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Chemical characterisation of produced water from four offshore oil production platforms in the North Sea
Chemosphere, Vol. 39, No.
15, pp. 2593-2606, 1999 © 1999 Elsevier Science Ltd. All rights reserved 0045-6535/99/$ - see front matter
Pergamon
PII: S0045-6535(99)00171-X CHEMIC~d.~ CHARACTERISATION OF PRODUCED WATER FROM FOUR OFFSHORE OIL PRODUCTION PLATFORMS IN THE NORTH SEA
Toril I. R~e Utvik
Norsk Hydro E&P Operations, Environmental Section, N-5020 Bergen, NORWAY Fax: +47 55 99 62 50, e-mail: [email protected] (Received in Germany 8 March 1999; accepted 9 April 1999)
ABSTRACT Samples of produced water from the offshore oil production platforms Oscbcrg Fcltscnter,Oscberg C, Bragc and Troll B were collcctcd during the period of Octobcr 1995 to August 1996. The samples were analysed for polycyclic aromatic hydrocarbons and phenols by gas chromatography with a mass-spcctromctric detector. Analysis of organic acids was done by isotachophorcsis, metal determinations by atomic absorption s]?ectromctry, and radioactivity mcasuremcnts by high resolution g a m m a spcctroscopy. The results wcre included in a database of chemical composition of produccd water, and wcrc then compared with data from othcr fields in thc Norwcgian scctor of thc North Sca using principal component analysis. The conccntrations of naphthalcnc, phcnanthrene, dibcnzothiophcne, and their CI-C3 alkyl homologues, and alkylated phcnols show decreasing Icvcls with increasing alkylation of the components for all fields. The results show that there is no correlation between the T H C content, which today is used as emission standard for environmental regulation, and the content of the aromatic compounds, which are assumed to be the most important contributors to toxicity. Field-specific detailed chemical characterisation of produced water from each platform is necessary in predicting fate and effects of the produced water discharged to the marine environment. © 1999 Elsevier Science Ltd. All fights reserved
INTRODUCTION Produced water is the largest wastewater stream in the oil exploration and production process. During oil production, water from the reservoir containing low levels of petroleum is pumped to the surface. Produced water is separated from the oil and either injected into a well or discharged after treatment to surface waters. Discharges of produced water from the Norwegian oil and gas production are expected to increase from 26 million m 3 irL 1993 to 120 million m 3 by the year 2000 (1). The fields Oseberg, Brage and Troll are Norsk Hydro's most important sources of produced water. A map of the North Sea with the discharge points of produced water in the Norwegian sector pointed out is shown in Figure 1. 2593
2594
Cat/
Figure 1. The North Sea with the Norwegian oil production fields Oseberg, Brage and Troll.
The amount of produced water discharged to the sea per year is shown in Table 1.
Table 1.
Oil production (m3/year, 1996) and produced water discharge volumes (m3/year, 1996) for Oseberg, Brage and Troll (2).
Field
Oil Production (m3/year)
Produced water discharged (m3/year)
Oseberg Brage Troll
29 000 000 7 000 000 14 500 000
1 432 000 1 856 000 4 382 000
The chemical composition of the discharged water is complex, including dispersed oil, dissolved hydrocarbons, organic acids, phenols, metals, and traces of chemicals added in the separation and
2595 production line, The composition is field-dependent, and several studies have been presented on chemical characterisation of produced water from fields in the North Sea (3-5), the Gulf of Mexico (6-8), and the coast of Canada (9). These studies show that the dissolved hydrocarbons are dominated by the volatile aromatic fraction of the oil, namely benzene, toluene, ethylbenzene and xylene (BTEX). The polynuclear aromatic hydrocarbons (PAHs) are dominated by naphthalene, phenanthrene and dibenzothiophene (NPD) and their C1-C3 alkyl homologues, but also compounds with higher molecular weight are reported such as chrysene and benzo(a)pyrene. The phenols reported may be alkylated up to C7. The organic acids are for most fields reported to be dominated by C1-C6 acids. The metal contents reported are varying, but often dominated by barium and iron. The analytical techniques used in previous studies are summarised in Table 2.
Table 2. Analytical techniques used for characterisation of produced water
Compound
Technique
Reference
"Total Oil" BTEX
IR Purge & trap, GLC-MS GC-FID GC-MS Direct GLC/FID Isotachophoresis Silylation]GLC-MS GC-MS Derivatisation/GC AAS ICP-AES ICP-MS
3,8 3 4,6 3,5,6,7.8,9 3 4 3 4,7,9 10 3,8 3 6
PAH Organic acids Phenols
Metals
Routine produced water analyses carried out offshore in the North Sea are based on freon extraction followed by determination of "total oil" by infrared spectroscopy. This is because legislation on the composition of discharged produced water offshore has, in general, been limited to "total oil" concentration (currently 40 mg/1 in the North Sea) and has been concerned mainly with monitoring the efficiency of oil/water sepaJ:ators (3).
The acute toxicity of produced water was studied by several authors in recent years (4,
11-13). The
main
conclusion from these studies is that the acute toxicity of produced water to marine organisms is low, and that the future work should be focusing on potential chronic effects of produced water components to marine organisms. This is also the conclusion of work initiated by the Norwegian Oil Industry Association in 1993
(5, 14, 15). The present work is conducted as
a part of a research program, which within the year 2000 is
supposed to end up in a new tool for environmental risk assessment of oil related compounds to marine organisms. The aim of the present study is to characterise produced water from four Norwegian oil production platforms, and to determine any correlation between the concentrations of the certain compounds within one field and in-between different fields.
2596 EXPERIMENTAL
Sampling Samples of produced water (2 x 1 1) was taken once a day for five days from each of the platforms rI'la~ samples were acidified (HC1), and kept dark and cool (4°C) during transport.
Sample preparation The samples were placed on a shaking table for one minute for homogenisation. One of the two litres from each sample was divided into 5 subsamples for determination of BTEX, organic acids, mercury, other metals and radioactivity, respectively (Table 3). The other litre were added internal standards of PAH and phenol compounds (50 mg/1).
Table 3. Sample volumes used in the chemical characterisation of produced water
Compound
Technique
Sample volume (ml)
BTEX Organic acids Hg Pb, Cr, Cu, Ni, Zn, Ba, Fe Radioactivity PAH/Phenols
GC headspace Isotachophoresis Cold vapour atomic absorption spectroscopy Atomic Absorption Spectroscopy (graphite furnace) Gamma spectroscopy GC-MS
10 10 100 250 500 1000
The samples for PAH/phenol analysis were extracted with dichloromethane. For Oseberg Feltsenter, Oseberg C and Troll, the 5 samples from each of the platforms were combined to one sample for determination of all parameters except PAHs and phenols. For the latter two compounds, the samples were extracted separately, and the extracts from each platform were combined. For Brage the 5 samples were treated separately.
Analysis The analysis with respect to BTEX was done by GC headspace (16) using a Varian 3700 Gas chromatograph with a DB-5, 30 m, 0.25 lam column (J&W) and a flame ionisation detector. Organic acids were determined by isotachophoresis as described by Barth (17). Mercury was analysed by cold vapour atomic absorption spectroscopy (CV-AAS) as described in the Norwegian Standard NS 4768 (18), with sample pre concentration on a gold trap. Lead, chromium, copper, and nickel were determined by atomic absorption spectrometry (graphite furnace) as described in Norwegian Standard NS 4780 (19) and 4781 (20). Zinc was determined by atomic al~sOrption spectrometry (flame technique) as described in Norwegian Standard NS 4771 (21). Barium and iron were analysed by inductively coupled plasma atom emission spectrometry (ICPAES) as described by Aase (22) and Broekaert (23). Radioactivity measurements were done by germanium detector gamma spectroscopy (Canberra) (24).
2597 PAils were determined by GC-MS as described in the EPA method 610 (25), and phenols were determined by GC-MS as described in EPA method 604 (26). Multivariate analysis of the analytical data was performed by using the multivariate statistical software "Unscrambler" (27).
RESULTS .~'D DISCUSSION The results of the chemical characterisation of the produced water samples from Brage, Oseberg Fehsenter, Oseberg C and Troll are given in Table 4. A comparison of daily samples taken during a five days period at Brage is shown in Table 5. Uncertainty in sampling and analysis, expressed as relative standard deviation (%) is shown :in Table 6.
Table 4. Chemical composition of produced water from the oil production platforms Oseberg Feltsenter, Oseberg C, Brage and Troll (lad:not detected,na: not analysed)
Component / Field
Unit
Brage
THC (platform)* S u m BTF.J[ Benzene Toluene Ethylbenzene Xylene S u m NPD Naphthalenes Naphthalene Cl-naphthalene C2-naphthalene C3-naphthalene Phena'athrenes Phenanthrene Cl-phenanthrene C2-phenanthrene C3-phenanthrene Diben',~othiophenes Dibenzothiophene Cl-dibenzothiophene C2-dibenzothiophene C3-dil~mzothiophene Acenaphtylene Acenaphthene Fluorene Fluoranthene Pyrene Chrysene Benz(a)anthracene Benzo(a)pyrene Benzo(ghi)perylene Benzo(k)fluoranthene S u m organic acids Formic: acid Propanoic acid
rag/1 mg/l mg/l mg/l mg/1 rag/1 mg/l mg/l mg/1 rag/1 mg/1 mg/1 llg/l ~g/1 tlg/1 t~g/1 ~g/1 pg/l tlg/1 ~g/1 pg/1 tlg/1 tlg/l pg/1 tlg/l tlg/l ~g/l ~g/1 pg/1 pg/] pg/l pg/1 mg/l mg/1 mg/1
58 9.0 4.5 3.5 0.3 0.7 0.93 0.86 0.35 0.26 0.15 0.10 50.9 16.4 20.3 6.3 7.9 17.5 2.5 5.7 6.1 3.2 nd 1.8 8.9 0.4 0.7 0.5 0.6 0.2 0.2 0.2 757 282 53
Oseberg F 44 8.3 4.6 2.7 0.6 0.4 1.27 1.2 0.43 0.38 0.25 0.14 99.8
27.2 32.3 27.2 13.1 37.8 8.8 10.8 10.6 7.6 nd 0.3 16.2 1.8 5.2 0.1 2.4 nd 0.6 0.8 1135 584 98
Oseberg C 60 5.8 3.7 1.5 0.3 0.2 1.60 1.06 0.46 0.30 0.19 0.12 76.3 27.4 30.2 8.9 9.7 na na na na na nd 5.1 2.7 7.8 8.6 0.4 1.9 0.1 0.1 0.2 717 299 56
Troll 33 2.4 0.8 1.0 0.4 0.2 1.32 1.23 0.53 0.42 0.20 0.08 60.2 18.8 18.7 15.3 7.4 28.0 6.2 8.6 7.2 6.0 nd 1.5 15.4 1.7 5.1 nd 2.0 nd nd 0.7 798 26 36
2598
Component / Field
Unit
Bra$e
Oseber~F
Oseber~C
Butanoic acid Pentanoic acid HexanHexanoic acid Organic acids > C6 Sum Phenols Phenol Cl-phenol C2-phenol C3-phenol C4-phenol Barium Lead Cadmium Chromium Iron Copper Mercury Nickel Zinc 214 Pb 214 Bi 228 Ac 212 Bi 212 Pb 226 Ra
mg/1 mg/1 mg/1 mg~/1 mg/l mg/1 mg/1 mgJ1 mg/1 mg/1 mg/1 lag,/1 lag/l lag/1 mg/1 lag/1 ng/1 lag/1 mg/1 Bql/1 Bql/1 Bql/1 Bql/1 Bql/1 Bql/1
28 5 nd 389"" 6.12 3.54 1.97 0.51 0.09 0.02 228 nd nd nd 11.3 nd 20 nd 0.20 9 8 17 <2 <2 9
46 33 nd 375"" 11.45 7.01 3.52 0.75 0.13 0.03 107 nd nd nd 4.2 nd 20 nd 0.03 6 6 11 <2 <2 6
22 8 nd 332"" 10.96 6.10 3.81 0.95 0.08 0.02 142 nd nd nd 7.7 nd 26 nd 0.34 7 6 <2 <2 <2 7
Troll nd nd nd 735"" 0.58 0.03 0.06 0.41 0.06 0.02 147 nd nd nd 4.3 nd 17 nd 0.012 6 6 7 <2 <2 6
nd: not detected (detection limits: Pb: 0.013 pg/l, Cd: 0.003 IJg/l, Cr: 0.03 lag/l, Cu: 0.03 lag/l, Ni: 0.03 lag/I).
* Analysis performed by works laboratory offshore** High values probably due to interferences from production chemicals.
Table 5. Chemical composition of produced water samples taken ever~ da:¢ in a 5 days period.
Component / Field
Unit Bra~e 1 Bra~e 2 Brage 3 Brage 4 Brage 5
THC (platform)* S u m BTEX Benzene Toluene Ethylbenzene Xylene S u m organic acids Formic acid Propanoic acid Butanoic acid Pentanoic acid HexanHexanoic acid Organic acids > C6 Barium Lead Cadmium Chromium Iron Copper Mercury Nickel Zinc 214 Pb 214 Bi 228~
mgeq mg/l mg/1 mg/1 mg/l mg/1 mg/l mg/] mg/1 mg/1 mg/1 mg/1 mg/1 mg/1 lag/1 lag/1 lag/1 mg/1 lag/1 lag/1 lag/l mg/1 Bql/1 Bql/1 Bql/1
38 7.3 3.6 2.9 0.2 0.6 765 252 48 34 12 nd 418"" 225 nd nd nd 10.6 nd 0.025 nd 0.003 10 10 17
70 8.8 4.1 3.2 0.8 0.7 740 258 63 12 17 nd 390" 226 nd nd nd 11.0 nd 0.016 nd 0.001 10 9 16
84 8.1 4.0 3.2 0.2 0.7 688 253 49 12 13 nd 361"" 226 nd nd nd 11.3 nd 0.023 nd 0.014 10 9 ,
< 1
50 8.2 4.1 3.3 0.2 0.6 875 255 53 14 19 nd 534"" 228 nd nd nd 11.3 nd 0.024 nd 0.004 11 11 16
58 9.0 4.5 3.5 0.3 0.7 757 282 53 28 5 nd 389"" 228 nd nd nd 11.3 nd 0.020 nd 0.20 9 8 17
2599
Component / Field
Unit
212 Bi
Bql/l Bql/l Bql/1
212 Pb
Bra~e I
Bra~e 2
Bra~e 3
Bra~e 4
Bra$e 5
< 2
< 2
< 2
< 2
< 2
< 2
< 2
< 2
< 2
< 2
10 I0 10 ii 9 nd: not detected (detection limits: Pb: 0.013 pg/l, Cd: 0.003 pg/l, Cr: O.03 lag~l, Cu: 0.03 I~g/l, Ni; O.03 pg/l) * Analysis performed by works laboratory offshore • * High values probably due to interferences from production chemicals 226
Ra
Table 6. Uncertainty in analysis and sampling, expressed as percent standard deviation for selected produced water components.
% st. dev. analysis
% st. dev. sampling
Phenols Organic acids Zn
5 % 1 - 12 % 1- 6%
4 - 67 % 1 - 51% 98%
H~R
5-22% 10%
17%
a
Table 5 show:~ that there are only minor variations in the concentration o f the produced water compounds. over a period o f five days. A statistical F-test showed that there were significant differences between the samples for c,ertain c o m p o u n d s , but these were only single values without any systematism for which samples this regarded. Based on this observation one may assume that the samples taken from the other platforms (once a day during 5 days, c o m b i n e d to one sample) are representative for the produced water composition o f that particular platform.
The chemical composition o f produced water from other fields in the Norwegian sector o f the North Sea is s h o w n in Table 7 (28). It must be emphasised that the quality o f these data is not known, because the analyses were done by different laboratories, and thus comparison between the fields has to be made with some care. Table 7. Chemical composition o f produced water from the fields Snorre, Ekofisk Tor, Ekofisk Edda, Ekofisk Kilo, Gullfaks A, Gullfaks B, Gullfaks C, Veslefrikk, Statfjord A, and Heidrun (5, 28)
Comp. THC BTEX NPD 3-6 r. PAH 16 PAH Phenols Org Acids Barium Lead Cadmium Iron Copper Mercury Nickel Zinc
Unit mg/l mg/l mg/1 lag/1 lag/l mg/l mg/l mg/l pg/1 pg/l mg/1 lag/l pg/l pg/l m~/l
Snorre 18.3 10.6 0.9 na 512 na 160 0.2 <10 4.6 0.1 na <0.l na na
Ekofisk Tor 2.7 5.5 0.3 Na 268 0.I Na 67 8.3 1.6 15 82 <0.5 1651 0.5
nr: not availabh~. * data from OLF report (5)
Ekofisk Edda 16 6.0 0.3 316 130 0.1 na 4 0.4 0.4 2.5 10 <0.5 193 <0.05
Ekofisk Kilo 4.2 2.0 0.4 413 218 0.2 na 21 0.7 0.4 2.1 40 <0.5 453 <0.05
Gullfaks B 35' 5.0 1.8 na 500 10.8 55* na <50* <10" na <2* 1.9" <40* 13"
Vesleffikk na 7.7 1.4 na 520 42.5 na na na na na na na na na
Sta(l)ord A 12 9.0 1.1 na 403 23.5 895* na <50* <10" na <2* 1.9" <40* 6.8*
Heidrun na 5.3 1.2 na 450 7.5 na na na na na na na na na
2600 The results from the analysis of produced water from Oseberg Feltsenter, Oseberg C, Brage and Troll show that the concentrations of all components are of the same order of magnitude as previously observed for other fields in the area. The results from all fields show that there is no correlation between the THC content, which is used as emission standard for environmental regulation, and the content of the aromatic compounds (which are assumed to contribute most to produced water toxicity).
There is little information available on the background level of inorganic and organic produced water compounds in the North Sea. The studies of Law et al (29) and Witt (30) represent the most comprehensive information available for PAHs in seawater around England/Wales and in the Baltic Sea, respectively. Background levels of PAHs in the North Sea have been recently measured by deployment of mussels and semipermeable membranes (SPMDs) along with direct water sampling for solvent and solid phase extraction, and in situ large volume water sampling (31, 32). A summary of background levels of produced water compounds in sea water is presented in Table 8.
Table 8. Background level of selected produced water compounds in the North Sea.
Compound PAH (15) PAH (16) Y. PAH (16) Barium Lead Cadmium Iron Copper Mercury Nickel Zinc Radioactivit~
Concentration 0.001 - 4.8 ng/1 <15 ng/1 1 - 45 ng/l <50 - 80 lag/l 22 [agJl 20 - 100 n~/1 4 - 28 ng/1 1.8 lags/1 20 - 410 ng/1 0.5 [a~/l 0.03 lag/l <2 - 10 lag/1 1.5 la~/l < 5 lag/1 823 lag/l 0.007-0.026 Bq/1
Reference 29 30 31,32 33 34 35 35 34 35 34 34 33 34 33 34 36
At first sight it seems that most of the organic components of the discharged water must be diluted 10 000 to 100 000 times to come down to the background level. Fortunately other processes than dilution are important for removal of these components from the water column. BTEX compounds are volatile, and normally will not be detected far away from the discharge point. Biodegradation by sea-water bacteria is another important process. Biodegradation is relatively rapid for organic acids, BTEX and NPD compounds (37). Other processes such as adsorption to surfaces of biological and non-biological material, and biological uptake by marine organisms will probably also influence on the concentration in the area surrounding the platforms (37).
2601 The results of the principal component analysis (PCA) of data from selected fields in the Norwegian sector are shown in figures 2 and 3. Due to lack of data, some of the compounds and fields had to be left out from the analysis o:["fields in the Norwegian sector of the North Sea (fig. 3). The fields included are: Oseberg Feltsenter, Oseberg C, Brage, Troll in figure 2, and Oseberg Feltsenter, Oseberg C, Brage, Troll, Ekofisk Kilo, Ekofisk Tor, Ekofisk Edda, Heidrun, Statfjord A, B, C (flotation units), Gullfaks A, and Veslefiikk in figure 3. The .compounds included are: BTEX, NPD, PAH, organic acids, phenols, Fe, Zn, Hg, Ba, and 22('Ra in Figure 2, aJad sum BTEX, sum phenols, sum 16 PAH, sum naphthalenes, sum phenanthrenes and sum dibenzothiophenes in Figure 3.
The biplot in Figure 2 shows that the first principal component (PC1) is stretched out by barium in the left end and the NPDs, PAHs, organic acids< C6 and phenols in the right end. The second principal component (PC2) is spanned out by organic acids >C6 in one end and benzene/chrysene in the other. The fields Oseberg Feltsenter, Oseberg C, Brage and Troll are all spread throughout the plot. This indicates that produced water from all field,,;differ from each other in chemical composition• Oseberg Feltsenter is located far to the right on PC1, which indicates that this field is different from the others by a higher level of NPDs, PAHs. organic acids< C6 and phenols, and a lower level of barium.
pc2................................................. ~:.~ot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • Chrysane
:
•
Benzene
: Fe.. Benzo(a)pyrene
Zn
• 22.eRa .
0.~
.
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• ~'lg@ .
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i
.....................................
- Acenaphthene
• BII
"0
IbIl~l
• C1-PKenol
e
: • C2-Phenol
:
~ ............
; . . . . . . . . . . . . . . . . . . . . . . .
~
• Fluoranthene
i
•'iBuWlic Phenolacid i:
i
• Xyten@
:
.
i
• "HgTolue~e
" ~(Le~l~lhalene
: •
i • Proponic acid : "hrBlen,zo(ghi)pery~ " hena, ot rene' • ~'nenantnfCall~,rlc acid. .'~.l:~P~ilnthrene
: p
:
C3:
U4-Pheno~
-
• |~/rene
!
i
i
' E~lt~tmzen
• :
• ~,eP~'~il~ -0.5 ¸
....................................
; ......................................
: ...............
!
• Fhiorene
:
'
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: .Troll...
" Naphthaione
O~'ganic acid>C6
.................
'
: "B t~fi ~.( &',, ~t;ffi;r~,~e ....
Beozo(k]l
CliNaph!hatene
:
. r .........
:
uo~en
:
.
:
:
" PC1
4..
" .o.8
-d.4
" -d.2
~
" o'.z
o'.4
"
o'.6
o'.~
~'.
X-expl:
Figure 2. Biplot from PCA of sumarised parameters for chemical composition of produced water from Oseberg Feltsenter, Oseberg C, Brage and Troll•
2602
PC2
Bi-plot . . . . . . . .
~tfjord
BTEX
. . . .
'
Statfj0rd
B
A •
StatfjOrd C
0.5
Sum
Ekofisk Edda
phencfls
• St~lll Na~hlha!ene~
• Bn • Oaebem
• Ekofisk Tor 'Ekoflsk Kilo
(~
•
Veslefrikk
• Heidrun • Sum
16!PAH
St~13 P t ~ e n ; ~ n l h r e ~ e ! ~
-0.5-
...............................................................
• Oseberg F : . . . . . . . . . . . . .
! ..........
• Tioll ..................................................................
:
B
• Sum
.......
Dii~enzothiephen(~s : .......
Gulffaks
A
: ....... PC1
' " T~,X-e~:
-d.a
-~.s
.d..
" -d.2
"
~ "
o'.2
o',4
"
o'.8
" o'..
"
~'.
Figure 3. Biplot from PCA of summarised parameters for chemical composition of produced water from selected fields in the Norwegian sector of the North Sea. Brage is located at the other end of PC1, with opposite characteristics than Oseberg Feltsenter. Troll is located at the low end of PC2, close to the organic acids >C6. This indicates a high level of these acids in the produced water samples from Troll, and a low level of the compounds placed at the high end of PC2; BTX, phenols and metals. Oseberg C is located at the centre of the plot, close to the BTXs, Hg, fluoranthene and acenaphthene, which indicates higher levels of these compounds in the samples from Osehcrg C than for the rest of the fields•
The biplot in Figure 3 shows that there is a group of fields located at the left side of PC1, Ekofisk Edda, Ekofisk Tor and Ekofisk Kilo. They are all characterised by a low level of NPDs and PAHs. Statfjord A, B and C are all located to the fight side of PC1, and high up on PC2, together with the BTEX compounds• Troll and Gullfaks A are grouped together, close to the dibenzothiophenes. Oseberg Feltsenter, Veslefrikk, Oseberg C, Heidrun and Brage are all grouped to the fight side of PC1, and are characterised by high concentrations of PAHs, naphthalenes, and phenanthrenes.
It is interesting to observe that fields, which are expected to be similar in chemical composition due to geographical position (Ekofisk and Statfjord fields), are grouped together in the biplot. This indicates that prediction of chemical composition of produced water based on geological or geographical information may
2603 be possible. The results are, however, influenced by the analytical variation between the laboratories for the compounds i:acluded in this principal component analysis, and it is possible that some of the variation between the fields may be explained by analytical variation.
CONCLUSIONS Chemical composition of produced water from the offshore oil production fields Oseberg Feltsenter. Oseberg C, Brage and Troll in the North Sea has been determined. The results were compared with chemical composition of produced water from other fields in the Norwegian sector of the North Sea. For all compounds, the concentrations were within the range shown among other fields. The concentration t~l naphthalenes, phenanthrenes, dibenzothiophenes, and phenols decrease with increasing alkylation t)f the components for all fields. The results show that there is no correlation between the THC content, which is used as emiszfion standard for environmental regulation, and the concent of the aromatic compounds. The results of the principal component analysis of chemical data for produced waters from selected fields in the Norwegian sector of the North Sea, showed that fields which are expected to be equal in chemical composition ,due to geographical position (Ekofisk and Statfjord fields), are grouped together in the biplot. This indicates that a rough prediction of chemical composition of produced water based on geological or geographical information may be possible. The results are, however, influenced by the analytical variation between the laboratories for the compounds included in this principal component analysis, and it is possible that some of the variation between the fields may be covered by the analytical variation. The overall conclusion is that individual detailed chemical characterisation of produced water from each platform is necessary in order to predict fate and effects of the discharges of produced water to the marine environment.
ACKNOWLEDGEMENTS Sampling of the produced water was done by Odd Aas at Troll B, Jan Pokorny at Oseberg C, Magnc van Riesen at Brage, and Tom Pedersen at Oseberg Feltsenter. The determination of organic acids were done by Tanja Barth at the University of Bergen, the radioactivity measurements were done by Tone Bergan at IFE, the metal determination were done by Arne ]ksheim, Gunhild Annette Iversen, and Harald St. Aase at Norsk Hydro Research Centre, and analyses with respect to BTEX were done by Gerd Aasen at Norsk Hydro Research Centre.
Thanks also to Erling Odden, Anita Seljelund, Siw T. Pettersen, and Grete Di at Norsk Hydro Research Centre for assistance during the determination of PAHs and phenols, Terje Karstang at Norsk Hydro Research Centre, Espen Langered and Mona Nyg~rd for help with the multivariate statistics, and Sthle Johnsen at Statoil Research Centre for good advices.
2604 REFERENCES 1.
The Norwegian Oil Industry Association, OLF, Environmental Report 1993-1995. Stavanger, Norway (1995).
2.
Environmental report, U&P Division, Norsk Hydro (in Norwegian) (1996).
3.
P.J.C. Tibbetts, I.T. Buchanan, L.J. Gawel, and R. Large, A comprehensive determination of produced water composition. In Produced water technological/Environmental lssues and Solutions (Edited by J.P. Ray and F.R. Engelhardt), Plenum Press, New York, p.97, (1992)
4.
J. Brendehaug, S. Johnsen, K.H. Bryne, A.L. Gj0se, T.H. Eide and E.Aamot, Toxicity testing and chemical characterisation of produced water - a preliminary study. In Produced water technological/Environmental lssues and Solutions (Edited by J.P. Ray and F.R. Engelhardt), Plenum Press, New York, p.245, (1992)
5.
T.I. ROe, S. Johnsen, T.K. Frost and G. Fumes, Discharges of produced water - effects in the water column, Phase IT 1996, Report to the Norwegian Oil Industry Association, OISF, Stavanger, Norway, (1996).
6.
N.N, Rabalais, B.A. McKee, D.J. Reed and J.C. Means, Fate and effects of nearshore discharges of OCS produced waters, Vol. 1-3, US Dept. of Interior, Minerals Management Service, Gulf of Mexico OCS Regional Office, New Orleans, (1991).
7.
J.S. Brown, T.C. Sauer Jr., M.J. Wade and J.M. Neff, Chemical and toxicological characterisation of produced water freon extracts. In Produced water technological/ Environmental Issues and Solutions (Edited by J.P. Ray and F.R. Engelhardt), Plenum Press, New York, p. 113, (1992).
8.
J.M. Neff, T.C. Sauer and N. Maciolek, Fate and effects of produced water discharges in nearshore marine waters, Vol. 1-2, Report to the American Petroleum Institute, Washington D.C., 1987.
9.
J. Carey, A. Zaidi and J. Ribo, Specific toxic organics in produced waters from in-situ heavy oil recovery operations in western Canada, In Produced water technological~Environmental Issues and Solutions (Edited by J.P. Ray and F.R. Engelhardt), Plenum Press, New York, p. 133, (1992).
10.
O. Grahl-Nilsen, Hydrocarbons and phenols in discharge water from offshore operations. Fate of hydrocarbons in the recipient. Sarsia, 72, 375-382, (1987).
11.
R.M. Montgomery, R.M. Parrish and S.D Friedman, Effects of produced (formation) water on mysids (Mysidopsis bahia), 8th annual meeting, Society o f Toxicology and Chemistry, Pensacola, FL, (1987).
12.
K.C. Schiff, D.J. Reish, J.W. Anderson and S.M. Bay, A comparative evaluation of produced water toxicity. In Produced water technological~Environmental Issues and Solutions (Edited by J.P. Ray and F.R. Engelhardt), Plenum Press, New York, p. 199, (1992).
2605 13.
T.C. Saner, T.J. Ward, J.S. Brown, S.O'Neil and M.J. Wade, Identification of toxicity in lowTDS produced waters. In Produced water technological/Environmental Issues and Solutions (Edited by J.P. Ray and F.R. Engelhardt), Plenum Press, New York, p.209, (1992).
14.
S. Johnsen, G. Fumes, O. Ringdal, B. Serigstad, Discharges of produced water - effects in the water column, Phase I, 1994. Report to the Norwegian Oil Industry Association, OLF, Stavanger, Norway, (1996).
15.
T.I. Roe, S. Johnsen, Discharges of produced water to the North Sea - effects in the water column. In Produced water 2, Environmental issues and mitigation technologies, M. Reed, S. Johnsen (eds.), Env. Sci. Res., vol. 52, Plenum Press, NY, USA, (1996).
16.
G. Aasen, Quatitative analysis of benzene, toluene, ethylbenzene and xylene by GCheadspace. Norsk Hydro, Analytical Method HRE-ANA-A 77, Porsgrunn, Norway (1995).
17.
T. Barth, Quantitative determination of volatile carboxylic acids in formation waters by is,atachophoresis. Analytical Chemistry, 59, p. 2232, (1987).
18.
Narges Standardiseringsforbund, Determination of mercury by cold vapour atomic absorption Sl:,ctrometry. Oxidation by nitric acid. Norwegian StandardNS 4768, Norway.
19.
Norges Standardiseringsforbund, Water Analysis. Metal content of water, sludge and sediment determined by tameless atomic absorption spectrometry. Electrothermal atomisation in a graphite furnace. General principles and guidelines. Norwegian StandardNS 4780, Norway (1988).
20.
N arges Standardiseringsforbund, Water Analysis. Metal content of water, sludge and sediment determined by tameless atomic absorption spectrometry. Electrothermal atomisation in a graphite furnace. Special guidelines for AI, Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb. Norwegian
Standard NS 4780, Norway (1988). 21.
N,arges Standardiseringsforbund, Water Analysis. Metal content of water, sludge and sediment by atomic absorption spectrometry, atomisation in flame, solvent extraction.
Norwegian Standard NS 4771, Norway. 22.
H. St. Aase, Rapid multielement analysis of metals in water by inductive coupled plasma atom eraission spectrometry, Norsk Hydro, Note 5916Q, (1985 ).
23.
J.A.C. Broekaert, Use of ICP atomic spectrometry for water analysis. Technisches Messen, 59, 4, p. 147-153, (1992).
24.
T.S. Bergan, Institute for Energy Technology, Analytical report to Norsk Hydro Research Centre, 12.12.1995.
25.
United States Environmental Protection Agency, Method 610.
26.
United States Environmental Protection Agency, Method 604.
27.
Unscrambler, Comercially available software for multivariate analysis.
2606 28.
OLF/Norsk HydrolStatoil: Database of chemical composition of produced water, Available at The Norwegian Oil Industry Association, Stavanger, Norway (1997).
29.
R.J. Law, V.J. Davies, R.J. Woodhead, P. Matthiessen, Polycyclic aromatic hydrocarbons (PAH) in seawater around England and Wales. Mar. Pollut. Bull., 34, 306-322, (1997).
30.
G. Witt, Polycyclic aromatic hydrocarbons in water and sediment of the Baltic Sea, Mar. Pollut. Bull., 31,237-248, (1995).
31.
S. Johnsen, T.I. ROe, G. Durell, Dilution and bioavallability of produced water compounds in the northern Nort Sea. A combined modeling and field study. SPE paper no. 46269. SPE International Conference on Health, Safety and Environment in Oil and Gas Exploration and Production, Caracas, Venezuela, 7-10: June, 1998.
32.
T.I. ROe, G.S. Durell, and S. Johnsen, Determining produced water originating polycyclic aromatic hydrocarbons in North Sea waters: Comparison of sampling techniques. In preparation (1999).
33.
P.J.C. Tibbetts, I.T. Buchanan, L.J. Gawel, R.Large, A comprehensive determination of produced water composition. In Produced water, Technological/Environmental lssues and Solutions, J.P. Ray and F.R. Engelhardt (eds.), Env. Sci. Res., vol. 46, Plenum Press, NY, USA, (1992).
34.
P.G. Brewer, Minor elements in seawater. In Chemical Oceanography, J.P. Riley, G. Skirrow, Eds., 2nd. ed., Academic Press, New York, USA, p.415-497 (1975).
35.
North Sea Task Force (NSTF). Assessment reports for Subregion 1 and 6. State Pollution Control Authority (sFr), Oslo, Norway, (1993).
36.
CSA, Measurement of Naturally Occuring Radioactive materials (NORM) at three produced water outfalls, Continental shelf associates, Inc., Jupiter, Florida, prepared for Mid- Continent Oil and Gas Association, Baton Rouge, Lousianna, USA, (1991).
37.
O.G. Brakstad, A.J. Olsen, T. Nordtug, T. Frost, T.K. Aunaas, S. Johnsen, Uptake and degradation of discharged produced water components in marine microorganisms, SPE paper 35910, New Orleans, USA (1996).
15, pp. 2593-2606, 1999 © 1999 Elsevier Science Ltd. All rights reserved 0045-6535/99/$ - see front matter
Pergamon
PII: S0045-6535(99)00171-X CHEMIC~d.~ CHARACTERISATION OF PRODUCED WATER FROM FOUR OFFSHORE OIL PRODUCTION PLATFORMS IN THE NORTH SEA
Toril I. R~e Utvik
Norsk Hydro E&P Operations, Environmental Section, N-5020 Bergen, NORWAY Fax: +47 55 99 62 50, e-mail: [email protected] (Received in Germany 8 March 1999; accepted 9 April 1999)
ABSTRACT Samples of produced water from the offshore oil production platforms Oscbcrg Fcltscnter,Oscberg C, Bragc and Troll B were collcctcd during the period of Octobcr 1995 to August 1996. The samples were analysed for polycyclic aromatic hydrocarbons and phenols by gas chromatography with a mass-spcctromctric detector. Analysis of organic acids was done by isotachophorcsis, metal determinations by atomic absorption s]?ectromctry, and radioactivity mcasuremcnts by high resolution g a m m a spcctroscopy. The results wcre included in a database of chemical composition of produccd water, and wcrc then compared with data from othcr fields in thc Norwcgian scctor of thc North Sca using principal component analysis. The conccntrations of naphthalcnc, phcnanthrene, dibcnzothiophcne, and their CI-C3 alkyl homologues, and alkylated phcnols show decreasing Icvcls with increasing alkylation of the components for all fields. The results show that there is no correlation between the T H C content, which today is used as emission standard for environmental regulation, and the content of the aromatic compounds, which are assumed to be the most important contributors to toxicity. Field-specific detailed chemical characterisation of produced water from each platform is necessary in predicting fate and effects of the produced water discharged to the marine environment. © 1999 Elsevier Science Ltd. All fights reserved
INTRODUCTION Produced water is the largest wastewater stream in the oil exploration and production process. During oil production, water from the reservoir containing low levels of petroleum is pumped to the surface. Produced water is separated from the oil and either injected into a well or discharged after treatment to surface waters. Discharges of produced water from the Norwegian oil and gas production are expected to increase from 26 million m 3 irL 1993 to 120 million m 3 by the year 2000 (1). The fields Oseberg, Brage and Troll are Norsk Hydro's most important sources of produced water. A map of the North Sea with the discharge points of produced water in the Norwegian sector pointed out is shown in Figure 1. 2593
2594
Cat/
Figure 1. The North Sea with the Norwegian oil production fields Oseberg, Brage and Troll.
The amount of produced water discharged to the sea per year is shown in Table 1.
Table 1.
Oil production (m3/year, 1996) and produced water discharge volumes (m3/year, 1996) for Oseberg, Brage and Troll (2).
Field
Oil Production (m3/year)
Produced water discharged (m3/year)
Oseberg Brage Troll
29 000 000 7 000 000 14 500 000
1 432 000 1 856 000 4 382 000
The chemical composition of the discharged water is complex, including dispersed oil, dissolved hydrocarbons, organic acids, phenols, metals, and traces of chemicals added in the separation and
2595 production line, The composition is field-dependent, and several studies have been presented on chemical characterisation of produced water from fields in the North Sea (3-5), the Gulf of Mexico (6-8), and the coast of Canada (9). These studies show that the dissolved hydrocarbons are dominated by the volatile aromatic fraction of the oil, namely benzene, toluene, ethylbenzene and xylene (BTEX). The polynuclear aromatic hydrocarbons (PAHs) are dominated by naphthalene, phenanthrene and dibenzothiophene (NPD) and their C1-C3 alkyl homologues, but also compounds with higher molecular weight are reported such as chrysene and benzo(a)pyrene. The phenols reported may be alkylated up to C7. The organic acids are for most fields reported to be dominated by C1-C6 acids. The metal contents reported are varying, but often dominated by barium and iron. The analytical techniques used in previous studies are summarised in Table 2.
Table 2. Analytical techniques used for characterisation of produced water
Compound
Technique
Reference
"Total Oil" BTEX
IR Purge & trap, GLC-MS GC-FID GC-MS Direct GLC/FID Isotachophoresis Silylation]GLC-MS GC-MS Derivatisation/GC AAS ICP-AES ICP-MS
3,8 3 4,6 3,5,6,7.8,9 3 4 3 4,7,9 10 3,8 3 6
PAH Organic acids Phenols
Metals
Routine produced water analyses carried out offshore in the North Sea are based on freon extraction followed by determination of "total oil" by infrared spectroscopy. This is because legislation on the composition of discharged produced water offshore has, in general, been limited to "total oil" concentration (currently 40 mg/1 in the North Sea) and has been concerned mainly with monitoring the efficiency of oil/water sepaJ:ators (3).
The acute toxicity of produced water was studied by several authors in recent years (4,
11-13). The
main
conclusion from these studies is that the acute toxicity of produced water to marine organisms is low, and that the future work should be focusing on potential chronic effects of produced water components to marine organisms. This is also the conclusion of work initiated by the Norwegian Oil Industry Association in 1993
(5, 14, 15). The present work is conducted as
a part of a research program, which within the year 2000 is
supposed to end up in a new tool for environmental risk assessment of oil related compounds to marine organisms. The aim of the present study is to characterise produced water from four Norwegian oil production platforms, and to determine any correlation between the concentrations of the certain compounds within one field and in-between different fields.
2596 EXPERIMENTAL
Sampling Samples of produced water (2 x 1 1) was taken once a day for five days from each of the platforms rI'la~ samples were acidified (HC1), and kept dark and cool (4°C) during transport.
Sample preparation The samples were placed on a shaking table for one minute for homogenisation. One of the two litres from each sample was divided into 5 subsamples for determination of BTEX, organic acids, mercury, other metals and radioactivity, respectively (Table 3). The other litre were added internal standards of PAH and phenol compounds (50 mg/1).
Table 3. Sample volumes used in the chemical characterisation of produced water
Compound
Technique
Sample volume (ml)
BTEX Organic acids Hg Pb, Cr, Cu, Ni, Zn, Ba, Fe Radioactivity PAH/Phenols
GC headspace Isotachophoresis Cold vapour atomic absorption spectroscopy Atomic Absorption Spectroscopy (graphite furnace) Gamma spectroscopy GC-MS
10 10 100 250 500 1000
The samples for PAH/phenol analysis were extracted with dichloromethane. For Oseberg Feltsenter, Oseberg C and Troll, the 5 samples from each of the platforms were combined to one sample for determination of all parameters except PAHs and phenols. For the latter two compounds, the samples were extracted separately, and the extracts from each platform were combined. For Brage the 5 samples were treated separately.
Analysis The analysis with respect to BTEX was done by GC headspace (16) using a Varian 3700 Gas chromatograph with a DB-5, 30 m, 0.25 lam column (J&W) and a flame ionisation detector. Organic acids were determined by isotachophoresis as described by Barth (17). Mercury was analysed by cold vapour atomic absorption spectroscopy (CV-AAS) as described in the Norwegian Standard NS 4768 (18), with sample pre concentration on a gold trap. Lead, chromium, copper, and nickel were determined by atomic absorption spectrometry (graphite furnace) as described in Norwegian Standard NS 4780 (19) and 4781 (20). Zinc was determined by atomic al~sOrption spectrometry (flame technique) as described in Norwegian Standard NS 4771 (21). Barium and iron were analysed by inductively coupled plasma atom emission spectrometry (ICPAES) as described by Aase (22) and Broekaert (23). Radioactivity measurements were done by germanium detector gamma spectroscopy (Canberra) (24).
2597 PAils were determined by GC-MS as described in the EPA method 610 (25), and phenols were determined by GC-MS as described in EPA method 604 (26). Multivariate analysis of the analytical data was performed by using the multivariate statistical software "Unscrambler" (27).
RESULTS .~'D DISCUSSION The results of the chemical characterisation of the produced water samples from Brage, Oseberg Fehsenter, Oseberg C and Troll are given in Table 4. A comparison of daily samples taken during a five days period at Brage is shown in Table 5. Uncertainty in sampling and analysis, expressed as relative standard deviation (%) is shown :in Table 6.
Table 4. Chemical composition of produced water from the oil production platforms Oseberg Feltsenter, Oseberg C, Brage and Troll (lad:not detected,na: not analysed)
Component / Field
Unit
Brage
THC (platform)* S u m BTF.J[ Benzene Toluene Ethylbenzene Xylene S u m NPD Naphthalenes Naphthalene Cl-naphthalene C2-naphthalene C3-naphthalene Phena'athrenes Phenanthrene Cl-phenanthrene C2-phenanthrene C3-phenanthrene Diben',~othiophenes Dibenzothiophene Cl-dibenzothiophene C2-dibenzothiophene C3-dil~mzothiophene Acenaphtylene Acenaphthene Fluorene Fluoranthene Pyrene Chrysene Benz(a)anthracene Benzo(a)pyrene Benzo(ghi)perylene Benzo(k)fluoranthene S u m organic acids Formic: acid Propanoic acid
rag/1 mg/l mg/l mg/l mg/1 rag/1 mg/l mg/l mg/1 rag/1 mg/1 mg/1 llg/l ~g/1 tlg/1 t~g/1 ~g/1 pg/l tlg/1 ~g/1 pg/1 tlg/1 tlg/l pg/1 tlg/l tlg/l ~g/l ~g/1 pg/1 pg/] pg/l pg/1 mg/l mg/1 mg/1
58 9.0 4.5 3.5 0.3 0.7 0.93 0.86 0.35 0.26 0.15 0.10 50.9 16.4 20.3 6.3 7.9 17.5 2.5 5.7 6.1 3.2 nd 1.8 8.9 0.4 0.7 0.5 0.6 0.2 0.2 0.2 757 282 53
Oseberg F 44 8.3 4.6 2.7 0.6 0.4 1.27 1.2 0.43 0.38 0.25 0.14 99.8
27.2 32.3 27.2 13.1 37.8 8.8 10.8 10.6 7.6 nd 0.3 16.2 1.8 5.2 0.1 2.4 nd 0.6 0.8 1135 584 98
Oseberg C 60 5.8 3.7 1.5 0.3 0.2 1.60 1.06 0.46 0.30 0.19 0.12 76.3 27.4 30.2 8.9 9.7 na na na na na nd 5.1 2.7 7.8 8.6 0.4 1.9 0.1 0.1 0.2 717 299 56
Troll 33 2.4 0.8 1.0 0.4 0.2 1.32 1.23 0.53 0.42 0.20 0.08 60.2 18.8 18.7 15.3 7.4 28.0 6.2 8.6 7.2 6.0 nd 1.5 15.4 1.7 5.1 nd 2.0 nd nd 0.7 798 26 36
2598
Component / Field
Unit
Bra$e
Oseber~F
Oseber~C
Butanoic acid Pentanoic acid HexanHexanoic acid Organic acids > C6 Sum Phenols Phenol Cl-phenol C2-phenol C3-phenol C4-phenol Barium Lead Cadmium Chromium Iron Copper Mercury Nickel Zinc 214 Pb 214 Bi 228 Ac 212 Bi 212 Pb 226 Ra
mg/1 mg/1 mg/1 mg~/1 mg/l mg/1 mg/1 mgJ1 mg/1 mg/1 mg/1 lag,/1 lag/l lag/1 mg/1 lag/1 ng/1 lag/1 mg/1 Bql/1 Bql/1 Bql/1 Bql/1 Bql/1 Bql/1
28 5 nd 389"" 6.12 3.54 1.97 0.51 0.09 0.02 228 nd nd nd 11.3 nd 20 nd 0.20 9 8 17 <2 <2 9
46 33 nd 375"" 11.45 7.01 3.52 0.75 0.13 0.03 107 nd nd nd 4.2 nd 20 nd 0.03 6 6 11 <2 <2 6
22 8 nd 332"" 10.96 6.10 3.81 0.95 0.08 0.02 142 nd nd nd 7.7 nd 26 nd 0.34 7 6 <2 <2 <2 7
Troll nd nd nd 735"" 0.58 0.03 0.06 0.41 0.06 0.02 147 nd nd nd 4.3 nd 17 nd 0.012 6 6 7 <2 <2 6
nd: not detected (detection limits: Pb: 0.013 pg/l, Cd: 0.003 IJg/l, Cr: 0.03 lag/l, Cu: 0.03 lag/l, Ni: 0.03 lag/I).
* Analysis performed by works laboratory offshore** High values probably due to interferences from production chemicals.
Table 5. Chemical composition of produced water samples taken ever~ da:¢ in a 5 days period.
Component / Field
Unit Bra~e 1 Bra~e 2 Brage 3 Brage 4 Brage 5
THC (platform)* S u m BTEX Benzene Toluene Ethylbenzene Xylene S u m organic acids Formic acid Propanoic acid Butanoic acid Pentanoic acid HexanHexanoic acid Organic acids > C6 Barium Lead Cadmium Chromium Iron Copper Mercury Nickel Zinc 214 Pb 214 Bi 228~
mgeq mg/l mg/1 mg/1 mg/l mg/1 mg/l mg/] mg/1 mg/1 mg/1 mg/1 mg/1 mg/1 lag/1 lag/1 lag/1 mg/1 lag/1 lag/1 lag/l mg/1 Bql/1 Bql/1 Bql/1
38 7.3 3.6 2.9 0.2 0.6 765 252 48 34 12 nd 418"" 225 nd nd nd 10.6 nd 0.025 nd 0.003 10 10 17
70 8.8 4.1 3.2 0.8 0.7 740 258 63 12 17 nd 390" 226 nd nd nd 11.0 nd 0.016 nd 0.001 10 9 16
84 8.1 4.0 3.2 0.2 0.7 688 253 49 12 13 nd 361"" 226 nd nd nd 11.3 nd 0.023 nd 0.014 10 9 ,
< 1
50 8.2 4.1 3.3 0.2 0.6 875 255 53 14 19 nd 534"" 228 nd nd nd 11.3 nd 0.024 nd 0.004 11 11 16
58 9.0 4.5 3.5 0.3 0.7 757 282 53 28 5 nd 389"" 228 nd nd nd 11.3 nd 0.020 nd 0.20 9 8 17
2599
Component / Field
Unit
212 Bi
Bql/l Bql/l Bql/1
212 Pb
Bra~e I
Bra~e 2
Bra~e 3
Bra~e 4
Bra$e 5
< 2
< 2
< 2
< 2
< 2
< 2
< 2
< 2
< 2
< 2
10 I0 10 ii 9 nd: not detected (detection limits: Pb: 0.013 pg/l, Cd: 0.003 pg/l, Cr: O.03 lag~l, Cu: 0.03 I~g/l, Ni; O.03 pg/l) * Analysis performed by works laboratory offshore • * High values probably due to interferences from production chemicals 226
Ra
Table 6. Uncertainty in analysis and sampling, expressed as percent standard deviation for selected produced water components.
% st. dev. analysis
% st. dev. sampling
Phenols Organic acids Zn
5 % 1 - 12 % 1- 6%
4 - 67 % 1 - 51% 98%
H~R
5-22% 10%
17%
a
Table 5 show:~ that there are only minor variations in the concentration o f the produced water compounds. over a period o f five days. A statistical F-test showed that there were significant differences between the samples for c,ertain c o m p o u n d s , but these were only single values without any systematism for which samples this regarded. Based on this observation one may assume that the samples taken from the other platforms (once a day during 5 days, c o m b i n e d to one sample) are representative for the produced water composition o f that particular platform.
The chemical composition o f produced water from other fields in the Norwegian sector o f the North Sea is s h o w n in Table 7 (28). It must be emphasised that the quality o f these data is not known, because the analyses were done by different laboratories, and thus comparison between the fields has to be made with some care. Table 7. Chemical composition o f produced water from the fields Snorre, Ekofisk Tor, Ekofisk Edda, Ekofisk Kilo, Gullfaks A, Gullfaks B, Gullfaks C, Veslefrikk, Statfjord A, and Heidrun (5, 28)
Comp. THC BTEX NPD 3-6 r. PAH 16 PAH Phenols Org Acids Barium Lead Cadmium Iron Copper Mercury Nickel Zinc
Unit mg/l mg/l mg/1 lag/1 lag/l mg/l mg/l mg/l pg/1 pg/l mg/1 lag/l pg/l pg/l m~/l
Snorre 18.3 10.6 0.9 na 512 na 160 0.2 <10 4.6 0.1 na <0.l na na
Ekofisk Tor 2.7 5.5 0.3 Na 268 0.I Na 67 8.3 1.6 15 82 <0.5 1651 0.5
nr: not availabh~. * data from OLF report (5)
Ekofisk Edda 16 6.0 0.3 316 130 0.1 na 4 0.4 0.4 2.5 10 <0.5 193 <0.05
Ekofisk Kilo 4.2 2.0 0.4 413 218 0.2 na 21 0.7 0.4 2.1 40 <0.5 453 <0.05
Gullfaks B 35' 5.0 1.8 na 500 10.8 55* na <50* <10" na <2* 1.9" <40* 13"
Vesleffikk na 7.7 1.4 na 520 42.5 na na na na na na na na na
Sta(l)ord A 12 9.0 1.1 na 403 23.5 895* na <50* <10" na <2* 1.9" <40* 6.8*
Heidrun na 5.3 1.2 na 450 7.5 na na na na na na na na na
2600 The results from the analysis of produced water from Oseberg Feltsenter, Oseberg C, Brage and Troll show that the concentrations of all components are of the same order of magnitude as previously observed for other fields in the area. The results from all fields show that there is no correlation between the THC content, which is used as emission standard for environmental regulation, and the content of the aromatic compounds (which are assumed to contribute most to produced water toxicity).
There is little information available on the background level of inorganic and organic produced water compounds in the North Sea. The studies of Law et al (29) and Witt (30) represent the most comprehensive information available for PAHs in seawater around England/Wales and in the Baltic Sea, respectively. Background levels of PAHs in the North Sea have been recently measured by deployment of mussels and semipermeable membranes (SPMDs) along with direct water sampling for solvent and solid phase extraction, and in situ large volume water sampling (31, 32). A summary of background levels of produced water compounds in sea water is presented in Table 8.
Table 8. Background level of selected produced water compounds in the North Sea.
Compound PAH (15) PAH (16) Y. PAH (16) Barium Lead Cadmium Iron Copper Mercury Nickel Zinc Radioactivit~
Concentration 0.001 - 4.8 ng/1 <15 ng/1 1 - 45 ng/l <50 - 80 lag/l 22 [agJl 20 - 100 n~/1 4 - 28 ng/1 1.8 lags/1 20 - 410 ng/1 0.5 [a~/l 0.03 lag/l <2 - 10 lag/1 1.5 la~/l < 5 lag/1 823 lag/l 0.007-0.026 Bq/1
Reference 29 30 31,32 33 34 35 35 34 35 34 34 33 34 33 34 36
At first sight it seems that most of the organic components of the discharged water must be diluted 10 000 to 100 000 times to come down to the background level. Fortunately other processes than dilution are important for removal of these components from the water column. BTEX compounds are volatile, and normally will not be detected far away from the discharge point. Biodegradation by sea-water bacteria is another important process. Biodegradation is relatively rapid for organic acids, BTEX and NPD compounds (37). Other processes such as adsorption to surfaces of biological and non-biological material, and biological uptake by marine organisms will probably also influence on the concentration in the area surrounding the platforms (37).
2601 The results of the principal component analysis (PCA) of data from selected fields in the Norwegian sector are shown in figures 2 and 3. Due to lack of data, some of the compounds and fields had to be left out from the analysis o:["fields in the Norwegian sector of the North Sea (fig. 3). The fields included are: Oseberg Feltsenter, Oseberg C, Brage, Troll in figure 2, and Oseberg Feltsenter, Oseberg C, Brage, Troll, Ekofisk Kilo, Ekofisk Tor, Ekofisk Edda, Heidrun, Statfjord A, B, C (flotation units), Gullfaks A, and Veslefiikk in figure 3. The .compounds included are: BTEX, NPD, PAH, organic acids, phenols, Fe, Zn, Hg, Ba, and 22('Ra in Figure 2, aJad sum BTEX, sum phenols, sum 16 PAH, sum naphthalenes, sum phenanthrenes and sum dibenzothiophenes in Figure 3.
The biplot in Figure 2 shows that the first principal component (PC1) is stretched out by barium in the left end and the NPDs, PAHs, organic acids< C6 and phenols in the right end. The second principal component (PC2) is spanned out by organic acids >C6 in one end and benzene/chrysene in the other. The fields Oseberg Feltsenter, Oseberg C, Brage and Troll are all spread throughout the plot. This indicates that produced water from all field,,;differ from each other in chemical composition• Oseberg Feltsenter is located far to the right on PC1, which indicates that this field is different from the others by a higher level of NPDs, PAHs. organic acids< C6 and phenols, and a lower level of barium.
pc2................................................. ~:.~ot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • Chrysane
:
•
Benzene
: Fe.. Benzo(a)pyrene
Zn
• 22.eRa .
0.~
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.
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.
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.
• ~'lg@ .
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i
.....................................
- Acenaphthene
• BII
"0
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• C1-PKenol
e
: • C2-Phenol
:
~ ............
; . . . . . . . . . . . . . . . . . . . . . . .
~
• Fluoranthene
i
•'iBuWlic Phenolacid i:
i
• Xyten@
:
.
i
• "HgTolue~e
" ~(Le~l~lhalene
: •
i • Proponic acid : "hrBlen,zo(ghi)pery~ " hena, ot rene' • ~'nenantnfCall~,rlc acid. .'~.l:~P~ilnthrene
: p
:
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: .Troll...
" Naphthaione
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.................
'
: "B t~fi ~.( &',, ~t;ffi;r~,~e ....
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. r .........
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uo~en
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:
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X-expl:
Figure 2. Biplot from PCA of sumarised parameters for chemical composition of produced water from Oseberg Feltsenter, Oseberg C, Brage and Troll•
2602
PC2
Bi-plot . . . . . . . .
~tfjord
BTEX
. . . .
'
Statfj0rd
B
A •
StatfjOrd C
0.5
Sum
Ekofisk Edda
phencfls
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Veslefrikk
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...............................................................
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~'.
Figure 3. Biplot from PCA of summarised parameters for chemical composition of produced water from selected fields in the Norwegian sector of the North Sea. Brage is located at the other end of PC1, with opposite characteristics than Oseberg Feltsenter. Troll is located at the low end of PC2, close to the organic acids >C6. This indicates a high level of these acids in the produced water samples from Troll, and a low level of the compounds placed at the high end of PC2; BTX, phenols and metals. Oseberg C is located at the centre of the plot, close to the BTXs, Hg, fluoranthene and acenaphthene, which indicates higher levels of these compounds in the samples from Osehcrg C than for the rest of the fields•
The biplot in Figure 3 shows that there is a group of fields located at the left side of PC1, Ekofisk Edda, Ekofisk Tor and Ekofisk Kilo. They are all characterised by a low level of NPDs and PAHs. Statfjord A, B and C are all located to the fight side of PC1, and high up on PC2, together with the BTEX compounds• Troll and Gullfaks A are grouped together, close to the dibenzothiophenes. Oseberg Feltsenter, Veslefrikk, Oseberg C, Heidrun and Brage are all grouped to the fight side of PC1, and are characterised by high concentrations of PAHs, naphthalenes, and phenanthrenes.
It is interesting to observe that fields, which are expected to be similar in chemical composition due to geographical position (Ekofisk and Statfjord fields), are grouped together in the biplot. This indicates that prediction of chemical composition of produced water based on geological or geographical information may
2603 be possible. The results are, however, influenced by the analytical variation between the laboratories for the compounds i:acluded in this principal component analysis, and it is possible that some of the variation between the fields may be explained by analytical variation.
CONCLUSIONS Chemical composition of produced water from the offshore oil production fields Oseberg Feltsenter. Oseberg C, Brage and Troll in the North Sea has been determined. The results were compared with chemical composition of produced water from other fields in the Norwegian sector of the North Sea. For all compounds, the concentrations were within the range shown among other fields. The concentration t~l naphthalenes, phenanthrenes, dibenzothiophenes, and phenols decrease with increasing alkylation t)f the components for all fields. The results show that there is no correlation between the THC content, which is used as emiszfion standard for environmental regulation, and the concent of the aromatic compounds. The results of the principal component analysis of chemical data for produced waters from selected fields in the Norwegian sector of the North Sea, showed that fields which are expected to be equal in chemical composition ,due to geographical position (Ekofisk and Statfjord fields), are grouped together in the biplot. This indicates that a rough prediction of chemical composition of produced water based on geological or geographical information may be possible. The results are, however, influenced by the analytical variation between the laboratories for the compounds included in this principal component analysis, and it is possible that some of the variation between the fields may be covered by the analytical variation. The overall conclusion is that individual detailed chemical characterisation of produced water from each platform is necessary in order to predict fate and effects of the discharges of produced water to the marine environment.
ACKNOWLEDGEMENTS Sampling of the produced water was done by Odd Aas at Troll B, Jan Pokorny at Oseberg C, Magnc van Riesen at Brage, and Tom Pedersen at Oseberg Feltsenter. The determination of organic acids were done by Tanja Barth at the University of Bergen, the radioactivity measurements were done by Tone Bergan at IFE, the metal determination were done by Arne ]ksheim, Gunhild Annette Iversen, and Harald St. Aase at Norsk Hydro Research Centre, and analyses with respect to BTEX were done by Gerd Aasen at Norsk Hydro Research Centre.
Thanks also to Erling Odden, Anita Seljelund, Siw T. Pettersen, and Grete Di at Norsk Hydro Research Centre for assistance during the determination of PAHs and phenols, Terje Karstang at Norsk Hydro Research Centre, Espen Langered and Mona Nyg~rd for help with the multivariate statistics, and Sthle Johnsen at Statoil Research Centre for good advices.
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