Paper

RADIATION PROTECTION AND RADIOACTIVE SCALES IN OIL

AND GAS PRODUCTION
C. Testa," D. Desideri,* M. A. Meli," C. Roselli,* A. Bassignani,t G. Colombo,t and
R. Fresca Fantonit

sludges that can reduce the plant efficiency, thus

Abstract-Low specific-activity scales consisting of alkaline hindering the extraction operations (Vetter 1976; El-
earth metal carbonates and sulfates are often present in some Hattab 1985; Raeburn et al. 1988).
gaseous and liquid hydrocarbon plants. These scales contain a The need to reduce the formation of scales and
certain concentrationof radium, uranium, and thorium which remove the accumulated scales has stimulated some
can cause a risk of gamma irradiation and internal radiocon-
studies about their nature and origin.
tamination when they must be mechanically removed. The
gamma dose rates and the 238U,232Th, 226Ra concentrations
were determined in sludges, scales, and waters of some gas and Origin, nature, and localization of the scales
oil hydrocarbon plants located in Italy, Congo, and Tunisia. Scale production in gas and oil field equipment is
238Uand "'Th concentrations were very low. The isotopes due to the precipitation of alkaline earth metal sulfates
23sUand 234Uresulted in radioactive equilibrium, while 232Th and carbonates, particularly CaCO, and SrBaSO,.
and "&rh were not always in equilibrium. A rough correlation Although many studies have been performed to
was found between the gamma dose rate and the 226Ra understand the scales' formation and growth mecha-
concentration. Some considerations and conclusions about nisms, a final solution to the problem has yet to be
radiation protection problems are pointed out. found. Some investigations, performed under con-
Health Phys. 67(1):34-38; 1994 trolled production conditions, proved that the factors
Key words: 238U;232Th; 226Ra; radiation protection that are greatly responsible for the formation of scales
are water composition, fluids temperature, and pres-
sure. The way in which the scales are deposited is
INTRODUCTION connected to the pipes, superficial features, fluid-
NATURAL radioactivity in water and soil is a well- dynamic phenomena, and crystallization kinetics.
known phenomenon connected with the presence of A recent study (Raeburn et al. 1988) showed that
variable concentrations of uranium- and thorium- the variation of sulfate and carbonate solubility, which
series radionuclides. Generally the relevant radiologi- can give rise to scale formation, are connected to a)
cal hazard is not important; however, some radiation temperature variation (5%), b) pressure changes
protection problems can occur in peculiar industrial (lo%), c) evaporation in the gas extraction pipes
processes involving the treatment of large material (lo%), and d) injection of incompatible waters (70%).
quantities. In this case a high concentration of radio- The water reinjected into the reservoirs to maintain
production pressure during the field exploitation
active substance, naturally occurring radioactive ma-
terials (NORM), can be found in specific points of the seems to be the principal cause for the scale formation
plant, in the manufactured by-products, and in the (Vetter and Philips 1970; Vetter et al. 1982; Shen and
wastes. Sometimes the natural radioactivity concen- Crosby 1983).
tration can be high enough to raise radiation protection Scales consistency is variable; they can be thin
problems which can be similar, in a sense, to the ones and hard, thick and soft, or muddy.
faced while handling, storing, and disposing of non- From a geological viewpoint, the scale phenome-
sealed radioactive sources. non generally occurs in the Jurassic reservoirs (Rae-
These processes often occur in the extraction and burn et al. 1988).
treatment of liquid and gaseous hydrocarbons and are
generally accompanied by the formation of scales and Low specific-activity scales and radiation protection
problems
*General Chemistry Institute, Urbino University, Piazza Rinas- The natural radionuclides present in the connate
cimento 6, 61029 Urbino, Italy; +National Radiation Protection waters can determine the radioactive scale formation,
Institute, Agip S.p.A., Via Emilia 1, S. Donato, Milan, Italy. generally called low specific activity (LSA) scales. In
(Manwcriptreceived 19 May 1993;revised manuscript received
25 October 1993, accepted 8 December 1993) particular, 226Raplays a fundamental role in the radio-
0017-9078/94/$3.00/0 active scales where it is incorporated by coprecipita-
Copyright 0 1994 Health Physics Society tion with barium, strontium, and calcium salts.
34
 Protection and radioactivity in oil and gas production 0 C. TESTA
ET AL. 35

As far as hydrocarbon extraction and treatment Uranium, thorium, and 226Rawere separated by
plants are concerned, LSA scales present both a extraction chromatography (Testa 1970; Braun and
gamma irradiation and an internal radiocontamination Ghersini 1972; Delle Site et al. 1985)with MicrotheneT
risk (Smith 1985; Bassignani et al. 1991). supporting trioctylphosphine oxide (TOPO). The nitric
The gamma irradiation risk concerns the workers solution was passed through a 1-cm inside diameter
remaining for a long time in certain plant areas. The (i.d.) chromatographic column. After washing the col-
internal radiocontamination risk especially arises dur- umn with water, the feeding and washing solutions
ing plant maintenance (valve or pipe replacement) and were kept to determine 226Ra,then thorium was eluted
particularly during the mechanical removal and dis- with 1 M HCI and uranium was eluted with 1 M
posal of the scales. In these processes whisks are (NH4)2c03-
generally used with a consequent production of a fine The HC1 and (NH4),C03 solutions were evapo-
powder which can be inhaled or ingested by the rated to dryness; the residue was dissolved in some
workers involved in these operations. To evaluate the concentrated H2S0,, and the pH was brought to 4 with
extent of the LSA scales problem, Agip SPA (Milan) ammonia. Then, the solution was transferred into an
and Urbino University have carried out a survey in electrolytic cell and electroplating was carried out for
some Italian and African plants (Testa et al. 1992). 5 h at 600 mA on a stainless steel disk (Talvitie 1972).
The alpha spectrometry was then performed by a
1,000-2,000-min counting time with a 300-mm2 solid-
MATERIALS AND METHODS state alpha detector' connected with a multichannel
The survey included three different phases: a) analyzer** and a digital plotter.tt
scales localization, b) gamma dose rate measurement 226Ra was determined by counting Ba(Ra)SO,
around the plants, and c) radioanalytical analyses of (Spezzano 1985) with a ZnS(Ag) detector.**
the scales. These techniques were preferred to gamma spec-
trometry because of the probable presence of radioac-
Scales localization tive disequilibrium in the 238Uand 232Thfamilies and
Portable scintillometers*were used to localize the because of the low level of activity to be detected.
LSA scales in the plants. These instruments are nor-
mally employed in uranium exploration and are char- RESULTS AND DISCUSSION
acterized by an excellent sensitivity to electromag-
netic radiations with energy >30 keV. The general survey involved 391 oil wells (Italy:
A gamma radiation survey of the LSA scales 171; Tunisia: 140; Congo: 80), 16 oil centers (Italy: 13;
permits checking inaccessible points while not stop- Tunisia: 2; Congo: l), 157 gas wells (Italy: 157), 38 gas
ping the plant's work. centers (Italy: 38), 42 platforms (Italy: 34; Congo: 8),
and 10 gas fields (Italy: 10) used as natural tanks to
Gamma dose rate store gases; however, LSA scales were found only in
A gamma dose rate survey was performed. Scin- some plants and mostly in phase separators. This
tillation detectors§ and proportional countersll were phenomenon is probably caused by the strong temper-
used at the locations where the LSA scales were ature and pressure changes that fluids undergo when
located. passing through this equipment and also by the slow
flow rates which facilitate the formation of muddy
scales. Tables 1, 2, and 3 show the gamma dose rate
Uranium, thorium, and 226Radetermination values measured where the scales have been located.
(radioanalytical procedure) The majority of the Italian scales consisted of
After addition of 232Uand '"Th as yield tracers, carbonates and often presented peculiar crystallization
the samples' pretreatment was as follows: a) sulfatic forms (aragonite). The gross gamma dose rates ranged
scales (500 mg of sample were dissolved by digestion between 0.1 and 6.0 pSv h-' (Table 1).
with NaOH followed by an attack with HCl; after The North African oil plants' scales (Tunisia)
drying, the residue was dissolved in 2 M HNO,); b) consisted of carbonates and sulfates, and the relevant
carbonate scales (the same quantity of scales was gross gamma dose rates ranged between 0.2 and 0.9
directly dissolved in HNO,); c) waters (200 mL of pSv h-' (Table 2).
samples were filtered, evaporated, and then had the
residue dissolved in 2 M HNO,).
' Microporous polyethylene, 50-100 mesh, Aldrich, Chemie
GmbH, P.O. Box 1120, W-7924 Steinheim, Germany.
* Scintillometre de prospection, model SPP 2NF, Saphymo # Model IPE 450-100-21, Wissenschaftliche Instrumente
Stel, Siege Social et Service Commerciaux, 2729 Avenue Carnot, GmbH, Struthweg 1, 6467 Hasselroth 1, Niedermittlau.
91302 Massy. * * Model Varro K8 Silena SPA, Via Firenze, 3-20063 Cer-
MicroRem Low Energy, Bicron Corporation, 12345 Kinsmon nusco S.N., Milan, Italy.
Road, Newbury, OH 44065-9677. tt Model 4550 Facit Data Products SPA Centro direzionale
11 Radiation Protection Dose Ratemeter, Model TOLE, Labo- Colleoni, 20041 Agrate Brianza, Mi, Italy.
ratorium Prof. Dott. Berthold Calm bacher-Srob 22, D 7547 ** Model Se-105, K. G. Elettronica Srl, Via Varese 4, 21050
Wildbad 1, Postfach 160. Cantello, Varese, Italy.
36 Health Physics July 1994, Volume 67, Number 1

Table 1. Gross gamma dose rate of the Italian plants where Table 2. Gross gamma dose rate of the Tunisian plants where
LSA scales were located. LSA scales were located.
Gross gamma Gross
dose rate Background gamma
Plant Location Oil/gas/water (pSv h- ') (ySv h-*) dose rate Background
Plant Location Oil/gas/water (pSv h-') (pSv h-')
Gas center: Separator Gas/water 0.17 0.03
Filtrator Gas 0.10 0.03 Oil center: L.P. collector Oil 0.3 0.10
Oil center: Flow line Oil/gas/water 0.20 0.09 M.P. collector Oil 0.3 0.10
Flow line Oil/gas/water 1.00 0.09 Gas scrubber Gas/water 0.4 0.10
Flow line Oil/gas/water 0.50 0.09 Separator Gas/water 0.8 0.10
Flow line Oil/gas/water 0.70 0.09 Separator Oil/gas/water 0.7 0.10
Flow line Oil/gas/water 0.40 0.09 Separator Gas/water 0.9 0.10
Flow line Oil/gas/water 1S O 0.09 Separator Oil/water 0.8 0.10
Water inj. well Water 0.40 0.09 Separator Oil/gas/water 0.7 0.10
Manifold Oil/gas/water 0.20 0.09 Separator Oil/gas/water 0.6 0.10
Separator Oil/gas/water 6.00 0.09 Separator Oil/gas/water 0.6 0.10
Separator Oil/gas/water 2.00 0.09 Oil washing Oil 0.4 0.10
Column Oil/water 2.00 0.09 Oil washing Oil/water 0.3 0.10
Separator Oil/gas 0.80 0.09 Manifold Oil 0.2 0.10
Separator Oil/gas 0.90 0.09 Separator Oil/water 0.3 0.10
Column Oil/gas 0.90 0.09 Water plant: Cooling system Water 0.6 0.10
Separator Oil/gas 0.80 0.09 Water injection Water 0.5 0.10
Manifold Oil/gas/water 1.00 0.09 1st filtration Water 0.4 0.10
Water tank Water 1.00 0.09 2nd filtration Water 0.3 0.10
Heat exchanger Oil/water 0.15 0.07 3rd filtration Water 0.3 0.10
Separator Oil/water 0.40 0.07
Water tank Water 0.20 0.07
Separator Oil/water 0.23 0.05
Separator Gas/water 0.30 0.09
Separator Oil/water 0.80 0.03 In all the analyzed samples, the principal uranium
Separator Oil/gas/water 0.60 0.03 isotopes (238Uand 234U)appeared to be in radioactive
Separator Oil/water 0.40 0.03 e uilibrium. For the majority of samples, 232Thand
Separator Oil/gas/water 0.80 0.03 "Th also resulted in radioactive equilibrium. A strong
Manifold Oil/water 0.30 0.03
Flow line Oil/gas/water 0.35 0.03
disequilibrium was found in some African scales, and
Separator Oil/water 0.25 0.03 the 22sTh:232Thratio reached a value as high as 700. This
Heat exchanger Oil/water 0.14 0.03 fact is certainly due to the presence of high concentra-
Separator Oil/gas/water 0.25 0.03 tions of 228Ra,which ultimately decays to 228Th;in this
Separator Oil/gas/water 0.20 0.03 case, the internal radiocontamination risk is due to 228Th
and its progeny (224Ra,'I2Bi, 220Rn,'16Po, and "'"Po). In
this paper, '" 'Ra was not measured as only alpha
Finally, Table 3 reports the gross gamma dose determinations were performed.
rate (0.02-0.06 +Sv h-l) in a Central African (Congo)
plant where the scales' formation was very poor and of
a sulfatic type. CONCLUSIONS
The results obtained for the radiochemical analy- The obtained results clearly show that the LSA
sis of scales, sludges, and waters are reported in
scales phenomenon does not present a severe radia-
Tables 4, 5, and 6, respectively, together with the
tion protection problem for the plants because work-
relevant gross gamma dose rates. The following re-
er's stay times around these plants are infrequent (only
marks can be made concerning these data:
a few hours per year). The gamma dose rates are low,
1. 238Uand 232Thconcentrations were either not
detectable or very low.
2. 226Raconcentration was higher in the oil ex-
traction plant scales (800-3,000 Bq kg-') than Table 3. Gross gamma dose rate of the Congo's plants where
in other plants. Because of its chemical behav- LSA scales were located.
ior, 226Raconcentration in waters was very low Gross
except for only one extraction deep well. g-a
dose rate Background
3. The correlation of 226Raconcentration with the Plant Location Oil/gas/water (pSv h-') (pSv h-')
relevant external gamma dose rate was not
always observed; this fact can be due to the Oil center: Desalter Oil 0.06 0.02
Desalter Oil 0.05 0.02
following: a) the gamma dose rate is correlated Separator Oil/water 0.04 0.02
not only to radium concentration but also to oil tank Oil 0.06 0.02
the scales' total amount; b) the variable depo- Platform: Separator Oil/water 0.06 0.01
sition and detection geometry; and c) the vari- Separator Oil/water 0.02 0.01
able gamma absorption due to different mate- Sea line Oil 0.04 0.01
Water injection Water 0.04 0.01
rials thickness and density.
 Protection and radioactivity in oil and gas production 0 C. TESTA
ET AL. 37

Table 4. 238U,232Th, and 226Ra concentration in scales and external gross gamma dose rate values.
Concentration (Bq kg-')
Extracted Depth Gross gamma Background
Plant features and site hydrocarbon (km) 2 3 8 ~
232Th 226Ra dose (pSv h-') ( ~ Sh-I)
V
Po valley, Italy
Extraction plant Liquid 6 <0.9 <0.8 2,890 f 578 0.20 0.05
Extraction plant Liquid 5 <0.9 ~0.8 1,126 f 225 1.00 0.09
Extraction plant Mixed 5 10.9 <0.8 120 2 24 0.20 0.09
Collection plant Gaseous - 23.8 2 4.3 18.9 2 3.8 30 2 6 0.10 0.03
Collection plant Gaseous - 53.8 f 10.8 ~0.8 <2.7 0.17 0.03
Southern Italy
Collection plant Liquid - 11.3 f 2.3 <0.8 110 f 22 0.05 0.05
Northern Sea
Offshore Liquid 3 10.9 ~0.8 780 & 156 - -
Platform
Tunisia
Extraction plant Liquid 1 1.2 2 0.2 10.8 1,189 2 238 0.25 0.10
Treatment plant Liquid 1 <0.9 ~0.8 31 f 6 - -
Phase separ. plant Liquid 1 7.6 f 1.5 5.1 f 1.0 64 f 12 - -
Congo
Phase separ. plant Liquid 1 <0.9 ~0.8 97 f 20 0.01 0.01
Oil storage reservoir Liquid 1 2.7 f 0.5 2.2 f 0.4 151 2 30 0.04 0.01

Table 5. 238U,232Th,and 226Raconcentration in some Tunisian sludges and external gross gamma dose rate values.
Concentration (Bq kg-')
Extracted Depth Gross gamma Background
Plant hydrocarbon (km) 2 3 8 ~
"Vh 226Ra dose ( ~ Sh-')
V ( ~ S h-')
V
Treatment plant (A) Liquid 1 5.3 f 1.1 6.1 f 1.3 66 f 14 0.30 0.10
Treatment plant (B) Liquid 1 5.0 f 1.0 10.0 f 2.0 169 f 35 0.30 0.10
Treatment plant (C) Liquid 1 6.6 f 1.3 2.6 ? 0.5 453 f 90 0.40 0.10

Table 6. 238U,232Th, and 226Raconcentration in waters and external gross gamma dose rate values.
Gross
Concentration (Bq kg-') gamma
Plant features Extracted Depth dose Background
and site hydrocarbon (km) 2 3 8 ~
'"Th 226Ra ( ~ S h-')
V ( ~ S h-')
V
Po valley, Italy
Extraction plant Mixed 5 14.5 x 10-3 (4.0 x lop3 2.0 X 10' 2 4.0 2.00 0.09
Extraction plant Liquid 2 1.5 x lo-' If: 3.0 X ~ 4 . 0x 2.3 X lo-' f 4.6 X lo-' 0.09 0.09
0
Adriatic Sea
Offshore platform Gas 1 7.3 x 10-3 2 1.5 x 1 0 - ~ <4.0 x 10-3 6.0 x 10-2 2 1.2 x lo-' 0.03 0.03
Congo
Offshore platform Liquid 1 <4.5 x 10-3 <4.0 x 10-3 5.1 2 1.2 0.01 0.01

and the action levels (2.5 pSv h-l, 5 mSv y-') sug- into the reservoirs should be seriously considered for
gested by international radiation protection rules the easily soluble substances (Fitzgerald 1991).
(ICRP 1990) are exceeded in only a few cases.
The routine maintenance of the plants presenting
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