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Characteristics of NORM in the oil industry from Eastern and Western deserts of Egypt
Applied Radiation and Isotopes 55 (2001) 135–139
Characteristics of NORM in the oil industry from Eastern and Western deserts of Egypt S. Shawkya,*, H. Amera, A.A. Nadab, T.M. Abd El-Maksoudb, N.M. Ibrahiema a
National Center for Nuclear Safety and Radiation Protection, P.O. Box 7551, Nasr City, 11762 Cairo, Egypt b Physics Department, Women’s College, Ain Shams University, Heliopolis, Cairo, Egypt Received 2 October 2000; accepted 13 October 2000
Abstract Naturally occurring radionuclides (NORs) from the 232Th- and 238U-series, which are omnipresent in the earth’s crust, can be concentrated by technical activities, particularly those involving natural resources. Although, a great deal of work has been done in the field of radiation protection and remedial action on uranium and other mines, recent concern has been devoted to the hazard arising from naturally occurring radioactive materials (NORM) in oil and gas facilities. NORM wastes associated with oil and gas operations from scale deposits, separated sludge and water at different oil fields in the eastern and western deserts were investigated. Concentrations of the uranium, thorium, and potassium (40K) series have been determined from high-resolution gamma-ray spectrometry. Total uranium content of samples was determined using laser fluorimetry. The levels of radioactivity were mainly due to enhanced levels of dissolved radium ions. Only minute quantities of uranium and thorium were present. The disequilibrium factor for 238 U/226Ra has been determined. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The presence of naturally occurring radioactive material (NORM) has been recognized since early 1930s in petroleum reservoirs, in oil and gas production, and in processing facilities. NORM was typically observed in barite scale that accumulated on the interior of oil production tubing and in storage tanks and heater-treated separation sludge. Recently, concern has been expressed over the health impacts from the uncontrolled release of NORM (Rood, 1996). Trace quantities of the radioactive elements 238U and 232Th as well as 40K have been present in the earth’s crust since its formation. Both 238U and 232Th are parents of a complex series of successive decays, producing many radioactive daughters. Some of these daughters may be co-produced with oil/gas well fluids and may concentrate in ordinary deposits (e.g. scale, sludge) and/or
*Corresponding author. Tel.: 2740236-7-9; fax: 2740238. E-mail address: [email protected] (S. Shawky).
leached in production water. There are several potential exposure pathways to humans from oilfield NORM, such as inhalation of radon gas. From the radiological point of view, the only important isotope of radium is 226 Ra which has an average concentration in the earth’s crust of about 10 12 g/g (40 Bq/kg).
1.1. Geological origin U and Th are present in the earth’s crust at an average concentration of 4.2 and 12.5 ppm, respectively (Wollenberg and Smith, 1990). When a geological formation containing 238U and 232Th has not been disturbed (closed system) for more that a million years, the members of the individual decay series will have the same activity (Bq/kg) which is known as secular equilibrium. However, when the geological formation is not closed to radionuclide migration, 226Ra can migrate and be deposited somewhere outside the formation. Then secular equilibrium will not exist and the growth of 226 Ra by radioactive decay of its ancestors will not
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 4 - X
136 Table 1 Average
S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
226
Ra,
238
Rock type
U,
232
Th and
a
K content (mBq/g)
226
238
Ra
(mBq/g) Sandstone Shales Limestone
40
26.3 39.9 15.5
232
U
(ppm) 0.12 0.18 0.07
(mBq/g) (a)
50 ,14.8 70(a),14.8 160(a),14.8
40
Th
(ppm) (a)
4.1 ,1.2 5.9(a),1.2 13(a),1.2
(mBq/g) (a)
40 ,24 70(a),41 10(a),5.2
K
(ppm) (a)
9.7 ,5.9 16.3(a),10 3(a),1.6
(mBq/g) (a)
700 ,325 1100(a),814 200(a),81.4
(%) 2.1(a),0.98 3.5(a),2.6 0.5(a),0.2
Mean values are given by Bloch and Key (1981), otherwise the values reported are give by NCRP (1975).
occur. 226Ra is said to be unsupported. Table 1 shows the average radium, uranium, thorium and potassium content in sedimentary rocks (NCRP, 1975). Oil is formed by thermal cracking of organic matter (kerogen) trapped in sedimentary rock. As oil migrates, naturally occurring radionuclides (NORs) may be taken up in the upflowing fluid stream and may be transferred into connate waters, which may be produced with oil as production water. During thermal cracking of kerogen, uranium or thorium remain with the residual organic matter and they will not be leached in a reducing environment into passing fluids. Radium will not leach into a hydrocarbon phase, but it may leach into the aqueous phase (Bloch and Key, 1981). In this work, NORs in scale/sludge originating from oil fields in the western desert and the Red Sea region have been characterized and compared with the exempt concentration levels given by the IAEA (IAEA, 1988, 1994).
in the same type and configuration was used to determine the absolute efficiency curve. For the 238U series peaks with gamma-ray energies of 63.3 keV (3.8%) 234 TH, 186.1 keV (3.3%) 226Ra (after the subtraction of the 185.7 keV 53% of 235U if present), 295.1 keV (19.2%), and 325.0 keV (37.1%) 214Pb, 609 keV (46.1%), 768.4 keV (5%), 934.0 keV (3.4%) and 1120 keV (15%) 214Bi were used to determine the concentrations of the assigned nuclides in the series. For the 232Th series peaks with gamma-ray energies of 463.1 keV (4.6%), 911.2 keV(29%) and 966.0 keV (2.3%) 228Ac, 727.3 keV (6.7%), 1620.7 keV (1.5%) 212 Bi and 583.0 keV (30.9%) 208Tl were used to determine the concentrations of the assigned nuclides in the series. The 1460.8 keV (10.7%) gamma-ray peak was used to determine the 40K concentration.
2.2. Low level alpha–beta counter 2. Experiment Three different techniques were applied to characterize the investigated samples; gamma-ray spectrometry, laser fluorimetry and total alpha–beta counting. 2.1. Gamma-Ray spectrometry The collected samples were individually weighed, dried at 1008C and pulverized. The homogenized samples were transferred to 750 or 500 ml Marinelli beakers as well as 100 ml polyethylene containers for gamma-ray measurements and sealed for 8 weeks to ensure secular equilibrium between radium and its radioactive progenies. A high-resolution gamma-ray spectrometer based on an hyper pure germanium detector (HpGe) from EG&G Ortec was used for the gamma-ray analysis. The HpGe crystal has a diameter of 64.5 mm and a 69.9 mm length, a relative efficiency of 50%, peak to Compton ratio of 62, and FWHM of 1.9 keV at the 1.33 MeV 60Co transition and 753 eV at the 122 keV 57Co tansition. The efficiency calibration for the system has been explained in a recent publication (Ibrahiem et al., 1999). In addition, a 226Ra (No3)2 solution in HNO3,
In screening and contamination measurements, it may not be necessary to know what an isotope is, but only that contamination is present and that cleanup is required. Low background counting provides a means to accomplish screening of samples for gross alpha and beta activity with relatively limited sample preparation, relatively good efficiency and exceptionally low background. A Berthold LB 770 10 channel low-level counter was used. The detector is a gas-flow proportional counter for measuring both alpha and beta activity of the sample. The sample detector features an ultra-thin entrance window that contains the P-10 counting gas (a mixture of argon and methane) within the detector chamber and offers minimum attenuation to particles of interest that pass through the window into the detector chamber. The detector is biased with a high voltage to collect the charge generated by the interaction of the radioactivity with the counting gas. The detector is surrounded by lead to shield it from the naturally occurring background radioactivity. While the lead shield is effective against naturally occurring background radiation, the attenuation is insufficient for shielding from high-energy cosmic radiation. A second gas-flow proportional detector is used to guard the detector from high-energy cosmic radiation.
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S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
Table 2 Concentration levels of NORM from oil-producing fields in Bq/kg dry weight (WD western desert, W water, S soil, RSW Red Sea water, RSS Red Sea soil, LDL lower than the detection limits) 226
232
Ra–U series
226
Ra
WDW1 WDW2 RSW Sludge WDS1 WDS2 WDS3 RSS
40 2 38 3 51 18,032 78 7541 126 143,262 2213 8005 143 194,489 441
40
Th–series
214
Pb
27 0.5 20 1 0.8 0.2 19,394 19 18,215 103 322,604 1311 18,785 113 437,960 2300
214
Bi
27 2 18 2 1.3 0.6 18,324 50 17,929 325 320,008 3460 17,627 89 434,435 659
228
Ac
59 2 55 3 1.1 0.34 13,257 70 35,460 164 661,328 4385 36,727 170 897,803 948
212
K
208
Bi
Tl
12 1 82 0.7 0.5 7398 56 LDL 368,654 4556 13,779 297 500,476 707
4 0.3 31 1.1 0.2 5105 14 LDL LDL 8615 82 LDL
29 3 43 2 19 2 1261 29 2914 138 45,882 2022 3013 152 LDL
Two grams of dried samples were leached in concentrated nitric and hydrochloric acids overnight at room temperature. Samples were centrifuged. The acid mixture was decanted in a pre-weighed planchet and the solution was evaporated to dryness.
Table 3 Exempt activity concentrations for the most relevant radionuclides in the case of NORM releases as established by the BSS (6) and activities found in extra-active industries. (a, Spezzano, 1993; b, Heaton and Lambley, 1995; c, Kolb and Wojciki, 1985; d, Paschoa, 1993)
2.3. Laser fluorimetry for uranium analysis
Radionuclide
The measurements were performed by laser fluorimetry (Sintrix UA-3) based on the fluorescence of a uranyl complex that was formed by the addition of a buffered inorganic complexing reagent FLURAN to the sample during analysis. Unlike conventional fluorimetry, the excitation source is UV light provided by a nitrogen laser tube, which emits an UV pulse (3371 A˚). The limit of detection of uranium in water is 0.05 ng/g and the precision is about 15% at the ng/g level (Veselsky et al., 1988). Samples (1 g) were dried and ashed at 6008C for 3 h. The residue, after fuming with nitric and perchloric acid, is taken up in a Ca(NO3)2 solution and the uranium is extracted with methyl isobutylketon (MIBK). After stripping uranium from the MIBK solution into 0.001 M nitric acid, it was measured by laser fluorimeter. More details are given by Shawky et al. (1994).
3. Results and discussion Table 2 shows the concentration levels of isotopes in the uranium–radium series, thorium series and potassium in Bq/kg dry weight. The activity levels for all investigated radionuclides ranged from natural background levels up to levels exceeding the exempt activity concentrations for NORM releases as established in the basic safety standards given by the IAEA (1994). A summary of the exempt activity concentrations for the most relevant radionuclides (plus their progeny) is given in Table 3. Uranium and thorium compounds are
Activity (kBq/kg) Exempt
222
Rn Ra 226 Ra 228 Ra 228 Th Th-natural 230 U U-natural 224
10 10 10 10 1 1 10 1
Extra-active industries
2.7a-658b 368c 1.1a-200c 0.7d-111a 3d-30a
mostly insoluble and as oil and gas are brought to the surface, they remain in the underground reservoir. As the natural pressure within the bearing formation falls, formation water present in the reservoir will also be extracted with oil and gas. Some radium and radium compounds are slightly soluble in water and may become mobilized when this production water is brought to the surface. This explains the elevated levels of radium over uranium in the investigated samples. 226 Ra is generally present in scale in equilibrium with its progeny, but 228Ra is not. This is clearly observed in the investigated samples presented in Table 2. Sample WDS1 shows the disequilibrium among the 232Th series, where 228Ac is the only daughter present in the series. The highest 226Ra and 228Ra activities were found in the RSS soil sample where the activities among the series ranged between 195 and 438 kBq/kg and 500 and 987 kBq/kg, respectively.
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S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
Table 4 Specific activity of U, 226Ra, U/Ra ratio and the total a–b counting in NORM wastes from petroleum origin (sample nomenclature is explained in Table 2) Sample
226
Ra (Bq/l)
Total uranium conc. ppb
RSW WDW1 WDW2 WDS1 WDS2 WDS3 RSS
5 1.1 40 2 38 3 226 Ra (Bq/kg) 7541 126 143262 2123 8005 143 194489 441
U/Ra
Total a
Total b
mBq/l
24.50 21.8 21.4 ppm 4.2 0.6 3.22 0.3 4.3 0.6 1.9 0.4
304 270 265 Bq/kg 49 1.9 40 3 53 6 24 4
6.1 10 6.8 10 7 10
2
6.5 10 3 10 6.6 10 1.2 10
3
} } }
3 3
} } }
92 4 187 2 84 2.2 610 2
4 3 4
214 2 486 32 214 1 1097 25
Table 5 Calculated enrichment factor (%) of radioactivity in soil samples (LDL lower than detection limit) Sample
Total U
226
214
214
228
212
208
40
WDS1 WDS2 WDS3 RSS
1.8 1.5 1.98 0.79
1.89 37.7 2.1 389
6.7 161 7.9 5475
6.6 178 7.8 3342
6.0 120 6.4 8162
LDL 461 13.8 7150
LDL LDL 24.6 LDL
1.00 10.7 0.8 LDL
Ra
Pb
Although the highest activity level found in the investigated samples exceeded the exempt activity concentration for 226Ra and 228Ra by a factor of 19.4 and 98.4, respectively, the levels are still within those given for the extra-active industries. The implications of the activity concentrations usually found in NORM wastes from the petroleum industry to the alpha radiation origin of petroleum were recently examined (Paschoa, 1997). The total uranium content, U/226Ra ratio and the total a–b counting of the investigated samples are given in Table 4. The disequilibrium factor for U/Ra went up to 6.8 10 3 in water samples and to 1.2 10 4 in soil samples, while some reported that disequilibrium factors have reached 1 10 5 (Snavely, 1989). The levels of radioactivity in production water were due to enhanced levels of dissolved radium ions, while only traces of uranium were present. Sample DWS2 and RSS show the highest total a–b counting in addition to their elevated differentiated U/Ra and 232Th series gamma-ray counting (Table 2). It has been reported that some connate waters showed relatively high concentrations of 210Pb (22y) in the absence of high 226Ra concentration, which suggests that a selective lead transport occurs (Hartog et al., 1995). However, for radiation protection and dose assessment purposes such characterization is required.
Bi
Ac
Bi
Tl
K
The long-term discharge of production water into the soil led to this enhanced level of natural radioactivity. The calculated enrichment factors of radioactivity in soil samples based on the activity in water are presented as percentages in Table 5. These factors ranged from 0.97% up to 8162% for total uranium and 228Ac, respectively. Sample RSS and WDS2 demonstrate the pronounced accumulation of radioactivity due to continuous discharge of production water, whereas WDS1 and WDS3 show comparable enrichment factors among both U238 and Th232 series. Although this study was not based on systematic sampling, randomly selected samples provided an overview reflecting the range of NORM produced by the oil industry. Each particular formation will produce NORM wastes with different concentrations of uranium and thorium daughters, depending not only on the extraction procedure but also on the initial concentrations and chemical form of the naturally occurring radioactive materials in the mineral matrices. Thus, the initial licensing for oil exploration should include estimates of the volume of NORM wastes to be produced as well as their concentrations and the techniques to be used to treat and/or manage such wastes. National guidelines concerning the potential environmental implications of NORM wastes have to be evaluated on a case by case basis.
S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
Acknowledgements The authors would like to express their sincere gratitude to Prof. Dr. Gaber Hassib for his valuable scientific comments, fruitful discussion and for providing the samples. The authors are also thankful to Prof. Dr. Hosnia Abu-Zeid Phys. Dept., Women’s College, Ain Shams University for performing the gamma-ray analysis.
References Bloch, S., Key, R.M., 1981. Am. Assoc. Petr. Geol. Bull. 65, 154–159. Hartog, F.A., Knaepen, W.A.I., Jonkers, G., 1995. Radioactive lead: An Underestimated Issue?, Proceedings of the API/GRI NORM Conference, Houston, USA, p. 59–69. Heaton, B., Lambley, J., 1995. Tenorm in oil, gas and mineral mining. App. Radiat. Isot. 46, 577–581. IAEA, 1988. Principles for the exemption of radiation and practices from regulatory control, Safety Series No.89, IAEA, Vienna. IAEA, 1994. International basic safety standards for the protection against ionizing radiation and for the safety of radiation sources, GOV/2715/94, IAEA, Vienna. Ibrahiem, N.M., Abd El-Maksoud, T.M., El-Ezaby, B., Nada, A., 1999. International Symposium of the Restoration
139
of Environments with Radiation Residues, Arlington, VA, USA, 29 November–3 December. Kolb, W.A., Wojciki, M., 1985. Enhanced radioactivity due to natural oil and gas production and related radiological problems. Sci. Total Environ. 45, 77–84. NCRP, 1975. Background radiation in USA, Report No. 45. Paschoa, A.S., 1993. Overview of environmental and waste management aspects of the monazite cycle. Radiat. Protect. Aust. 11, 170–173. Paschoa, A.S., 1997. Naturally occurring radioactive materials (NORM) and petroleum origin. Appl. Radiat. Isot. 48, 1391–1396. Rood, A.S., 1996. Measurement of 222Rn flux, 222Rn emanation and 226Ra concentration from injection well pipe scale. 29th Midyear Topical Meeting of the Health Physics Society, Scottsdale, AZ, United States, 7–10 January OSTI as DE96007275, p. 10. Shawky, S., Ibrahiem, N., Farouk, A., Ghods, A., 1994. Appl. Radiat. Isot. 45 (11), 1079. Snavely, E.S., 1989. Radionuclides in produced water (a literature review), Report of the American Petroleum Institute, Dallas, TX, USA. Spezzano, P., 1993. Determination of alpha emitting radionuclides of uranium, thorium and radium in zircon sands, Radiat. Protect. Aust. 11, 117–121. Veselsky, J.C., Kwiecnska, B., Wehrstien, E., Suschy, O., 1988. Analyst 113, 451. Wollenberg, H.A., Smith, A.R., 1990. A geochemical assessment of terrestrial g-ray absorbed dose rates. Health Phys. 58, 183–189.
Characteristics of NORM in the oil industry from Eastern and Western deserts of Egypt S. Shawkya,*, H. Amera, A.A. Nadab, T.M. Abd El-Maksoudb, N.M. Ibrahiema a
National Center for Nuclear Safety and Radiation Protection, P.O. Box 7551, Nasr City, 11762 Cairo, Egypt b Physics Department, Women’s College, Ain Shams University, Heliopolis, Cairo, Egypt Received 2 October 2000; accepted 13 October 2000
Abstract Naturally occurring radionuclides (NORs) from the 232Th- and 238U-series, which are omnipresent in the earth’s crust, can be concentrated by technical activities, particularly those involving natural resources. Although, a great deal of work has been done in the field of radiation protection and remedial action on uranium and other mines, recent concern has been devoted to the hazard arising from naturally occurring radioactive materials (NORM) in oil and gas facilities. NORM wastes associated with oil and gas operations from scale deposits, separated sludge and water at different oil fields in the eastern and western deserts were investigated. Concentrations of the uranium, thorium, and potassium (40K) series have been determined from high-resolution gamma-ray spectrometry. Total uranium content of samples was determined using laser fluorimetry. The levels of radioactivity were mainly due to enhanced levels of dissolved radium ions. Only minute quantities of uranium and thorium were present. The disequilibrium factor for 238 U/226Ra has been determined. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The presence of naturally occurring radioactive material (NORM) has been recognized since early 1930s in petroleum reservoirs, in oil and gas production, and in processing facilities. NORM was typically observed in barite scale that accumulated on the interior of oil production tubing and in storage tanks and heater-treated separation sludge. Recently, concern has been expressed over the health impacts from the uncontrolled release of NORM (Rood, 1996). Trace quantities of the radioactive elements 238U and 232Th as well as 40K have been present in the earth’s crust since its formation. Both 238U and 232Th are parents of a complex series of successive decays, producing many radioactive daughters. Some of these daughters may be co-produced with oil/gas well fluids and may concentrate in ordinary deposits (e.g. scale, sludge) and/or
*Corresponding author. Tel.: 2740236-7-9; fax: 2740238. E-mail address: [email protected] (S. Shawky).
leached in production water. There are several potential exposure pathways to humans from oilfield NORM, such as inhalation of radon gas. From the radiological point of view, the only important isotope of radium is 226 Ra which has an average concentration in the earth’s crust of about 10 12 g/g (40 Bq/kg).
1.1. Geological origin U and Th are present in the earth’s crust at an average concentration of 4.2 and 12.5 ppm, respectively (Wollenberg and Smith, 1990). When a geological formation containing 238U and 232Th has not been disturbed (closed system) for more that a million years, the members of the individual decay series will have the same activity (Bq/kg) which is known as secular equilibrium. However, when the geological formation is not closed to radionuclide migration, 226Ra can migrate and be deposited somewhere outside the formation. Then secular equilibrium will not exist and the growth of 226 Ra by radioactive decay of its ancestors will not
0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 4 3 ( 0 0 ) 0 0 3 6 4 - X
136 Table 1 Average
S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
226
Ra,
238
Rock type
U,
232
Th and
a
K content (mBq/g)
226
238
Ra
(mBq/g) Sandstone Shales Limestone
40
26.3 39.9 15.5
232
U
(ppm) 0.12 0.18 0.07
(mBq/g) (a)
50 ,14.8 70(a),14.8 160(a),14.8
40
Th
(ppm) (a)
4.1 ,1.2 5.9(a),1.2 13(a),1.2
(mBq/g) (a)
40 ,24 70(a),41 10(a),5.2
K
(ppm) (a)
9.7 ,5.9 16.3(a),10 3(a),1.6
(mBq/g) (a)
700 ,325 1100(a),814 200(a),81.4
(%) 2.1(a),0.98 3.5(a),2.6 0.5(a),0.2
Mean values are given by Bloch and Key (1981), otherwise the values reported are give by NCRP (1975).
occur. 226Ra is said to be unsupported. Table 1 shows the average radium, uranium, thorium and potassium content in sedimentary rocks (NCRP, 1975). Oil is formed by thermal cracking of organic matter (kerogen) trapped in sedimentary rock. As oil migrates, naturally occurring radionuclides (NORs) may be taken up in the upflowing fluid stream and may be transferred into connate waters, which may be produced with oil as production water. During thermal cracking of kerogen, uranium or thorium remain with the residual organic matter and they will not be leached in a reducing environment into passing fluids. Radium will not leach into a hydrocarbon phase, but it may leach into the aqueous phase (Bloch and Key, 1981). In this work, NORs in scale/sludge originating from oil fields in the western desert and the Red Sea region have been characterized and compared with the exempt concentration levels given by the IAEA (IAEA, 1988, 1994).
in the same type and configuration was used to determine the absolute efficiency curve. For the 238U series peaks with gamma-ray energies of 63.3 keV (3.8%) 234 TH, 186.1 keV (3.3%) 226Ra (after the subtraction of the 185.7 keV 53% of 235U if present), 295.1 keV (19.2%), and 325.0 keV (37.1%) 214Pb, 609 keV (46.1%), 768.4 keV (5%), 934.0 keV (3.4%) and 1120 keV (15%) 214Bi were used to determine the concentrations of the assigned nuclides in the series. For the 232Th series peaks with gamma-ray energies of 463.1 keV (4.6%), 911.2 keV(29%) and 966.0 keV (2.3%) 228Ac, 727.3 keV (6.7%), 1620.7 keV (1.5%) 212 Bi and 583.0 keV (30.9%) 208Tl were used to determine the concentrations of the assigned nuclides in the series. The 1460.8 keV (10.7%) gamma-ray peak was used to determine the 40K concentration.
2.2. Low level alpha–beta counter 2. Experiment Three different techniques were applied to characterize the investigated samples; gamma-ray spectrometry, laser fluorimetry and total alpha–beta counting. 2.1. Gamma-Ray spectrometry The collected samples were individually weighed, dried at 1008C and pulverized. The homogenized samples were transferred to 750 or 500 ml Marinelli beakers as well as 100 ml polyethylene containers for gamma-ray measurements and sealed for 8 weeks to ensure secular equilibrium between radium and its radioactive progenies. A high-resolution gamma-ray spectrometer based on an hyper pure germanium detector (HpGe) from EG&G Ortec was used for the gamma-ray analysis. The HpGe crystal has a diameter of 64.5 mm and a 69.9 mm length, a relative efficiency of 50%, peak to Compton ratio of 62, and FWHM of 1.9 keV at the 1.33 MeV 60Co transition and 753 eV at the 122 keV 57Co tansition. The efficiency calibration for the system has been explained in a recent publication (Ibrahiem et al., 1999). In addition, a 226Ra (No3)2 solution in HNO3,
In screening and contamination measurements, it may not be necessary to know what an isotope is, but only that contamination is present and that cleanup is required. Low background counting provides a means to accomplish screening of samples for gross alpha and beta activity with relatively limited sample preparation, relatively good efficiency and exceptionally low background. A Berthold LB 770 10 channel low-level counter was used. The detector is a gas-flow proportional counter for measuring both alpha and beta activity of the sample. The sample detector features an ultra-thin entrance window that contains the P-10 counting gas (a mixture of argon and methane) within the detector chamber and offers minimum attenuation to particles of interest that pass through the window into the detector chamber. The detector is biased with a high voltage to collect the charge generated by the interaction of the radioactivity with the counting gas. The detector is surrounded by lead to shield it from the naturally occurring background radioactivity. While the lead shield is effective against naturally occurring background radiation, the attenuation is insufficient for shielding from high-energy cosmic radiation. A second gas-flow proportional detector is used to guard the detector from high-energy cosmic radiation.
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S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
Table 2 Concentration levels of NORM from oil-producing fields in Bq/kg dry weight (WD western desert, W water, S soil, RSW Red Sea water, RSS Red Sea soil, LDL lower than the detection limits) 226
232
Ra–U series
226
Ra
WDW1 WDW2 RSW Sludge WDS1 WDS2 WDS3 RSS
40 2 38 3 51 18,032 78 7541 126 143,262 2213 8005 143 194,489 441
40
Th–series
214
Pb
27 0.5 20 1 0.8 0.2 19,394 19 18,215 103 322,604 1311 18,785 113 437,960 2300
214
Bi
27 2 18 2 1.3 0.6 18,324 50 17,929 325 320,008 3460 17,627 89 434,435 659
228
Ac
59 2 55 3 1.1 0.34 13,257 70 35,460 164 661,328 4385 36,727 170 897,803 948
212
K
208
Bi
Tl
12 1 82 0.7 0.5 7398 56 LDL 368,654 4556 13,779 297 500,476 707
4 0.3 31 1.1 0.2 5105 14 LDL LDL 8615 82 LDL
29 3 43 2 19 2 1261 29 2914 138 45,882 2022 3013 152 LDL
Two grams of dried samples were leached in concentrated nitric and hydrochloric acids overnight at room temperature. Samples were centrifuged. The acid mixture was decanted in a pre-weighed planchet and the solution was evaporated to dryness.
Table 3 Exempt activity concentrations for the most relevant radionuclides in the case of NORM releases as established by the BSS (6) and activities found in extra-active industries. (a, Spezzano, 1993; b, Heaton and Lambley, 1995; c, Kolb and Wojciki, 1985; d, Paschoa, 1993)
2.3. Laser fluorimetry for uranium analysis
Radionuclide
The measurements were performed by laser fluorimetry (Sintrix UA-3) based on the fluorescence of a uranyl complex that was formed by the addition of a buffered inorganic complexing reagent FLURAN to the sample during analysis. Unlike conventional fluorimetry, the excitation source is UV light provided by a nitrogen laser tube, which emits an UV pulse (3371 A˚). The limit of detection of uranium in water is 0.05 ng/g and the precision is about 15% at the ng/g level (Veselsky et al., 1988). Samples (1 g) were dried and ashed at 6008C for 3 h. The residue, after fuming with nitric and perchloric acid, is taken up in a Ca(NO3)2 solution and the uranium is extracted with methyl isobutylketon (MIBK). After stripping uranium from the MIBK solution into 0.001 M nitric acid, it was measured by laser fluorimeter. More details are given by Shawky et al. (1994).
3. Results and discussion Table 2 shows the concentration levels of isotopes in the uranium–radium series, thorium series and potassium in Bq/kg dry weight. The activity levels for all investigated radionuclides ranged from natural background levels up to levels exceeding the exempt activity concentrations for NORM releases as established in the basic safety standards given by the IAEA (1994). A summary of the exempt activity concentrations for the most relevant radionuclides (plus their progeny) is given in Table 3. Uranium and thorium compounds are
Activity (kBq/kg) Exempt
222
Rn Ra 226 Ra 228 Ra 228 Th Th-natural 230 U U-natural 224
10 10 10 10 1 1 10 1
Extra-active industries
2.7a-658b 368c 1.1a-200c 0.7d-111a 3d-30a
mostly insoluble and as oil and gas are brought to the surface, they remain in the underground reservoir. As the natural pressure within the bearing formation falls, formation water present in the reservoir will also be extracted with oil and gas. Some radium and radium compounds are slightly soluble in water and may become mobilized when this production water is brought to the surface. This explains the elevated levels of radium over uranium in the investigated samples. 226 Ra is generally present in scale in equilibrium with its progeny, but 228Ra is not. This is clearly observed in the investigated samples presented in Table 2. Sample WDS1 shows the disequilibrium among the 232Th series, where 228Ac is the only daughter present in the series. The highest 226Ra and 228Ra activities were found in the RSS soil sample where the activities among the series ranged between 195 and 438 kBq/kg and 500 and 987 kBq/kg, respectively.
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S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
Table 4 Specific activity of U, 226Ra, U/Ra ratio and the total a–b counting in NORM wastes from petroleum origin (sample nomenclature is explained in Table 2) Sample
226
Ra (Bq/l)
Total uranium conc. ppb
RSW WDW1 WDW2 WDS1 WDS2 WDS3 RSS
5 1.1 40 2 38 3 226 Ra (Bq/kg) 7541 126 143262 2123 8005 143 194489 441
U/Ra
Total a
Total b
mBq/l
24.50 21.8 21.4 ppm 4.2 0.6 3.22 0.3 4.3 0.6 1.9 0.4
304 270 265 Bq/kg 49 1.9 40 3 53 6 24 4
6.1 10 6.8 10 7 10
2
6.5 10 3 10 6.6 10 1.2 10
3
} } }
3 3
} } }
92 4 187 2 84 2.2 610 2
4 3 4
214 2 486 32 214 1 1097 25
Table 5 Calculated enrichment factor (%) of radioactivity in soil samples (LDL lower than detection limit) Sample
Total U
226
214
214
228
212
208
40
WDS1 WDS2 WDS3 RSS
1.8 1.5 1.98 0.79
1.89 37.7 2.1 389
6.7 161 7.9 5475
6.6 178 7.8 3342
6.0 120 6.4 8162
LDL 461 13.8 7150
LDL LDL 24.6 LDL
1.00 10.7 0.8 LDL
Ra
Pb
Although the highest activity level found in the investigated samples exceeded the exempt activity concentration for 226Ra and 228Ra by a factor of 19.4 and 98.4, respectively, the levels are still within those given for the extra-active industries. The implications of the activity concentrations usually found in NORM wastes from the petroleum industry to the alpha radiation origin of petroleum were recently examined (Paschoa, 1997). The total uranium content, U/226Ra ratio and the total a–b counting of the investigated samples are given in Table 4. The disequilibrium factor for U/Ra went up to 6.8 10 3 in water samples and to 1.2 10 4 in soil samples, while some reported that disequilibrium factors have reached 1 10 5 (Snavely, 1989). The levels of radioactivity in production water were due to enhanced levels of dissolved radium ions, while only traces of uranium were present. Sample DWS2 and RSS show the highest total a–b counting in addition to their elevated differentiated U/Ra and 232Th series gamma-ray counting (Table 2). It has been reported that some connate waters showed relatively high concentrations of 210Pb (22y) in the absence of high 226Ra concentration, which suggests that a selective lead transport occurs (Hartog et al., 1995). However, for radiation protection and dose assessment purposes such characterization is required.
Bi
Ac
Bi
Tl
K
The long-term discharge of production water into the soil led to this enhanced level of natural radioactivity. The calculated enrichment factors of radioactivity in soil samples based on the activity in water are presented as percentages in Table 5. These factors ranged from 0.97% up to 8162% for total uranium and 228Ac, respectively. Sample RSS and WDS2 demonstrate the pronounced accumulation of radioactivity due to continuous discharge of production water, whereas WDS1 and WDS3 show comparable enrichment factors among both U238 and Th232 series. Although this study was not based on systematic sampling, randomly selected samples provided an overview reflecting the range of NORM produced by the oil industry. Each particular formation will produce NORM wastes with different concentrations of uranium and thorium daughters, depending not only on the extraction procedure but also on the initial concentrations and chemical form of the naturally occurring radioactive materials in the mineral matrices. Thus, the initial licensing for oil exploration should include estimates of the volume of NORM wastes to be produced as well as their concentrations and the techniques to be used to treat and/or manage such wastes. National guidelines concerning the potential environmental implications of NORM wastes have to be evaluated on a case by case basis.
S. Shawky et al. / Applied Radiation and Isotopes 55 (2001) 135–139
Acknowledgements The authors would like to express their sincere gratitude to Prof. Dr. Gaber Hassib for his valuable scientific comments, fruitful discussion and for providing the samples. The authors are also thankful to Prof. Dr. Hosnia Abu-Zeid Phys. Dept., Women’s College, Ain Shams University for performing the gamma-ray analysis.
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