrup-tion. However, the experimental evidence for disturbance of hormone regulation is quite lim-ited. Table 5 gives an overview over different chronic biological effects linked to the aromatic hydrocarbons discussed in the present paper.

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Table 5: Toxicological characteristics of selected aromatic compounds (chronic effects). Based on data from Varanasi, 1989.

Compound Carcinogenicity Genotoxicity Other effects

BTEX

Benzene - - -

Toluene - - -

Ethylbenzene - - -

Xylene - - -

NPD

Naphthalene - - -

1,2-methylnaphthalene - - -

Phenanthrene -/? - -

PAH

Acenaphthylene -/? + -

Anthracene - - -

Fluorene - - -

Benz(a)anthracene + + -

Chrysene + + -

Fluoranthene +/? + -

Pyrene -/? + -

Benzo(a)pyrene + + Embryotoxic, Teratogenic

Dibenz(a,h)anthracene + + -

Perylene -/? + -

Indeno(1,2,3-c,d)pyrene + + -Note: an Entry of ‘-’ means ‘no reported toxicity; an entry of ‘+’ means ‘reported effect’; ‘? ’ means the

presence/absence of an effect is unresolved.

Non-polar narcosis or baseline toxicity is a non-specifi c mode of toxicity of non-polar organic compounds and is thought to result from accumulation in biological membranes. Some PAHs may be more reactive and have a non-specifi c mode of action and thus have a more specifi c mode of toxicity than baseline toxicity. If a narcosis-like mode of action is responsible for the toxicity of PAHs, the toxicity is expected to increase with increasing molecular weight and hydrophobicity (indicated by log POW - octanol/water partition coeffi cient) of the indi-vidual PAH (McCarty et al, 1992; Verhaar et al, 1992). Acute toxicity can be predicted on the basis of Quantitative Structure-Activity Relationships (QSAR) , developed for narcotic chemicals, by applying the log LC50/log POW regression (McCarty et al, 1992).

The literature contains a considerable amount of information on effect values obtained in short-term toxicity studies. Under such conditions, non-polar narcosis is the most likely mechanism of toxicity to be observed. Effects due to more specifi c toxicity mechanisms, such as biochemical activation and subsequent mutagenic and carcinogenic effects, and distur-bance of hormone regulation, may occur in the environment as a result of prolonged expo-sure to relatively low concentrations of PAH. Chronic toxicity data from long-term exposure to PAH are scarce. Based on fi ndings that fi sh and shellfi sh living in their natural environ-ments near produced water discharges do not accumulate PAH to an environmentally sig-nifi cant extent (Offshore Operators Committee, 1997), the potential extent of exposure to PAH and potential toxicity from that exposure route can be considered low.

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Frost et al (2001) have carried out a comprehensive literature review of the effects of PAH in the aquatic environment. The purpose of this study was to establish a basis for the deter-mination of Predicted No Effect Concentrations (PNECs) for PAH, to develop a produced water management system called the Environmental Impact Factor (EIF) to be used by the Norwegian oil industry (Johnsen et al, 2000). In this work, different toxic effect levels of aromatic compounds (mono-aromatic, di-aromatic and PAH) on marine and freshwater spe-cies were collected from several databases (Johnsen et al, 2000; Frost et al, 2001). To secure the relevance and reliability of data for the PNEC calculations, certain quality criteria were applied for the selection and approval of the data. Toxicity studies with experimental design in agreement with international accepted guidelines/test methods (OECD, EEC, EPA, ISO or other) were considered to be reliable. The use of results from studies not following stand-ard guidelines were based on selected quality criteria applied by ECETOC in the establish-ment of ECETOCs aquatic toxicity database (ECETOC, 1993).

The PAH toxicity data used as the basis for the EIF modelling are shown in Table 6. The table shows the lowest estimated PAH concentration that will cause a toxic effect in the aquatic environment (ie PNEC values). All literature data meet the quality criteria outlined above.

Table 6 Lowest toxicity level of aromatic hydrocarbons in the aquatic environment (Frost et al. 2001)

Compounds Endpoint Trophic level Conc. value(µg l-1) Assessment factor PNEC(µg l-1)

BTEX

Benzene NOEC (20 d) Crustacea (M) 170 10 17

Ethylbenzene LC50 (96 h) Crustacea (M) 490 1000 0.49

Toluene NOEC (21 d) Crustacea (F) 1000 10 100

Xylene LC50 (96 h) Fish (F) 1200 1000 1.2

Naphthalenes

Naphthalene NOEC (40 d) Crustacea (M) 21 10 2.1

PAH (2-3 rings)

Phenanthrene NOEC (60 d) Fish (F) 1.5 10 0.15

Anthracene NOEC (21 d) Crustacea (F) 0.63 50 0.013

PAH (4+)

Chrysene NOEC (21 d) Crustacea (F) 1.4 100 0.014

Pyrene - - - - -

Benzo(a)pyrene NOEC (42 d) Fish (F) 6.3 100 0.063

In general, lower molecular weight aromatics are less toxic to aquatic organisms than higher molecular weight PAH. This is directly linked to the ability of compounds to bioaccumulate, which is known to increase with increasing molecular weight. However, when the molecu-lar weight reaches a certain level, PAHs are unable to pass through cell membranes in a living organism, due simply to the size of the molecules. In practice, this limit lies around the molecular weight corresponding to 6-ring compounds. Lowest observed chronic toxic-ity of aromatics is found to vary from 0.65 µg l-1 through the aromatic compound family; mono-aromatics, being the less toxic group (ie having higher numerical toxicity concentra-tion values). The table also shows assessment factors and corresponding PNEC values for the aromatic compounds, as determined for EIF values, based on the Technical Guidance Document (TGD) principles (EC, 1996).

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Acute and chronic toxicity may also be determined theoretically from the log KOW and molecular weights by applying QSAR techniques (McCarty et al, 1992). Neff and Johnsen (2001) have determined the acute and chronic toxicity of a large number of PAH found in produced water. The results of these calculations are shown in Table 7. Chronic toxicity levels determined by this method may be directly comparable to the PNEC values determined by the TGD method.

Table 7 Acute and chronic toxicity levels of aromatic hydrocarbons determined from the method described by McCarthy et al (1992). (all numbers given in µg l-1)

Compound Log KOW Acute toxicity Chronic toxicity

NPD

Naphthalene 3.37 4872 195

C1-Naphthalenes 3.87 1422 569

C2-Naphthalenes 4.37 406 16.2

C3-Naphthalenes 5.0 83.4 3.3

Phenanthrene 4.57 274 11.0

C1-Phenanthrenes 5.14 64.4 2.6



Dibenzothiophene 4.49 352 14.1

C1-Dibenzothiophenes 4.86 141 5.6

C2-Dibenzothiophenes 5.5 27.2 1.1

C3-Dibenzothiophenes 5.73 15.6 0.62

PAH

Acenaphthylene 4.07 898 35.9

Fluorene 4.18 731 29.3

Anthracene 4.54 298 11.9

Fluoranthene 5.22 54.6 2.2

Pyrene 5.18 60.7 2.4

Benz(a)anthracene 5.91 9.8 0.39

Chrysene 5.86 11.2 0.45

Benzo(b)fl uoranthene 5.8 14.4 0.57

Benzo(k)fl uoranthene 6.0 8.6 0.34

Benzo(a)pyrene 6.04 7.6 0.30

Benzo(ghi)perylene 6.5 2.4 0.09

The chronic toxicity levels determined by this method, using the acute:chronic toxicity ratio (about 20) for a limited number of PAHs are, in general, lower than the values reported in the literature. As seen from the table, chronic toxicity levels range from approximately 0.5 mg l-1 for the naphthalenes (2-ring aromatics) to 0.05 µg l-1 for 5 and 6-ring PAH. The difference is probably caused by the fact that most theoretical approaches to toxicity calcula-tions tend to involve more conservative assumptions, where different safety or assessment factors are included to account for uncertainties in the comparison of laboratory tests and fi eld conditions (Neff and Johnsen, 2001). The high level of conservatism can be assessed by considering measurements of acute and chronic toxicity of produced water samples. These indicate that chronic (eg 7-day) no effect concentrations are only about 2-4 times lower than short-term (eg 96 hour) LC 50s (Moffi tt et al, 1992). As a result, comparison of the chronic toxicity values from Neff and Johnsen to the PNEC values from the TGD method may be more relevant.

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Environmental impact of aromatic hydrocarbons

To assess the environmental impact or potential effect of any pollutant, the most common approach is to compare the expected effect level derived from laboratory tests to the con-centration and distribution of the substance in the environment. This is the basis of most risk assessment methods applied for the purpose of discharge management. In practice, this implies a comparison of the toxicity levels derived for different compounds to the environ-mental concentrations derived from monitoring programmes or dilution modelling. Risk assessment, as defi ned by the EU Technical Guideline Document (TGD), introduces a set of assessment factors to account for the uncertainty in, and expected difference between, laboratory tests and real-life conditions (EC, 1996). This method is also the basis for the development of the EIF, a produced water management tool developed and used by both the oil industry and authorities in Norway.

A direct comparison between the levels of aromatic hydrocarbons measured and modelled in the vicinity of production fi elds in the North Sea shows that concentration levels in the envi-ronment exceeding the toxicity threshold levels derived from tables 6 and 7 are only found inside a distance of 500 metres from the discharge point. The distance is, of course, depend-ent on the actual current/hydrodynamic situation and may vary considerably between loca-tions and over time. Also, this does not imply that such concentrations occur everywhere within the 500-metre circle around an installation. The water body required to dilute the discharges below the calculated threshold limit is, however, only dependent on the discharge concentration. In general, a dilution factor of 1000 or less is suffi cient to reach the environ-mental threshold concentration (PNEC or chronic toxicity level) for aromatic hydrocarbons. All monitoring and modelling data from offshore fi elds in the North Sea show that this is found within a few hundred metres away of discharge point.

Dilution models also show that a 1000-fold dilution will occur within a few minutes after the produced water plume hits the sea. Thus the exposure time to aromatic hydrocarbon concen-trations higher than the PNEC or chronic toxicity levels to marine organisms is short. The limited potential for exposure of organisms living near discharges is supported by the obser-vation that fi sh and shellfi sh living near produced water discharges do not accumulate PAHs to an environmentally signifi cant degree (Offshore Operators Committee, 1997). From this, one can conclude that the potential risk associated with aromatic hydrocarbons in produced water to the environment is low.

Risk assessment studies performed by calculating the Environmental Impact Factor (EIF) for all operating fi elds in the Norwegian sector support this observation (Johnsen et al, 2000). The tool can be used to undertake fi eld and region specifi c evaluation of produced water environmental effects, and to identify measures to reduce the environmental risk posed by these discharges if needed.

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Best available techniques (BAT) for the treatment of produced water (in particular aromatics)

Treatment technology

The techniques and technologies used by industry to remove oil from produced water have changed considerably over the last twenty years. In part this has been driven by regulatory compliance, but substantial improvements have come about voluntarily from within the industry, by the need to achieve more consistent levels of performance and thus improve overall operational effi ciency.

A range of technologies exists for the treatment of produced water and many of these are often classed as best available techniques (BAT). A recent report from the Netherlands (SEBA 99/8/3) lists and details various treatment technologies for produced water, their principle of operation and scale of application along with typical performance levels. The main technologies for produced water treatment used or under evaluation by the industry are highlighted in Table 8.

As expected the majority of these will remove dispersed oil and some are also able to remove or reduce the aromatic components from the produced water. However, the performance achieved by each system will greatly depend on the process variables at each installation. These variables include: reservoir type, temperature, pressure, oil type and viscosity, emul-sion stability, oil droplet size, water salinity, fl ow rate etc, and each of these variables can signifi cantly affect the performance of any of the treatment technologies outlined in the BAT listing.

Table 8: Current Produced Water treatment technologies

Technology type Can be used for removal of: Dispersed oil BTEX NPD PAH

Physical separation

Flotation Yes No No Some

Sparging Yes No No Some

Coalescence Yes No No Some

Enhanced separation

Hydrocyclones Yes No No Some

Hydrocyclones + coalescence (Pect-F, Mare’s Tail) Yes No No Some

Centrifuges Yes No No Some

Alternative (new) technology

Absorption (polymers, MPPE, clays) Yes Yes Yes Yes

Adsorption (carbon, natural fi bres) Yes Yes ? ?

Hydrocyclones + solvent extraction (C-Tour) Yes No Yes Yes

Membranes Yes Yes Yes Yes

Steam Stripping No Yes No No

Biological (aerobic degradation) Yes Yes ? ?

PWRI Yes Yes Yes Yes

Dispersed and aromatic species

The majority of the technologies listed in the Netherlands report submitted to the OSPAR Working Group on Sea-Based Activities (SEBA, document 99/8/3)) as BAT (or more cor-rectly as techniques that may form part of a BAT strategy), are designed to remove dis-persed hydrocarbons only. Their principle of operation is based on gravity or enhanced separation, and utilises the difference in specifi c gravity between the oil and water. Over the last decade, enhanced gravity separation technologies have become more common (hydro-cyclones and centrifuges), enabling further improvements in overboard water quality to be achieved. However, these technologies primarily remove dispersed oil and cannot normally remove soluble oil or dissolved aromatic components.

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As highlighted in a previous section of this report (Defi nition of aromatics), the aromatic frac-tion in produced water is dominated by BTEX and NPD. As outlined, these compounds are the most water-soluble, whereas the higher molecular weight PAHs are less water soluble, and will be present mainly in, or associated with, the dispersed aliphatic oil droplets. This indicates that, at least for the PAH fraction of the total aromatic content, a correlation exists between PAH concentration and the currently measured dispersed oil content, ie a lower dispersed oil content yields a lower PAH concentration.

Limits of performance for dispersed oil removal

Currently most offshore water treatment facilities can achieve <40 mg l-1, with the most common treatment package utilising hydrocyclone technology, followed by some type of simple polishing technology, such as a degasser vessel. Typically, this type of combined system may be able to achieve a 15-30 mg l-1 discharge, depending on process variables and oil type.

Further removal of dispersed oil from produced water has also been demonstrated in some operations by modifying the oil droplet characteristics in the upstream feed to the primary treatment system. This includes the use of mechanical coalescing systems (Pect-F, Mare’s tail) and chemical fl occulation and coagulation systems (G-Floc and CodeFlo). Additional (polishing) treatment technologies are also commercially available to reduce the dispersed oil content further. These include centrifuges, absorbents, membranes and biological treatment systems. However, their use is limited on throughput, weight, space and economic grounds.

Key factors for offshore operations are weight and space limitations. The application of hydrocyclone technology (most widely used in the oil industry) enables large water volumes to be processed, often without the need for any additional power input or chemical require-ments. The short residence (hold-up) time for hydrocyclones is very low, often seconds, and as such can contribute to signifi cant reductions in the size, weight and thus the cost of the treatment facilities. In addition, because of the low weight and size requirements, the industry has been able to retrofi t this technology into older installations, resulting in improved levels of performance compared with previously employed treatment technologies (tilted plate separators, skim tanks, fl otation units and coalescers), thus improving the overall industry performance.

Options & limitations for removal of aromatic species

As shown in Table 8, all of the technologies outlined can remove the dispersed oil fraction, with the actual performance being dependent on the fl uid properties and process conditions. As a result of the removal of the dispersed oil droplets, an associated reduction in the PAH concentration is to be expected. However, what is evident is that there is only a small number of technologies suitable for the removal of dissolved aromatic hydrocarbons.

Using data from Table 8 and from industry reports looking at treatment options: (E&P Forum, Report 2.71/247 - Technologies for handling produced water in the offshore environ-ment, September 1996; OIC Document 01/8/8-E (L) - Options for Reducing Aromatic Hydro-carbons in Offshore Produced Water submitted by Denmark in 2001; and SEBA Document 99/8/3 - Factsheets on Current BAT and Emerging Candidates for BAT for Produced Water Management on Offshore Oil and Gas Installations, submitted by The Netherlands); there are several treatment options that have been identifi ed which can reduce and/or remove aromatic components from the produced water and their advantages and disadvantages of each of these are highlighted. These include:

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• C-Tour

• Absorbents - macro porous polymer extraction (MPPE)

• Membranes

• Stripping

• Biodegradation

• PWRI

However, before considering these options, it must be borne in mind at the onset that, in general, these technologies would need to be applied as a fi nal “polishing or tertiary” treatment after the removal of the dispersed oil phase, and on low volume applications. As such, the ability to incorporate these technologies into oil operations in particular would be severely limited due to the weight and space limitations.

Absorbents

Principle:

There are two types of absorbent available: those that cannot be regenerated (modifi ed clays, wood and fi bres), and those that can (activated carbon, MPPE, etc). Both systems can be designed for either a high removal effi ciency of specifi c hydrocarbons (ie aromatics) or dis-solved species in general.

Non-regenerative:

Once all available material is used, the medium needs to be removed and replaced, and the waste absorbent material shipped to land for disposal. The amount of dissolved hydrocar-bons in the produced water will dictate the replacement frequency of a particular medium.

Regenerative - MPPE:

In principle, the technique can remove all organic substances with a different polarity from water. In the technique, hydrocarbon contaminated water is passed through a column packed with MPPE particles. An extraction liquid immobilised within the polymer matrix removes the hydrocarbons from the water. The purifi ed water passes out of the column directly for reuse or discharge. In-situ regeneration of the extraction liquid containing MPPE particles is accomplished using low-pressure steam: the steam volatilises the hydrocarbons. Volatilised hydrocarbons are condensed and then separated by gravity. The hydrocarbon phase is recovered, and the water phase is recycled to the system.

Advantages:• MPPE technology has been successfully evaluated and installed by operators in Norway

and onshore in the Netherlands for the removal of aromatics from produced water.

• The process can remove the majority of all aromatics (BTEX, NPD and PAHs) in pro-duced water.

• Several operators in the Netherlands have plans to install MPPE systems on specifi c off-shore installations, mainly gas fi elds.

Disadvantages:• The system requires a minimum of 2 vessels, so that one can be regenerated while the

other is in operation, and a separate steam generation facility.

• This system is likely to be used as an additional water treatment stage (polishing).

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• The process is seen to be limited to low volume/high aromatic loadings, ie gas opera-tions, due to size and weight factors. For example, a unit capable of treating 10 m3 h-1, will be 5m × 3m × 4.5m, and weigh 22 tonnes.

• The MPPE particles can degenerate over time and need replacement. The supplier claims 1.5 to 3 years lifetime in industrial units.

• Feed water temperature of 60°C is required for optimal operation.

• Energy intensive (approx 6 kWh m-3 water), with high cost and maintenance.

C-Tour

Principle:

The C-Tour Process System is an enhancement to hydrocyclone technology based on the extraction of hydrocarbons from water using gas condensate. The injected gas condensate acts as an extraction-solvent. The principle in extraction processes is to add an immiscible solvent in a solution that will absorb the solute (in this case the aromatics) because of their higher affi nity towards the extraction solvent. The solvent extracts the dissolved hydrocar-bons from the water phase and these are then removed in the hydrocyclone.

Advantages:• This could be an enhancement to existing hydrocyclone technology, and thus potentially

could be used in high volume applications.

• It is a potentially single stage process for both dispersed and aromatics removal.

• May reduce the need for some production chemicals.

• The injected solvent can reduce the NPD and PAHs (reported up to 95% in the lab), and possibly some BTEX.

Disadvantages:• Only a specifi c grade of gas condensate will suffi ce as the solvent, and this limits platform

selection.

• The process will require a ratio of approx 2% condensate to water.

• High-pressure re-circulation equipment (> 10 bar) is required.

• Pressure in produced water must be above 10 bar.

• This technology has yet to be validated in the fi eld (an initial fi eld test in Norway was unsuccessful, but encouraging), and further testing is planned after recent laboratory tests and modifi cations.

• The condensate used as the solvent can increase the fi nal concentration of BTEX com-ponents in the effl uent stream, although these are deemed to fl ash-off rapidly upon dis-charge.

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Membranes

Principle:

Removes dissolved hydrocarbons at the molecular level. All hydrocarbons larger than the membrane material used will be rejected (including dispersed oil).

Advantages:• Dependent on membrane type, the process can remove the majority of all aromatics

(BTEX, NPD and PAHs) in produced water.

Disadvantages:• Unproven in oil fi eld-type applications.

• Fouling of the membranes is a major problem and aromatic hydrocarbons are known to be incompatible with some materials (due to swelling, etc).

• This system could only be used as an additional water treatment stage (polishing).

• Membranes with a low molecular cut-off often have a low fl ux rate. Therefore, additional capacity is required for sustaining long-term use (weight and space then become a prob-lem).

Steam stripping

Principle:

Stripping and distillation is based on vapour-liquid equilibrium. Produced water components with boiling points lower than the boiling point for water, such as BTEX and other light hydrocarbons, can normally be separated using a simple stripper confi guration. In a typical stripper column a vapour stream enters at the bottom while the liquid enters the top. The vapour stream, which could be steam, air or another gas, will then pick up the most volatile components in the liquid stream.

A small number of systems has been installed offshore, not for produced water treatment, but mainly for the treating the process water (condensate) recovered from the di-ethylene glycol (DEG) and tri-ethylene glycol (TEG) regeneration units on the platforms.

Advantages:• It is possible to reduce the effl uent concentration of the dispersed oil and the BTEX to

<1 ppm.

Disadvantages:• The system has not been evaluated commercially for produced water treatment or for the

removal of other organics (NPD and PAHs).

• The system has a high space and weight requirement and is therefore suitable only for low volume, high contaminant loading applications.

• The process is vulnerable to process disturbances, therefore pre-treatment of the pro-duced water is preferable, eg by using hydrocyclones.

• The air/hydrocarbon mixture must be treated before release or the mixture must be fl ared.

• Steam stripping requires that steam is available and the process is energy intensive.

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Biodegradation

Principle:

The use of microbiological organisms to break down and remove oil and aromatics from produced water. The system requires a medium and long residence time for the organisms to grow and stabilise for effective operation.

Advantages:• This is an effective method of reducing dissolved hydrocarbons in water.

• Proven technology onshore for the treatment of produced water prior to discharge to rivers.

• Best suited to onshore installations where space and weight are not limitations.

Disadvantages:• Due to the large water hold-up volume and culture contact time, these systems are very

large and heavy, and therefore only suitable offshore for low volume applications (if at all).

Produced water re-injection (PWRI)

Principle:

After separation of the water and oil, and dependent on water quality required to maintain effective injectivity, produced water can be injected into a disposal well, or preferably the same reservoir from whence the water originated.

Advantages:• PWRI is widely applied across the industry in a number of regions, particularly

onshore.

• Several operators already inject PW where technically feasible, either as part of the over-all waterfl ood strategy, or as a disposal option.

• The application of PWRI, where possible, reduces both dispersed and aromatic oil com-ponents.

Disadvantages:• Need to have a suitable injection zone.

• Re-injection can be very energy intensive due to high pump pressure requirements, and thereby cause increased greenhouse gas emissions.

• PWRI will not enable an operator to meet performance standard in the portion of pro-duced water being discharged

• Potential high capex/opex option, dependent upon fi eld/reservoir fl uid conditions.

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Best management practices for produced water treatment

To date, the focus has been on the treatment and removal of dispersed oil fractions in the produced water, and over the last decade with the application of more advanced technology, the average discharge in the North Sea is now around 23 mg l-1.

Additional “polishing” technologies are available which could reduce the dispersed oil con-tents further, and in some cases reduce the level of associated soluble “aromatic” species. However, these technologies often require additional energy and process chemicals to achieve the lower oil discharge levels, and in many cases their application would be limited, due to weight and space constraints, to low volume applications (<1000 m3d-1).

An alternative would be to reduce the volumes of water being produced or to re-inject the water, thus reducing the overall discharge of both dispersed and soluble aromatic species to the marine environment.

Each option described is potentially technically feasible to a degree and, in most cases, each has been tested, mainly on a pilot plant basis in the fi eld. However, no single process or technology is applicable to all fi elds, nor can the performance of any one technology be guar-anteed. It must also be borne in mind that many of these systems rely on the use of addi-tional capacity/residence time (posing weight and space constraints); chemicals (fl occulants, coagulants, de-oilers); energy (heat and pressure, which will need to be generated) to achieve the desired performance levels. As such, the removal of soluble aromatic species from the produced water, although technically feasible, would have a signifi cant operational cost and environmental impact, particularly in high volume applications.

Summary

• Aromatic hydrocarbons are mainly soluble species and cannot be removed by conven-tional gravity separation technology.

• There are technologies available with a clear potential for a signifi cantly reduction of the concentration of aromatic hydrocarbon compounds and PAH in the discharged pro-duced water, although their application within existing oil operations (in particular) and gas operations could be severely limited due to the weight and space limitations.

• Where proven, technologies to reduce or remove the soluble aromatic species are best applied as a fi nal “polishing or tertiary” treatment after the removal of the dispersed oil phase.

• PWRI, where technically feasible, would reduce the amount of aromatics from produced water but, in many cases, will not be a viable option due to high costs or high energy consumption.

• The application of treatment technologies to remove aromatics from the produced water would present to the operators a higher degree of complexity, risk and cost to the opera-tions, and potentially reduce the viability of many mature operations.

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References

Definition of aromatics

Brendehaug, J, Johnsen, S, Bryne, K H, Gjøse, A L; Eide, T H and Aamot, E (1992). Toxicity testing and chemi-cal characterisation of produced water - a preliminary study. In Produced Water: Technological/Environmental Issues and Solutions (Edited by J P Ray and F R Engelhardt), Plenum Press, NY, 245-256.

Cofi no, W P, Slager, L K and Van Hattum, B (1993). Environmental aspects of produced water discharges from oil and gas production in the Dutch continental shelf. Part 1: Overview of surveys on the composition of produced waters conducted on the Dutch continental shelf, NOGEPA, ISBN 90-5192-021-0.

E&P Forum (1994). Fate and effects of produced water discharges. Report no. 2.62/204.

E&P Forum (1997). Aqueous discharges in the North Sea: An update from the E&P Industry. E&P Forum submission to the OSPAR workshop in Hague 6-8.10.1997.

Flynn, S A, Butler, E J and Vance, I (1996). Produced water composition, toxicity, and fate. A review of recent BP North Sea studies. In Produced water 2: Environmental Issues and Mitigation Technologies (M Reed and S Johnsen, eds), Plenum Press, NY, 69-100.

Jacobs, R P W M, Grant, R O H, Kwant, J, Marquenie, J M and Mentzer, E (1992). The composition of produced water from Shell operated oil and gas production in the North Sea. In Produced water: Technological/Environmental Issues and Solutions (J P Ray and F R Engelhardt eds), Plenum Press, NY, 13-22.

Sørstrøm, S E (1993). Produced water - Chemistry, toxicity, and dispersion. Results from a study of three oil producing fi elds in the North Sea. A report by IKU for the Norwegian Oil Industry Association (OLF), Norway.

Utvik, T I R (1999). Chemical characterisation of produced water from four offshore oil production fi elds in the North Sea. Chemosphere 39, (15), 2593-2606.

Fate and effects of aromatic hydrocarbons in produced water

Benville, P E Jr and Korn, S (1977). The acute toxicity of six monocyclic aromatic crude oil components to striped bass (Morone saxatilis) and bay shrimp (Crago franciscorum). California Fish and Game, 63, 204-209.

Caldwell, R S, Caldarone, E M and Mallon M H (1977). Effects of seawater-soluble fraction of cook inlet crude oil and its major aromatic components on larval stages of Dungeness crab, Cancer magister. Proceedings of the Dana Symposium: Fate and Effects of Petroleum Hydrocarbons on Marine Ecosystems. Chapter 22, 210-220.

EC (1996). Technical guidance document in support of commission directive 93/67/EEC on risk assessment for new notifi ed substances and Commission regulation (EC) No. 1488/94 on risk assessment for existing substances. Part I to IV, Offi ce for offi cial publications of the European Communities. ISBN 92-827-8011-8012.

Frost, T K (2001). Development of Predicted No Effect Concentrations (PNECs) for risk assessment of produced water discharges to the marine environment. In preparation.

Furuholt, E and Kinn, S J (1998). Regional Environmental Impact Assessments. SPE paper SPE 46470, Presented at the 1998 HSE meeting, Caracas, Venezuela.

Galassi, S, Mingazzini, M, Vigano, L, Cesareo, D and Tosato, M L (1988). Approaches to modelling toxic responses of aquatic organisms to aromatic hydrocarbons. Ecotoxicology and Environmental Safety, 16, 158-169.

Hooftman, R N and Evers de Ruiter, A (1992). Early life stage tests with Brachdanio rerio and several polycyclic aromatic hydrocarbons using an intermittent fl ow-through system. TNO-rep IMW-R 9/253.

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Hooftman, R N (1991). The toxicity and uptake of chrysene in a reproduction test with Daphnia magna. TNO-report no. R91/217.

Holst, L L and Giesy, J P (1989). Chronic effects of the photo enhanced toxicity of anthracene on Daphnia magna reproduction. Environmental Toxicology and Chemistry, 8, 933-942.

Johnsen, S, Røe T I, Durell, G and Reed, M (1998). Dilution and Bioavailability of Produced Water Compounds in the Northern North Sea. A Combined Modelling and Field Study. SPE paper SPE 46269. Presented at the HSE meeting, Caracas, Venezuela.

Johnsen, S T K, Frost, M, Hjelsvold, T R and Utvik, T I R (2000). The Environmental Impact Factor - a proposed tool for produced water impact reduction, management and regulation. SPE 61178, Presented at the SPE HSE conference, Stavanger 2000.

Kühn, R M, Pattard, K, Pernak, D and Winter A (1989). Results of the harmful effects of water pollutants to Daphnia magna in the 21-day reproduction test. Water Research,. 23, 501.

Moffi tt, C M, Rhea M R, Dorn, P B, Hall, J F, Bruney J M and Evans S H (1992). Short term chronic toxicity of produced water and its variability as a function of sample time and discharge rate”. In “Produced Water: Technological/Environmental Issues and Solutions, (Ray J P and Engelhardt, R eds), Plenum Press, New York, 1992, 235-244.

Neff, J and Johnsen, S (2001). Comparison of environmental risk assessment by modelled and measured data from the North Sea environmental monitoring studies 1997 - 2000. In preparation.

Offshore Operators Committee (1997). Gulf of Mexico produced water bioaccumulation study. Conducted by Continental Shelf Associates, Jupiter, Florida for the Offshore operators Committee PO Box 50751, New Orleans, 5 volumes.

OLF, 1998. Produced water discharges to the North Sea: Fate and effects in the water column. Summary report.

Riksheim, H and Johnsen, S (1994). Determination of produced water constituents in the vicinity of production fi elds in the North Sea. SPE paper 27150. Presented at the SPE conference in Jakarta 1994.

Savino, J F and Tanabe L L (1989). Sublethal effects of phenanthrene, nicotine, and pinane on Daphnia pulex. Bulletin of Environmental Contamination and Toxicology, 42, 778-784.

Terrends G W and Tait R D (1996). Monitoring Ocean Concentrations of Aromatic Hydrocarbons from produced formation water discharges to Bass Strait Australia. SPE paper 36033, presented at the 3rd SPE Health Safety and Environment Conference, New Orleans, 1996.

Utvik, T I R, Durell, G S and Johnsen, S (1999). Determining produced water originating PAH in North Sea water: Comparison of sampling techniques. Marine Pollution Bulletin. In press.

Van Brummelen, T, van Hattum, B, Crommentuijn, T and Kalf, D F (1998). Bioavailability and Ecotoxicity of PAHs. Handbook of Environmental Chemistry, Volume 3, Part J, Chapter 13. Springer Verlag, Berlin and Heidelberg, 1998.

Varanasi, U (1989). Metabolism of polycyclic aromatic compounds in the aquatic environment. CRC Press, Boca Raton, FL, USA, pp.43-44.


© OGP

What is OGP?

e International Association of Oil & Gas Producers encompasses the world’s leading private and state-owned oil & gas companies, their national and regional associations, and major upstream contractors and suppliers.

Vision

• To work on behalf of all the world’s upstream companies to promote responsible and profitable operations.

Mission

• To represent the interests of the upstream industry to international regulatory and legislative bodies.

• To achieve continuous improvement in safety, health and environmental performance and in the engineering and operation of upstream ventures.

• To promote awareness of Corporate Social Responsibility issues within the industry and among stakeholders.

Objectives

• To improve understanding of the upstream oil and gas industry, its achievements and challenges and its views on pertinent issues.

• To encourage international regulators and other parties to take account of the industry’s views in developing proposals that are effective and workable.

• To become a more visible, accessible and effective source of information about the global industry, both externally and within member organisations.

• To develop and disseminate best practices in safety, health and environmental performance and the engineering and operation of upstream ventures.

• To improve the collection, analysis and dissemination of safety, health and environmental performance data.

• To provide a forum for sharing experience and debating emerging issues.• To enhance the industry’s ability to influence by increasing the size and diversity of

the membership.• To liaise with other industry associations to ensure consistent and effective approaches

to common issues.

209-215 Blackfriars RoadLondon SE1 8NLUnited KingdomTelephone: +44 (0)20 7633 0272Fax: +44 (0)20 7633 2350

165 Bd du Souverain4th FloorB-1160 Brussels, BelgiumTelephone: +32 (0)2 566 9150Fax: +32 (0)2 566 9159

Internet site: www.ogp.org.uke-mail: [email protected]

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