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Characterization of NORM solid waste produced from the petroleum industry a
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Lina Al Attar , Wael Doubal , Jamal Al Abdullah , Hussam Khalily , Basem Abdul Ghani & Bassam Safia
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Department of Protection and Safety, Atomic Energy Commission of Syria, Damascus, P.O. Box 6091, Syria Accepted author version posted online: 31 Oct 2014.Published online: 26 Nov 2014.
Click for updates To cite this article: Lina Al Attar, Wael Doubal, Jamal Al Abdullah, Hussam Khalily, Basem Abdul Ghani & Bassam Safia (2015) Characterization of NORM solid waste produced from the petroleum industry, Environmental Technology, 36:9, 1104-1113, DOI: 10.1080/09593330.2014.982713 To link to this article: http://dx.doi.org/10.1080/09593330.2014.982713
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Environmental Technology, 2015 Vol. 36, No. 9, 1104–1113, http://dx.doi.org/10.1080/09593330.2014.982713
Characterization of NORM solid waste produced from the petroleum industry Lina Al Attar ∗ , Wael Doubal, Jamal Al Abdullah, Hussam Khalily, Basem Abdul Ghani and Bassam Safia Department of Protection and Safety, Atomic Energy Commission of Syria, Damascus, P.O. Box 6091, Syria
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(Received 28 July 2013; accepted 31 August 2014 ) The accumulation of scales in the production pipe lines is a common problem in the oil industry, reducing fluid flow and leading to costly remediation and disposal programmes. Thus, an accurate determination of the activity of the radionuclides in scale samples is essential for environmental protection. The present study focuses on the characterization of naturally occurring radioactive materials (NORM) in scales generated from the petroleum industry to develop a suitable NORM waste management plan. The activity concentrations of 226 Ra, 228 Ra and 210 Pb in 32 representative samples, collected from a number of drums at the NORM Decontamination Facility storage, were determined using gamma spectrometry. It was found that the highest concentrations were 2922, 254 and 1794 Bq g−1 for 226 Ra, 228 Ra and 210 Pb, respectively. A comparison to the reported worldwide values was made. Statistical approaches, namely Box plot, ANOVA and principal components analysis were applied on the total results. Maximal correlation was demonstrated by 226 Ra activity concentration and count per second (cps) to density ratio. To obtain an accurate characterization of the radionuclides studied in the scale samples, method validation of gamma measurement procedure was carried out, in which minimum detectable activity, repeatability, intermediate precision and assessment of uncertainty were the parameters investigated. The work is a forefront for the proper and safe disposal of such radioactive wastes. Keywords: scales; oil industry; characterization; NORM; gamma spectrometry
Introduction Oil is one of the main contributors to the national economy with a production of 300 × 103 barrels daily in 2010. There are six shared oil companies in Syria, with the major offshore platforms for exploration and production facilities located in the North East. During oil production, naturally occurring radioactive materials (NORM) from 232 Th and 238 U series can be concentrated and accumulated in the form of scale deposits, sludge and produced water. Radium, the predominant radionuclide, can either stay in produced water or co-precipitate with barium forming complex sulphate compounds, carbonates and silicates. 210 Pb is also found in considerable concentrations. Only minute quantities of uranium and thorium may exist in sludge and scales due to their relative insolubility.[1–5] Scale formation is due to solubility variations of alkaline earth metal–sulphates and carbonates and is associated with pH and temperature variations, pressure changes, evaporation in the gas extraction pipes and injection of incompatible water. The amount of precipitate is dependent on the physical–chemical characteristics of the water (formation or injection).[3,4,6–8] Scales deposit on the interior surface of the production components (such as pipes, filters, injection wellhead equipment, pumps and
*Corresponding author. Email: prscientifi[email protected] © 2014 Taylor & Francis
valves) and are mainly responsible for restricted oil extraction due to plugging perforations, clogging tubular and valves.[9,10] Scales are basically barium and strontium sulphates and calcium carbonates, in addition to radium compounds. The use of scale inhibitors reduces the formation of scales.[2,11,12] Hence, operating installations and equipment have to be frequently maintained and refurbished prior to re-use and should be decontaminated to avoid their classification as either radioactive waste or contaminated objects.[13] In this content, there has been a good deal of work done worldwide on the characterization and measurement of the produced solid radioactive wastes.[1,3,10,14] In Syria, the Atomic Energy Commission (AECS) in co-operation with the oil and gas industry have taken an action, in 1998, for the treatment of NORM waste. This enabled implementation of remediation projects for sludge and low-level 226 Ra-contaminated soil (land farming, steam separation and landfill) [9,15] in accordance with the Legislative Decree No. 64, Radiological and Nuclear Regulatory Office,[16] the application of reinjection water, in addition to constructing and commissioning the NORM Decontamination Facility (NDF), which in turn creates scale waste. To date, 60 t of scales has been generated as a consequence of decontaminating
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Environmental Technology the radioactive components using high-pressure water jetting process. They are kept in plastic drums and stored in an interim licensed storage at the NDF. The present work draws a baseline in radiochemical characterization of these solid wastes to develop a suitable NORM waste management plan. Gamma spectrometry is the main technique used for the determination of uranium and thorium progeny in scales. However, radio-characterization would be a challenging task for such samples that contain activities greatly exceeding the environmental levels, with complex matrix composition and high density. Thus, an extensive effort is made, herein, to obtain an accurate radiochemical characterization taking into account energy calibration, detection efficiency and corrections for selfabsorption. Further work will proceed to compare the results with those could be simulated using the X-com programme.[17]
Experimental Scale sampling and preparation In March 2011, a total of 32 scale samples were collected from a number of drums, at the NDF storage, i.e. 500 km to the northeast of Damascus (Figure 1). The drums studied were chosen according to their measured gamma dose rate (at contact, using Rados 110) so that the entire recoded range of the stored 300 drums, i.e. 1–700 μSv h−1 , was covered. Sampling was carried out using a pressurizedstainless steel Auger (inner diameter of 8 cm) along the centre of the drum. In some cases, duplicate or triplicate cores were taken from the same drum to assure the success of sampling process. After spreading the sampled core on a polyethylene sheet, 20 g from each 10-cm core was taken and mixed, forming a representative sample of final weight ca. 80–120 g. It is worth pointing out that the scales were of various physical properties (colour, density and hardness) due to the volatilization of hydrocarbon compounds upon storage. Each scale sample was double-bagged in plastic and tightly sealed to ensure secure containment. A dust mask, disposal gloves and coveralls, glasses, helmet and heavy boots were used as personal protection equipment (PPE) during the sampling work, to comply with the Health Safety Legislations in the oilfields. Workers external exposure dose (measured via. TLD films) throughout the entire study did not exceed the permissible limit, i.e. 20 mSv y−1 . After transportation to laboratory, the scale samples were air-dried, ground and homogenized using Turbula mixer (Basel/Schweiz) for 24 h, and placed in a sterilized Falcon Petri dish (Model 351006, USA). Next, each sample was sealed and stored for about 4 weeks, allowing establishment of secular equilibrium between 222 Rn and progeny, prior to counting via HPGe gamma spectrometry.
Laboratory gamma spectrometry measurements of scale samples Activity concentrations of 210 Pb at 46.5 keV (4.25%) and 226 Ra through its progeny, namely 214 Pb at 295 (18.15) and 351 keV (35.9%) and 214 Bi at 1764 keV (15.1%), and 228 Ra through 228 Ac at 338.6 (11.27%) and 911 keV (25.8%) in the scale samples were determined using NType HPGe detector (Bruker company, with relative efficiency of 60% and FWHM of 0.89 and 2.0 keV at 122 and 1332.5 keV, respectively). Estimation of the activity concentration of the studied radionuclides was performed, with an acquisition live time of 1000 s, for each sample, using the following equation [18]: AEi =
NEi εE γ t m K1 K2 Ki
(1)
where AEi is the activity concentration (Bq g−1 ) of the nuclide i at energy E, N Ei is the net count in the full energy E peak, εE is the detector efficiency determined at the peak energy (gamma line) E, γ is the gamma line branching ratio (intensity) at energy E (%), t is the counting live-time (in second), m is the sample mass (g) and K 1 , K 2 and K i are correction factors for self-attenuation, nuclide decay time and dead time, respectively, etc. The spectra obtained were analysed by InterWinner7 PC software (ITECH Instruments, 2011). Measured activity concentrations of the nuclides studied were then expressed as weighted means.[19] Self-absorption correction was made for the scale samples that were found to have densities in the range of 1.0–3.1 g cm−3 , and complex matrices according to Kitto,[20] in which a point source of 152 Eu (reference date 1 July 1984) provided by the International Atomic Energy Agency, IAEA, with an activity of 376 kBq was used for the transmission measurements. Background correction was made by counting for 100,000 s. Depending on the preliminary counting activity (total cps) of the scale samples, it was necessary to place the active samples away from the face of the detector to achieve dead time < 1%. This, in turn, resulted in three counting distances, ca. 0, 5 and 10 cm. Efficiency calibration for the three practically determined counting levels, detector–sample distances (0, 5 and 10 cm) and energies, was made using CRM standard solution QCYB40, obtained from AEA Technology, UK, containing 210 Pb and 241 Am (reference date 1 January 2004), in addition to the use of CRM QCY48 solution, supplied by AEA Technology, UK, containing 57 Co, 60 Co, 85 Sr, 88 Y, 109 Cd, 113 Sn, 137 Cs, 139 Ce, 203 Hg and 241 Am (reference date 1 December 2008) with sufficient gamma ray lines lie along the working energy range (i.e. 59.5–1836 keV). Efficiency calibration was verified using RGU-1 (supplied by the IAEA) and the in-house NORM contaminated soil sample from the oilfields, designated as SYR-NORM-2005.[21]
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Figure 1. The location of NORM decontamination facility (NDF) in Deir Ezzor city, in Syria.
Minimum detectable activity, precision and uncertainty of gamma measurements Method parameters used in this study were minimum detection activity, repeatability, intermediate precision and measurement uncertainty.[22] Minimum detectable activity (MDA) for the nuclides of interest was derived from background measurements, at the three sample–detector distances, and automatically calculated via InterWinner7 PC software according to ISO 11929.[23] The two most common precision measures are repeatability and reproducibility (the so-called intermediate precision). They were determined, herein, for the radionuclides studied at the three counting levels and expressed in terms of relative standard deviations (RSDr and RSDR , respectively) according to Eurachem.[22] For better estimation of repeatability, a number of chosen samples within the same counting level (sample–detector distance) was counted, each for ten times on one piece of equipment by the same analyst and over a short timescale. Intermediate precision was performed by different analysts over extended timescales within a single laboratory. Assessment of uncertainty was established by counting a chosen sample at 5 cm detector distance, since it was the suitable geometry for counting the majority of scale samples in this work. With reference to the IAEA,[18] combined standard uncertainty (σ Ai ) in determining the activity concentration, of a given radionuclide,
by gamma spectroscopy could be derived using the following equation: σ A i = Ai
σN N
2
+
σ εi εi
2
+
σ γi γi
2
+
σt t
2 +
σ m 2 m
.
(2)
Results and discussion Efficient calibration To obtain a representative efficiency for the physical measurements, it was necessary to calibrate the detector with regard to the three counting levels considered in this study, i.e. sample at the detector face (0 cm) and 5 and 10 cm away from the detector. Experimental efficiency calibration curves are shown in Figure 2. It could be seen that the efficiency decreased, at a specific gamma-energy peak, with increasing sample–detector distance. Moreover, the calibration curves presented a maximum at low range of gamma energy; thus, care should be taken when correcting for low energy lines, such as 210 Pb (at 46.5 keV). Laboratory gamma spectrometry measurements of scale samples As stated in the Experimental Section, the activity concentrations of radium (226 Ra and 228 Ra) and 210 Pb in the
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respectively, distributed. In the case of 228 Ra, the majority of scale samples (90.6%) had activity concentrations < 150 Bq g−1 , with 9.4% of samples falling within 151– 450 Bq g−1 . A quite different pattern was observed for the activity distribution of 210 Pb. One half of the samples had activity concentration < 450 Bq g−1 , whereas the other 50% were in the activity concentration range of 450– 1900 Bq g−1 . Table 1 shows the minimum, maximum and mean values of the activity concentrations of the radionuclides investigated in the scale samples with regard to the counting levels of gamma-measurements taking into account dose rate and the ratio of cps/density. An increase of one order of magnitude was recorded in the mean activity concentration of 226 Ra with elevation of cps/density, reaching 2577 Bq g−1 at the ratio 4642. Conversely, hardly any variation was observed in the mean concentration of 228 Ra at 5 and 10 cm distance (ca. 94 and 85 Bq g−1 , respectively), which could be related to the low of counting efficiency as a consequence of the sample distance from detector, taking into account the low content of the radionuclide in the samples. The mean concentration of 210 Pb was essentially doubled in the practical counting levels used (i.e. in 5 and 10 cm). Others [11] determined radium content in scale samples collected, using a scraper, from contaminated equipment and found that the maximal concentrations are 1520 Bq g−1 (for 226 Ra) and 868 Bq g−1 (for 228 Ra). The variation of data in comparison to the results of this study could be related to the source of scale samples and method of measurement. The results obtained here were statistically evaluated using the so-called explanatory data analysis method; one example is the Box Plot used herein (Figure 4). Generally, the median value was more representative than the mean as the former is less affected by outliers. Clearly, the variation of the activity concentrations of 226 Ra and 210 Pb in the scale samples was high, since median values drifted away from the means; however, less variation was observed for 228 Ra, which in turn reflected its low concentration. The median values were 393, 56 and 503 Bq g−1 for 226 Ra, 228 Ra and 210 Pb, respectively. The distribution of activity concentration ranges, described above (see Figure 3), may reflect the obtained medians, with the fact that about 56% of scales had 226 Ra content ranging from 0 to 450 Bq g−1 .
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Figure 2. Experimental data of the variation of the detector efficiency with different γ -energy lines for three sample–detector distances (0, 5 and 10 cm).
Figure 3. Distribution of the activity concentrations of 226 Ra, 228 Ra and 210 Pb in the scale samples that produced from the petroleum industry.
scale samples produced from the oil industry were determined using transmission measurement and expressed as weighted means (Bq g−1 ). The distribution of the activity concentrations of the radionuclides investigated in the scales, regardless of form and source, is shown in Figure 3. The data presented illustrated that 31.3% of the scale samples had activity concentrations of 226 Ra in the range of 0–50 Bq g−1 and 25% were within the range 151–450 Bq g−1 . At higher ranges (451–1900 and 1901–2925 Bq g−1 ), 28% and 15.6% of samples were,
Table 1. Minimum, maximum and mean values of cps/density, dose rate and the concentrations (in Bq g−1 ) of the radionuclides studied in the scale samples, according to the counting levels of gamma measurements. 0 cm Counting level
Min
Max
cps/density Dose rate (μSv h−1 ) 226 Ra 228 Ra 210 Pb
8 1 2 0.3 < MDA
159 11 37 3 349
5 cm Mean
Min
Max
± ± ± ± ±
406 80 184 17 202
3858 600 1584 254 1478
71 6 20 2 175
38 3 10 1 145
10 cm Mean
Min
Max
± ± ± ± ±
3924 440 2341 79 1108
6117 700 2922 94 1794
1747 307 771 94 696
1116 172 527 72 435
Mean 4642 588 2577 85 1462
± ± ± ± ±
912 93 308 6 311
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Figure 4. Box plot of the activity concentrations (Bq g−1 ) of 226 Ra, 228 Ra and 210 Pb in the scale samples.
Gazineu and Hazin [1] reported the median values of 897 Bq g−1 (for 226 Ra) and 680 Bq g−1 (for 228 Ra) when estimating radium contents in scales produced from Brazilian oil industry, although they investigated limited number of samples (i.e. 12). The high order of uncertainty of the activity concentrations of the radionuclide studied in the scale samples herein was in agreement with that noted in the literature.[1,2,11] Generally, statistical approaches are essential tools to illustrate the correlation among variables studied. Principal component analysis (PCA) and analysis of variance (ANOVA) were carried out for the 32 scale samples using XLStat software.[24] The variables investigated were the activity concentrations of 226 Ra, 228 Ra, 210 Pb, dose rate and cps/density. The latter was used to account for the matrix effect resulting from the complex chemical composition of the samples. Data are represented as a correlation circle (on axes F1–F2) (Figure 5) and plots of variance are shown in Figure 6. Correlation coefficients are summarized in Pearson table (Table 2). Looking at Figure 5 shows that the applied projection explained 92.86% of variability, with τ 1 = 81.82% and τ 2 = 11.04%. Intercorrelation of the four variables (i.e. 226 Ra, 210 Pb, cps/density and dose rate) is clearly seen. However, low correlation was noted in the case of 228 Ra with the rest of variables, which could mainly be related to the low concentration of the radionuclide in the samples. Since both 226 Ra and dose rate are far from the centre and close to cps/density, they show a significant positive correlation to cps/density with correlation coefficients of 0.983 and 0.908, respectively (Table 2). The maximal correlation demonstrated by 226 Ra with cps/density, in comparison to 228 Ra (0.658) and 210 Pb (0.812), could be a reflection of the high concentrations of the former radionuclide in the samples. A considerable correlation was observed between 226 Ra and dose rate (r = 0.885), which may mirror the high
Figure 5. Correlation circle (on axes F1–F2) of the activity concentrations of 226 Ra, 228 Ra, 210 Pb as well as dose rate and cps/density in the scale samples.
peak energies of gamma lines that are detectable by the dosimeter used. A similar scenario could be applicable in the case of 228 Ra and dose rate, with correlation coefficient of 0.751. Plots of variance (Figure 6) also showed linear proportion between cps/density and the main components studied (i.e. concentration of 226 Ra, 228 Ra, 210 Pb and dose rate), with the maximum correlation achieved between cps/density and the activity concentration of 226 Ra (R2 = 0.966). Linearity of the dose rate with cps/density gave a regression coefficient of 0.896. These results were compatible with those obtained, previously, by PCA and Pearson. A literature survey of radium contents in scales of petroleum industry in various countries is shown in Table 3. Scales generated from Riyadh city refinery tanks in Saudi Arabia [25] were found to contain the lowest concentrations, not exceeding 1.5 and 3.2 Bq kg−1 for 226 Ra and 228 Ra, respectively. The highest concentration of 226 Ra reported herein was somewhat, comparable to that observed in the Brazilian scales collected from oil exploration unit, i.e. 2110 Bq g−1 .[1] However, USEPA stated that the maximal activity concentration of 226 Ra in scales could reach up to 15,170 Bq g−1 and ca. 1000 Bq g−1 for 210 Pb.[26] The variation observed in radium concentration ranges (or means) within the same country (e.g. Egypt, Brazil or UK) reflected the differences of scale sources and measurement method used, as noted for Syria in the preceding section. Consequently, the results of this study were found to fit within the worldwide range, notwithstanding the fact that no barrel of the 32 had activity concentration below 1 Bq g−1 , which is the exemption/clearance level proposed by the European Union.[27] Considerable concern was, therefore, recognized by the national waste
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Environmental Technology (a)
(b)
(c)
(d)
Figure 6. ANOVA of cps/density of the scale samples with the main variables, 226 Ra (a), 228 Ra (b), 210 Pb (c) and dose rate (d). Table 2. samples.
Correlation matrix of Pearson ‘n’ of the studied parameters in the scale
Variables Dose rate cps/density 226 Ra 228 Ra 210 Pb
Dose rate
cps/density
226 Ra
228 Ra
210 Pb
1 0.908 0.885 0.751 0.767
1 0.983 0.658 0.812
1 0.549 0.802
1 0.542
1
Note: Values in bold are different from 0 with a significance level α = 0.001.
management authority to establish special treatment and safe disposal of such waste.
Method validation of gamma measurements Minimum detectable activity Determination of the minimum detection activity of the radionuclides of interest was performed using Currie formula at the three counting levels stated. The results are
shown in Table 4. Higher MDA values, determined at the detector face, for 210 Pb (ca. 65 Bq kg−1 ) were observed in comparison to those of 226 Ra and 228 Ra (i.e. 23 and 22 Bq kg−1 , respectively). The increase in MDAs was reasonable since Currie formula includes several parameters related to the nuclear counting measurement of the sample. These parameters are counting time, the counting of a blank, sample mass and counting efficiency of the system at the specific peak energy. Therefore, the high MDAs for 210 Pb could be due mainly to its low-energy peak and
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Worldwide values of 226 Ra and 228 Ra concentrations in scales produced from oil 226 Ra
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Country Egypt (Eastern bank of Suez gulf) Egypt (Abu Rudeis) Egypt (Gabal El Zeit) Egypt (Badr El Din) Egypt (Red Sea) Egypt (Western desert) KSA (Riyadh city refinery tanks) Australia Brazil Brazil (Sergipe and Alagoas) Malaysia USA USA (EPA) Norway Germany The Netherlands UK Algeria Tunisia Libya Syria Syria
(Bq g−1 )
493–519 68.9 14.8 31.4 195 59.2 (0.8–1.5) × 10−3 20–70 16.2–93.2 77.9–2110 0.5–434 (114.3)* (76.2)* 15,170 8–100 100–500 0–900 20–400 1–950 4.3–658 23–221 0.3–1520 (174)* 2–2922 (818)*
228 Ra
(Bq g−1 )
32–50 24 4.3 43.3 897.8 244.5 (0.1–3.1) × 10−3 – 4–36.9 101.5–1550 0.9–479 (130.1)* – – 5–30 40–200 0–400 20–300 – – 1.1–111 0.6–868 (91)* 0.3–254 (64)*
References [28] [29] [5] [25] [19] [14] [1] [12] [30] [26] [31]
[3] [32] [33] [11] This work
*Arithmetic mean is given in parentheses. Table 4. Determined MDAs (in Bq kg−1 ) for the radionuclides studied in accordance with the sample–detector distances. Sample–detector distance (cm) Sample mass (g) 226 Ra 228 Ra 210 Pb
0
5
10
6 8 10 23 89 192 22 74 145 65 261 635
Note: Detector efficiency 60%; counting time 100,000 s.
counting efficiency. In a similar manner, enhanced MDA values were observed with increasing sample–detector distance for a particular radionuclide. For instance, the MDA for 226 Ra was four times higher when sample–detector distance was raised to 5 cm than those determined at the detector face and was doubled when distance increased to 10 cm. This behaviour mirrored the relative decrease of counting efficiency with increasing sample–detector distance as shown previously in Figure 2. Previous studies [34,35] described the experience of the AECS in method validation for environmental radiochemical measurements. They stated that the MDA for 226 Ra determination was found to be 2 Bq kg−1 with sample mass of 160 g, detector efficiency 80% and counting time of 80,000 s. The MDA increased to reach 40 Bq kg−1 when sample mass ranged from 25 to 30 g under the same experimental conditions (counting efficiency and time). In the case of 210 Pb, the authors found that the MDA were within the range of 12–15 Bq kg−1 for soil samples of 40 g
weight using HPGe detector of 10% efficiency. In comparison to the MDAs determined herein, it may be concluded that the MDAs obtained for 226 Ra (23 Bq kg−1 ) and 210 Pb (65 Bq kg−1 ), at the detector face, were quite sensible for sample mass ranging from 6 to 10 g.
Gamma measurement precision Repeatability and reproducibility measurements were performed by choosing two, four and three samples for the 0 cm counting level (at the detector face), 5 and 10 cm sample–detector distances, respectively, to allow better estimation to be drawn. Because scales were enormously varied in chemical composition, density and radiochemical concentrations, samples were selected to cover the entire activity concentration range in each counting level. Tables 5 and 6 summarize the ranges of mean, standard deviation as well as repeatability and reproducibility relative standard deviation (RSDr and RSDR ) of the replicates. The data showed an improvement of the RSDr for 226 Ra with elevated sample–detector distance, reaching 0.4–0.6 at the maximal distance. More or less, constant values were observed for 228 Ra, giving a range of 5.7–8.4 at the highest counting level, which could be due to low radionuclide content in the scale samples compared with that of 226 Ra. Meaning that the high concentration of 226 Ra and its progeny caused an increase in the baseline of the sample spectrum, which in turn made the determination of 228 Ra quite difficult. As expected, a converse scenario was revealed for 210 Pb where RSDr increased when samples
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Environmental Technology Table 5. Ranges of mean, standard deviation and relative standard deviation of the repeatability measurements for 210 Pb with reference to the counting levels. Sample–detector distance
0 cm
5 cm
226 Ra, 228 Ra
and
10 cm
Nuclide
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
Meanr (Bq g−1 ) Sr (Bq g−1 ) RSDr
18–31 0.3–0.6 1.8–2.0
3–4 0.2–0.4 7.8–9.8
25–304 0.5–4.2 1.4–2.0
382–1414 3.7–5.1 0.3–1.3
35–97 1.8–6.3 3.3–8.4
304–1367 4.1–21.4 1.4–3.7
2338–2907 8.8–13.8 0.4–0.6
75–96 5.3–6.7 5.7–8.4
1118–1781 69.9–119.2 6.2–7.1
Table 6. Ranges of mean, standard deviation and relative standard deviation of the reproducibility measurements for 226 Ra, 228 Ra and 210 Pb with reference to the counting levels.
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Sample–detector distance
0 cm
5 cm
10 cm
Nuclide
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
MeanR (Bq g−1 ) SR (Bq g−1 ) RSDR
18–31 0.2–0.5 1.2–1.7
3–4 0.2 5.5–6.8
25–300 0.6–4.5 1.5–2.4
379–1424 2.8–9.2 0.6–1.4
33–100 1.6–3.5 2.2–9.0
302–1126 12.3–26.9 2.4–4.2
2319–2887 9.4–20.8 0.4–0.7
70–96 6.5–7.7 6.7–9.9
1015–1738 56.8–97.3 5.3–5.6
Table 7. Percentage of the contribution of the studied components to the total uncertainty, for the determination of the activity concentrations of 226 Ra, 228 Ra and 210 Pb in the scale samples. Radionuclide 226 Ra 228 Ra 210 Pb
Net count
γ line-intensity
Efficiency
Sample mass
Counting time
99.60 99.51 99.73
0.0007 0.0069 0.0004
0.36 0.31 0.24
0.03 0.10 0.02
0.02 0.08 0.01
were counted further away from the detector, which could be a consequence of increasing the attenuation of its peak energy. Reproducibility RSDR for the radionuclides of interest showed comparable trends to those reported for RSDr . For example, RSDr and RSDR for 226 Ra were 0.4–0.6 and 0.4–0.7 at the 10 cm sample–detector distance, so little variation could be seen within the parameters studied (analyst and time scale). This did not include 228 Ra, which was counted at the face of the detector because of its relatively low activity concentration (3–4 Bq g−1 ). Finally, the RSDr and RSDR obtained herein (at 10 cm counting level) were comparable to the values interpolated from the literature [34,35] for soil samples measured by gamma spectroscopy at the AECS, i.e. 0.6 and 0.7 for 226 Ra (at the detector face). The authors reported higher RSDr and RSDR for 210 Pb (2.2 and 2.6, respectively), for larger sample mass (ca. 30–40 g) than those used in this study (i.e. 5–10 g) giving maximal values of 2.0 and 2.4 (at the detector face), respectively. Uncertainty of gamma measurements Total uncertainty of gamma measurement of the studied radionuclides was calculated using Equation (2). Sample mass, counting time, gamma peak intensity, and efficiency and count rate were the main components of total uncertainty. Contribution of these components to
the total uncertainty was calculated using the Spreadsheet method.[36] The total uncertainty for 2σ of the activity concentrations 373, 51.5 and 193 Bq g−1 of 226 Ra, 228 Ra and 210 Pb, respectively, was found to be 7%, 4% and 9%, respectively. The uncertainty due to net counting was the highest contributor to the total uncertainty for the radionuclides studied, which exceeded 99% (see Table 7). Uncertainly of counting efficiency was the next contributor, ranging between 0.24 and 0.36%, whereas uncertainty related to sample mass was ≤ 0.1%. In consistence with the literature,[34] it was found that the relative uncertainty by gamma spectrometry increased linearly with decreasing activity concentration of the radionuclides studied. Linear coefficients were found, herein, to be 0.996, 0.999 and 0.616 for 226 Ra, 228 Ra and 210 Pb, respectively.
Conclusion The present work represents a preliminary investigation of the oilfield scales using a gamma spectrometry, to pave the way for establishing a suitable radioactive waste management plan. The maximum activity concentrations of 226 Ra, 228 Ra and 210 Pb were determined using HPGe detector and found to be 2922, 254 and 1794 Bq g−1 for 226 Ra, 228 Ra and 210 Pb, respectively. These results were within the
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literature stated values. Application of Box plots, ANOVA and PCA as statistical approaches was made, in which the medians were 393, 56 and 503 for 226 Ra, 228 Ra and 210 Pb, respectively. 226 Ra and cps/density showed the best correlation (with correlation coefficient of 0.983), while less correlation was observed for the other radionuclides. Since scale samples are known to have high densities, complex chemical compositions and high activities of radionuclides, method validation of gamma measurements was carried out, in which MDA, intermediate precision, repeatability and assessment of uncertainty were the parameters investigated. The MDAs were comparable to those reported in the gamma-ray spectroscopy laboratory at the AECS for soil samples. RSDr and RSDR were improved by increasing the sample–detector distance for 226 Ra, whereas an opposite effect was noted for 210 Pb. Finally, the approach used in this study could be applied for accurate radiochemical characterization of scale samples. Acknowledgements The authors thank Mr. W. Ahmad and the administrative team at the NDF for the assistance in sampling. The Director General of the Atomic Energy Commission of Syria is highly appreciated for his support throughout the work. Disclosure statement No potential conflict of interest was reported by the author(s). References [1] Gazineu MHP, Hazin CA. Radium and potassium-40 in solid wastes from the oil industry. Appl Radiat Isotopes. 2008;66:90–94. [2] Gazineu MHP, de Araújo AA, Brandão YB, Hazin CA, de Godoy OJM. Radioactivity concentration in liquid and solid phases of scale and sludge generated in the petroleum industry. J Environ Radioactiv. 2005;81:47–54. [3] Hamlat MS, Djeffal S, Kadi H. Assessment of radiation exposures from naturally occurring radioactive materials in the oil and gas industry. Appl Radiat Isotopes. 2001;55:141– 146. [4] OGP. Guidelines for the management of naturally occurring radioactive material (NORM) in the oil & gas industry – OGP. England and Wales: The International Association of Oil & Gas Publications; 2008. [5] Shawky S, Amer H, Nada AA, Abd El-Maksoud TM, Ibrahiem NM. Characteristics of NORM in the oil industry from Eastern and Western deserts of Egypt. Appl Radiat Isotopes. 2001;55:135–139. [6] IAEA. Extent of environmental contamination by naturally occurring radioactive material (NORM) and technological options for mitigation. Technical Reports Series No. 419. IAEA, Vienna, 2003. [7] Smith KP. An overview of naturally occurring radioactive materials (NORM) in the petroleum industry. Illinois: Department of Energy; 2008. [8] IAEA. Radiation protection and the management of radioactive waste in the oil and gas industry. Safety Reports Series, No. 49. Vienna, Austria: IAEA; 2004.
[9] Al-Masri MS, Suman H. NORM waste management in the oil and gas industry: the Syrian experience. J Radioanal Nucl Chem. 2003;256:159–162. [10] El-Hattab MI. Scale deposition in surface and subsurface production equipment in the Gulf of Suez. J Petrol Technol. 1985;37:1640–1652. [11] Al-Masri MS, Aba A. Distribution of scales containing NORM in different oilfields equipment. Appl Radiat Isotopes. 2005;63:457–463. [12] Omar M, Ali HM, Abu MP, Kontol KM, Ahmad Z, Ahmad SHSS, Sulaiman I, Hamzah R. Distribution of radium in oil and gas industry wastes from Malaysia. Appl Radiat Isotopes. 2004;60:779–782. [13] Strand T, Lysebo I. NORM in oil and gas production waste management and disposal alternatives. International Symposium of the Treatment of Naturally Occurring Radioactive Materials, Krefeld, Germany; 1998. [14] Matta EL, Godoy MJ, Reis MC. 226 Ra, 228 Ra and 228 Th in scale and sludge samples from the Campos Basin Oilfield E&P Activities. Radiat Prot Dosim. 2002;102:175–178. [15] Othman I, Al-Masri MS. Disposal strategy for NORM waste generated by the Syrian oil industry. International Symposium on Disposal of Low Activity Radioactive Waste, Cordoba, Spain; 2004. [16] Radiological and Nuclear Regulatory Office (AECS, 2005). Legislative Decree, No. 64. Issued by the Prime Minister, Syria; 2007. [17] Berger MJ, Hubbell JH, Seltzer SM, Chang J, Coursey JS, Sukumar R, Zucker DS, Olsen K. XCOM: photon cross sections database [cited 2013 March 28]. Available from: http://www.nist.gov/pml/data/xcom/index.cfm, 1990. [18] IAEA. Quantifying uncertainty in nuclear analytical measurements, Technical Document No. 1401, IAEA, Vienna; 2004. [19] Debertin K, Helmer RG. Gamma- and X-ray spectrometry with semiconductor detectors. Amsterdam: North Holland; 1988. [20] Kitto ME. Determination of photon self-absorption corrections for soil samples. Appl Radiat Isotopes. 1991;42:835– 839. [21] Al-Masri MS, Aba A, Al-Hamwi A, Shakhashiro A. Preparation of in-house reference soil sample containing high levels of naturally occurring radioactive materials from the oil industry. Appl Radiat Isotopes. 2004;61:1397–1402. [22] Eurachem. The fitness for purpose of analytical methods: a laboratory guide to method validation and related topics. 1st edn. Middlesex: LGC Teddington Ltd; 1998. [23] ISO. Determination of the characteristic limits (decision threshold, detection limit and limits of the confidence interval) for measurements of ionizing radiation: fundamentals and application, ISO/DIS 11929. Genova; International Standard Organisation; 2010. [24] Addinsoft. Principal component analysis-XLSTAT-Support [cited 2013 Feb 27]. Available from: http://www.xlstat.com/ en/, 2013. [25] Al-Saleh FS, Al-Harshan GA. Measurements of radiation level in petroleum products and wastes in Riyadh City Refinery. J Environ Radioactiv. 2008;99:1026–1031. [26] US-EPA. Oil and gas production wastes [cited 2013 Feb 27]. Available from: http://www.epa.gov/radiation/tenorm/oilan dgas.html, 2001. [27] IAEA. Radiation protection and safety of radiation sources: international basic safety standards. Interim edn. Vienna: IAEA; 2011. [28] Abo-Elmagd M, Soliman HA, Salman KA, El-Masry NM. Radiological hazards of TENORM in the wasted petroleum pipes. J Environ Radioactiv. 2010;101:51–54.
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[29] El Afifi EM, Awwad NS. Characterization of the TE-NORM waste associated with oil and natural gas production in Abu Rudeis, Egypt. J Environ Radioactiv. 2005;82:7–19. [30] White GJ, Rood AS. Radon emanation from NORMcontaminated pipe scale and soil at petroleum industry sites. J Environ Radioactiv. 2001;54:401–413. [31] Weers AW van, Butter KR, Lysebo I, Meijne EIM, Pace I, Sterker T, Strand T, Watkins S. Current practice of dealing with natural radioactivity from oil and gas production in EU Member States. Final Report. Luxembourg: European Communities; 1997. [32] Heaton B, Lambley J. TENORM in the oil, gas and mineral mining industry. Appl Radiat Isotopes. 1995;46: 577–581.
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[33] Habib AS, Shutt AL, Regan PH, Matthews MC, lsulaiti HA, Bradley DA. Characterization of naturally occurring radioactive materials in libyan oil pipe scale using a germanium detector and Monte Carlo simulation. Radiat Phys Chem. 2014;95:352–355. [34] Al-Masri MS, Hassan M, Amin Y. A comparison of two nuclear analytical techniques for determination of 210 Pbspecific activity in solid environmental samples. Accredit Qual Assur. 2010;15:163–170. [35] Al-Masri MS, Shakhashiro A, Amin Y. Method validation procedures for environmental radiochemical measurements at AECS. Accredit Qual Assur. 2004;9:361–368. [36] AECS. Uncertainty estimation (No. PM-09/1). Syria: Atomic Energy Commission of Syria; 2012.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Characterization of NORM solid waste produced from the petroleum industry a
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Lina Al Attar , Wael Doubal , Jamal Al Abdullah , Hussam Khalily , Basem Abdul Ghani & Bassam Safia
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Department of Protection and Safety, Atomic Energy Commission of Syria, Damascus, P.O. Box 6091, Syria Accepted author version posted online: 31 Oct 2014.Published online: 26 Nov 2014.
Click for updates To cite this article: Lina Al Attar, Wael Doubal, Jamal Al Abdullah, Hussam Khalily, Basem Abdul Ghani & Bassam Safia (2015) Characterization of NORM solid waste produced from the petroleum industry, Environmental Technology, 36:9, 1104-1113, DOI: 10.1080/09593330.2014.982713 To link to this article: http://dx.doi.org/10.1080/09593330.2014.982713
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Environmental Technology, 2015 Vol. 36, No. 9, 1104–1113, http://dx.doi.org/10.1080/09593330.2014.982713
Characterization of NORM solid waste produced from the petroleum industry Lina Al Attar ∗ , Wael Doubal, Jamal Al Abdullah, Hussam Khalily, Basem Abdul Ghani and Bassam Safia Department of Protection and Safety, Atomic Energy Commission of Syria, Damascus, P.O. Box 6091, Syria
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(Received 28 July 2013; accepted 31 August 2014 ) The accumulation of scales in the production pipe lines is a common problem in the oil industry, reducing fluid flow and leading to costly remediation and disposal programmes. Thus, an accurate determination of the activity of the radionuclides in scale samples is essential for environmental protection. The present study focuses on the characterization of naturally occurring radioactive materials (NORM) in scales generated from the petroleum industry to develop a suitable NORM waste management plan. The activity concentrations of 226 Ra, 228 Ra and 210 Pb in 32 representative samples, collected from a number of drums at the NORM Decontamination Facility storage, were determined using gamma spectrometry. It was found that the highest concentrations were 2922, 254 and 1794 Bq g−1 for 226 Ra, 228 Ra and 210 Pb, respectively. A comparison to the reported worldwide values was made. Statistical approaches, namely Box plot, ANOVA and principal components analysis were applied on the total results. Maximal correlation was demonstrated by 226 Ra activity concentration and count per second (cps) to density ratio. To obtain an accurate characterization of the radionuclides studied in the scale samples, method validation of gamma measurement procedure was carried out, in which minimum detectable activity, repeatability, intermediate precision and assessment of uncertainty were the parameters investigated. The work is a forefront for the proper and safe disposal of such radioactive wastes. Keywords: scales; oil industry; characterization; NORM; gamma spectrometry
Introduction Oil is one of the main contributors to the national economy with a production of 300 × 103 barrels daily in 2010. There are six shared oil companies in Syria, with the major offshore platforms for exploration and production facilities located in the North East. During oil production, naturally occurring radioactive materials (NORM) from 232 Th and 238 U series can be concentrated and accumulated in the form of scale deposits, sludge and produced water. Radium, the predominant radionuclide, can either stay in produced water or co-precipitate with barium forming complex sulphate compounds, carbonates and silicates. 210 Pb is also found in considerable concentrations. Only minute quantities of uranium and thorium may exist in sludge and scales due to their relative insolubility.[1–5] Scale formation is due to solubility variations of alkaline earth metal–sulphates and carbonates and is associated with pH and temperature variations, pressure changes, evaporation in the gas extraction pipes and injection of incompatible water. The amount of precipitate is dependent on the physical–chemical characteristics of the water (formation or injection).[3,4,6–8] Scales deposit on the interior surface of the production components (such as pipes, filters, injection wellhead equipment, pumps and
*Corresponding author. Email: prscientifi[email protected] © 2014 Taylor & Francis
valves) and are mainly responsible for restricted oil extraction due to plugging perforations, clogging tubular and valves.[9,10] Scales are basically barium and strontium sulphates and calcium carbonates, in addition to radium compounds. The use of scale inhibitors reduces the formation of scales.[2,11,12] Hence, operating installations and equipment have to be frequently maintained and refurbished prior to re-use and should be decontaminated to avoid their classification as either radioactive waste or contaminated objects.[13] In this content, there has been a good deal of work done worldwide on the characterization and measurement of the produced solid radioactive wastes.[1,3,10,14] In Syria, the Atomic Energy Commission (AECS) in co-operation with the oil and gas industry have taken an action, in 1998, for the treatment of NORM waste. This enabled implementation of remediation projects for sludge and low-level 226 Ra-contaminated soil (land farming, steam separation and landfill) [9,15] in accordance with the Legislative Decree No. 64, Radiological and Nuclear Regulatory Office,[16] the application of reinjection water, in addition to constructing and commissioning the NORM Decontamination Facility (NDF), which in turn creates scale waste. To date, 60 t of scales has been generated as a consequence of decontaminating
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Environmental Technology the radioactive components using high-pressure water jetting process. They are kept in plastic drums and stored in an interim licensed storage at the NDF. The present work draws a baseline in radiochemical characterization of these solid wastes to develop a suitable NORM waste management plan. Gamma spectrometry is the main technique used for the determination of uranium and thorium progeny in scales. However, radio-characterization would be a challenging task for such samples that contain activities greatly exceeding the environmental levels, with complex matrix composition and high density. Thus, an extensive effort is made, herein, to obtain an accurate radiochemical characterization taking into account energy calibration, detection efficiency and corrections for selfabsorption. Further work will proceed to compare the results with those could be simulated using the X-com programme.[17]
Experimental Scale sampling and preparation In March 2011, a total of 32 scale samples were collected from a number of drums, at the NDF storage, i.e. 500 km to the northeast of Damascus (Figure 1). The drums studied were chosen according to their measured gamma dose rate (at contact, using Rados 110) so that the entire recoded range of the stored 300 drums, i.e. 1–700 μSv h−1 , was covered. Sampling was carried out using a pressurizedstainless steel Auger (inner diameter of 8 cm) along the centre of the drum. In some cases, duplicate or triplicate cores were taken from the same drum to assure the success of sampling process. After spreading the sampled core on a polyethylene sheet, 20 g from each 10-cm core was taken and mixed, forming a representative sample of final weight ca. 80–120 g. It is worth pointing out that the scales were of various physical properties (colour, density and hardness) due to the volatilization of hydrocarbon compounds upon storage. Each scale sample was double-bagged in plastic and tightly sealed to ensure secure containment. A dust mask, disposal gloves and coveralls, glasses, helmet and heavy boots were used as personal protection equipment (PPE) during the sampling work, to comply with the Health Safety Legislations in the oilfields. Workers external exposure dose (measured via. TLD films) throughout the entire study did not exceed the permissible limit, i.e. 20 mSv y−1 . After transportation to laboratory, the scale samples were air-dried, ground and homogenized using Turbula mixer (Basel/Schweiz) for 24 h, and placed in a sterilized Falcon Petri dish (Model 351006, USA). Next, each sample was sealed and stored for about 4 weeks, allowing establishment of secular equilibrium between 222 Rn and progeny, prior to counting via HPGe gamma spectrometry.
Laboratory gamma spectrometry measurements of scale samples Activity concentrations of 210 Pb at 46.5 keV (4.25%) and 226 Ra through its progeny, namely 214 Pb at 295 (18.15) and 351 keV (35.9%) and 214 Bi at 1764 keV (15.1%), and 228 Ra through 228 Ac at 338.6 (11.27%) and 911 keV (25.8%) in the scale samples were determined using NType HPGe detector (Bruker company, with relative efficiency of 60% and FWHM of 0.89 and 2.0 keV at 122 and 1332.5 keV, respectively). Estimation of the activity concentration of the studied radionuclides was performed, with an acquisition live time of 1000 s, for each sample, using the following equation [18]: AEi =
NEi εE γ t m K1 K2 Ki
(1)
where AEi is the activity concentration (Bq g−1 ) of the nuclide i at energy E, N Ei is the net count in the full energy E peak, εE is the detector efficiency determined at the peak energy (gamma line) E, γ is the gamma line branching ratio (intensity) at energy E (%), t is the counting live-time (in second), m is the sample mass (g) and K 1 , K 2 and K i are correction factors for self-attenuation, nuclide decay time and dead time, respectively, etc. The spectra obtained were analysed by InterWinner7 PC software (ITECH Instruments, 2011). Measured activity concentrations of the nuclides studied were then expressed as weighted means.[19] Self-absorption correction was made for the scale samples that were found to have densities in the range of 1.0–3.1 g cm−3 , and complex matrices according to Kitto,[20] in which a point source of 152 Eu (reference date 1 July 1984) provided by the International Atomic Energy Agency, IAEA, with an activity of 376 kBq was used for the transmission measurements. Background correction was made by counting for 100,000 s. Depending on the preliminary counting activity (total cps) of the scale samples, it was necessary to place the active samples away from the face of the detector to achieve dead time < 1%. This, in turn, resulted in three counting distances, ca. 0, 5 and 10 cm. Efficiency calibration for the three practically determined counting levels, detector–sample distances (0, 5 and 10 cm) and energies, was made using CRM standard solution QCYB40, obtained from AEA Technology, UK, containing 210 Pb and 241 Am (reference date 1 January 2004), in addition to the use of CRM QCY48 solution, supplied by AEA Technology, UK, containing 57 Co, 60 Co, 85 Sr, 88 Y, 109 Cd, 113 Sn, 137 Cs, 139 Ce, 203 Hg and 241 Am (reference date 1 December 2008) with sufficient gamma ray lines lie along the working energy range (i.e. 59.5–1836 keV). Efficiency calibration was verified using RGU-1 (supplied by the IAEA) and the in-house NORM contaminated soil sample from the oilfields, designated as SYR-NORM-2005.[21]
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Figure 1. The location of NORM decontamination facility (NDF) in Deir Ezzor city, in Syria.
Minimum detectable activity, precision and uncertainty of gamma measurements Method parameters used in this study were minimum detection activity, repeatability, intermediate precision and measurement uncertainty.[22] Minimum detectable activity (MDA) for the nuclides of interest was derived from background measurements, at the three sample–detector distances, and automatically calculated via InterWinner7 PC software according to ISO 11929.[23] The two most common precision measures are repeatability and reproducibility (the so-called intermediate precision). They were determined, herein, for the radionuclides studied at the three counting levels and expressed in terms of relative standard deviations (RSDr and RSDR , respectively) according to Eurachem.[22] For better estimation of repeatability, a number of chosen samples within the same counting level (sample–detector distance) was counted, each for ten times on one piece of equipment by the same analyst and over a short timescale. Intermediate precision was performed by different analysts over extended timescales within a single laboratory. Assessment of uncertainty was established by counting a chosen sample at 5 cm detector distance, since it was the suitable geometry for counting the majority of scale samples in this work. With reference to the IAEA,[18] combined standard uncertainty (σ Ai ) in determining the activity concentration, of a given radionuclide,
by gamma spectroscopy could be derived using the following equation: σ A i = Ai
σN N
2
+
σ εi εi
2
+
σ γi γi
2
+
σt t
2 +
σ m 2 m
.
(2)
Results and discussion Efficient calibration To obtain a representative efficiency for the physical measurements, it was necessary to calibrate the detector with regard to the three counting levels considered in this study, i.e. sample at the detector face (0 cm) and 5 and 10 cm away from the detector. Experimental efficiency calibration curves are shown in Figure 2. It could be seen that the efficiency decreased, at a specific gamma-energy peak, with increasing sample–detector distance. Moreover, the calibration curves presented a maximum at low range of gamma energy; thus, care should be taken when correcting for low energy lines, such as 210 Pb (at 46.5 keV). Laboratory gamma spectrometry measurements of scale samples As stated in the Experimental Section, the activity concentrations of radium (226 Ra and 228 Ra) and 210 Pb in the
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respectively, distributed. In the case of 228 Ra, the majority of scale samples (90.6%) had activity concentrations < 150 Bq g−1 , with 9.4% of samples falling within 151– 450 Bq g−1 . A quite different pattern was observed for the activity distribution of 210 Pb. One half of the samples had activity concentration < 450 Bq g−1 , whereas the other 50% were in the activity concentration range of 450– 1900 Bq g−1 . Table 1 shows the minimum, maximum and mean values of the activity concentrations of the radionuclides investigated in the scale samples with regard to the counting levels of gamma-measurements taking into account dose rate and the ratio of cps/density. An increase of one order of magnitude was recorded in the mean activity concentration of 226 Ra with elevation of cps/density, reaching 2577 Bq g−1 at the ratio 4642. Conversely, hardly any variation was observed in the mean concentration of 228 Ra at 5 and 10 cm distance (ca. 94 and 85 Bq g−1 , respectively), which could be related to the low of counting efficiency as a consequence of the sample distance from detector, taking into account the low content of the radionuclide in the samples. The mean concentration of 210 Pb was essentially doubled in the practical counting levels used (i.e. in 5 and 10 cm). Others [11] determined radium content in scale samples collected, using a scraper, from contaminated equipment and found that the maximal concentrations are 1520 Bq g−1 (for 226 Ra) and 868 Bq g−1 (for 228 Ra). The variation of data in comparison to the results of this study could be related to the source of scale samples and method of measurement. The results obtained here were statistically evaluated using the so-called explanatory data analysis method; one example is the Box Plot used herein (Figure 4). Generally, the median value was more representative than the mean as the former is less affected by outliers. Clearly, the variation of the activity concentrations of 226 Ra and 210 Pb in the scale samples was high, since median values drifted away from the means; however, less variation was observed for 228 Ra, which in turn reflected its low concentration. The median values were 393, 56 and 503 Bq g−1 for 226 Ra, 228 Ra and 210 Pb, respectively. The distribution of activity concentration ranges, described above (see Figure 3), may reflect the obtained medians, with the fact that about 56% of scales had 226 Ra content ranging from 0 to 450 Bq g−1 .
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Figure 2. Experimental data of the variation of the detector efficiency with different γ -energy lines for three sample–detector distances (0, 5 and 10 cm).
Figure 3. Distribution of the activity concentrations of 226 Ra, 228 Ra and 210 Pb in the scale samples that produced from the petroleum industry.
scale samples produced from the oil industry were determined using transmission measurement and expressed as weighted means (Bq g−1 ). The distribution of the activity concentrations of the radionuclides investigated in the scales, regardless of form and source, is shown in Figure 3. The data presented illustrated that 31.3% of the scale samples had activity concentrations of 226 Ra in the range of 0–50 Bq g−1 and 25% were within the range 151–450 Bq g−1 . At higher ranges (451–1900 and 1901–2925 Bq g−1 ), 28% and 15.6% of samples were,
Table 1. Minimum, maximum and mean values of cps/density, dose rate and the concentrations (in Bq g−1 ) of the radionuclides studied in the scale samples, according to the counting levels of gamma measurements. 0 cm Counting level
Min
Max
cps/density Dose rate (μSv h−1 ) 226 Ra 228 Ra 210 Pb
8 1 2 0.3 < MDA
159 11 37 3 349
5 cm Mean
Min
Max
± ± ± ± ±
406 80 184 17 202
3858 600 1584 254 1478
71 6 20 2 175
38 3 10 1 145
10 cm Mean
Min
Max
± ± ± ± ±
3924 440 2341 79 1108
6117 700 2922 94 1794
1747 307 771 94 696
1116 172 527 72 435
Mean 4642 588 2577 85 1462
± ± ± ± ±
912 93 308 6 311
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Figure 4. Box plot of the activity concentrations (Bq g−1 ) of 226 Ra, 228 Ra and 210 Pb in the scale samples.
Gazineu and Hazin [1] reported the median values of 897 Bq g−1 (for 226 Ra) and 680 Bq g−1 (for 228 Ra) when estimating radium contents in scales produced from Brazilian oil industry, although they investigated limited number of samples (i.e. 12). The high order of uncertainty of the activity concentrations of the radionuclide studied in the scale samples herein was in agreement with that noted in the literature.[1,2,11] Generally, statistical approaches are essential tools to illustrate the correlation among variables studied. Principal component analysis (PCA) and analysis of variance (ANOVA) were carried out for the 32 scale samples using XLStat software.[24] The variables investigated were the activity concentrations of 226 Ra, 228 Ra, 210 Pb, dose rate and cps/density. The latter was used to account for the matrix effect resulting from the complex chemical composition of the samples. Data are represented as a correlation circle (on axes F1–F2) (Figure 5) and plots of variance are shown in Figure 6. Correlation coefficients are summarized in Pearson table (Table 2). Looking at Figure 5 shows that the applied projection explained 92.86% of variability, with τ 1 = 81.82% and τ 2 = 11.04%. Intercorrelation of the four variables (i.e. 226 Ra, 210 Pb, cps/density and dose rate) is clearly seen. However, low correlation was noted in the case of 228 Ra with the rest of variables, which could mainly be related to the low concentration of the radionuclide in the samples. Since both 226 Ra and dose rate are far from the centre and close to cps/density, they show a significant positive correlation to cps/density with correlation coefficients of 0.983 and 0.908, respectively (Table 2). The maximal correlation demonstrated by 226 Ra with cps/density, in comparison to 228 Ra (0.658) and 210 Pb (0.812), could be a reflection of the high concentrations of the former radionuclide in the samples. A considerable correlation was observed between 226 Ra and dose rate (r = 0.885), which may mirror the high
Figure 5. Correlation circle (on axes F1–F2) of the activity concentrations of 226 Ra, 228 Ra, 210 Pb as well as dose rate and cps/density in the scale samples.
peak energies of gamma lines that are detectable by the dosimeter used. A similar scenario could be applicable in the case of 228 Ra and dose rate, with correlation coefficient of 0.751. Plots of variance (Figure 6) also showed linear proportion between cps/density and the main components studied (i.e. concentration of 226 Ra, 228 Ra, 210 Pb and dose rate), with the maximum correlation achieved between cps/density and the activity concentration of 226 Ra (R2 = 0.966). Linearity of the dose rate with cps/density gave a regression coefficient of 0.896. These results were compatible with those obtained, previously, by PCA and Pearson. A literature survey of radium contents in scales of petroleum industry in various countries is shown in Table 3. Scales generated from Riyadh city refinery tanks in Saudi Arabia [25] were found to contain the lowest concentrations, not exceeding 1.5 and 3.2 Bq kg−1 for 226 Ra and 228 Ra, respectively. The highest concentration of 226 Ra reported herein was somewhat, comparable to that observed in the Brazilian scales collected from oil exploration unit, i.e. 2110 Bq g−1 .[1] However, USEPA stated that the maximal activity concentration of 226 Ra in scales could reach up to 15,170 Bq g−1 and ca. 1000 Bq g−1 for 210 Pb.[26] The variation observed in radium concentration ranges (or means) within the same country (e.g. Egypt, Brazil or UK) reflected the differences of scale sources and measurement method used, as noted for Syria in the preceding section. Consequently, the results of this study were found to fit within the worldwide range, notwithstanding the fact that no barrel of the 32 had activity concentration below 1 Bq g−1 , which is the exemption/clearance level proposed by the European Union.[27] Considerable concern was, therefore, recognized by the national waste
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Environmental Technology (a)
(b)
(c)
(d)
Figure 6. ANOVA of cps/density of the scale samples with the main variables, 226 Ra (a), 228 Ra (b), 210 Pb (c) and dose rate (d). Table 2. samples.
Correlation matrix of Pearson ‘n’ of the studied parameters in the scale
Variables Dose rate cps/density 226 Ra 228 Ra 210 Pb
Dose rate
cps/density
226 Ra
228 Ra
210 Pb
1 0.908 0.885 0.751 0.767
1 0.983 0.658 0.812
1 0.549 0.802
1 0.542
1
Note: Values in bold are different from 0 with a significance level α = 0.001.
management authority to establish special treatment and safe disposal of such waste.
Method validation of gamma measurements Minimum detectable activity Determination of the minimum detection activity of the radionuclides of interest was performed using Currie formula at the three counting levels stated. The results are
shown in Table 4. Higher MDA values, determined at the detector face, for 210 Pb (ca. 65 Bq kg−1 ) were observed in comparison to those of 226 Ra and 228 Ra (i.e. 23 and 22 Bq kg−1 , respectively). The increase in MDAs was reasonable since Currie formula includes several parameters related to the nuclear counting measurement of the sample. These parameters are counting time, the counting of a blank, sample mass and counting efficiency of the system at the specific peak energy. Therefore, the high MDAs for 210 Pb could be due mainly to its low-energy peak and
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Worldwide values of 226 Ra and 228 Ra concentrations in scales produced from oil 226 Ra
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Country Egypt (Eastern bank of Suez gulf) Egypt (Abu Rudeis) Egypt (Gabal El Zeit) Egypt (Badr El Din) Egypt (Red Sea) Egypt (Western desert) KSA (Riyadh city refinery tanks) Australia Brazil Brazil (Sergipe and Alagoas) Malaysia USA USA (EPA) Norway Germany The Netherlands UK Algeria Tunisia Libya Syria Syria
(Bq g−1 )
493–519 68.9 14.8 31.4 195 59.2 (0.8–1.5) × 10−3 20–70 16.2–93.2 77.9–2110 0.5–434 (114.3)* (76.2)* 15,170 8–100 100–500 0–900 20–400 1–950 4.3–658 23–221 0.3–1520 (174)* 2–2922 (818)*
228 Ra
(Bq g−1 )
32–50 24 4.3 43.3 897.8 244.5 (0.1–3.1) × 10−3 – 4–36.9 101.5–1550 0.9–479 (130.1)* – – 5–30 40–200 0–400 20–300 – – 1.1–111 0.6–868 (91)* 0.3–254 (64)*
References [28] [29] [5] [25] [19] [14] [1] [12] [30] [26] [31]
[3] [32] [33] [11] This work
*Arithmetic mean is given in parentheses. Table 4. Determined MDAs (in Bq kg−1 ) for the radionuclides studied in accordance with the sample–detector distances. Sample–detector distance (cm) Sample mass (g) 226 Ra 228 Ra 210 Pb
0
5
10
6 8 10 23 89 192 22 74 145 65 261 635
Note: Detector efficiency 60%; counting time 100,000 s.
counting efficiency. In a similar manner, enhanced MDA values were observed with increasing sample–detector distance for a particular radionuclide. For instance, the MDA for 226 Ra was four times higher when sample–detector distance was raised to 5 cm than those determined at the detector face and was doubled when distance increased to 10 cm. This behaviour mirrored the relative decrease of counting efficiency with increasing sample–detector distance as shown previously in Figure 2. Previous studies [34,35] described the experience of the AECS in method validation for environmental radiochemical measurements. They stated that the MDA for 226 Ra determination was found to be 2 Bq kg−1 with sample mass of 160 g, detector efficiency 80% and counting time of 80,000 s. The MDA increased to reach 40 Bq kg−1 when sample mass ranged from 25 to 30 g under the same experimental conditions (counting efficiency and time). In the case of 210 Pb, the authors found that the MDA were within the range of 12–15 Bq kg−1 for soil samples of 40 g
weight using HPGe detector of 10% efficiency. In comparison to the MDAs determined herein, it may be concluded that the MDAs obtained for 226 Ra (23 Bq kg−1 ) and 210 Pb (65 Bq kg−1 ), at the detector face, were quite sensible for sample mass ranging from 6 to 10 g.
Gamma measurement precision Repeatability and reproducibility measurements were performed by choosing two, four and three samples for the 0 cm counting level (at the detector face), 5 and 10 cm sample–detector distances, respectively, to allow better estimation to be drawn. Because scales were enormously varied in chemical composition, density and radiochemical concentrations, samples were selected to cover the entire activity concentration range in each counting level. Tables 5 and 6 summarize the ranges of mean, standard deviation as well as repeatability and reproducibility relative standard deviation (RSDr and RSDR ) of the replicates. The data showed an improvement of the RSDr for 226 Ra with elevated sample–detector distance, reaching 0.4–0.6 at the maximal distance. More or less, constant values were observed for 228 Ra, giving a range of 5.7–8.4 at the highest counting level, which could be due to low radionuclide content in the scale samples compared with that of 226 Ra. Meaning that the high concentration of 226 Ra and its progeny caused an increase in the baseline of the sample spectrum, which in turn made the determination of 228 Ra quite difficult. As expected, a converse scenario was revealed for 210 Pb where RSDr increased when samples
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Environmental Technology Table 5. Ranges of mean, standard deviation and relative standard deviation of the repeatability measurements for 210 Pb with reference to the counting levels. Sample–detector distance
0 cm
5 cm
226 Ra, 228 Ra
and
10 cm
Nuclide
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
Meanr (Bq g−1 ) Sr (Bq g−1 ) RSDr
18–31 0.3–0.6 1.8–2.0
3–4 0.2–0.4 7.8–9.8
25–304 0.5–4.2 1.4–2.0
382–1414 3.7–5.1 0.3–1.3
35–97 1.8–6.3 3.3–8.4
304–1367 4.1–21.4 1.4–3.7
2338–2907 8.8–13.8 0.4–0.6
75–96 5.3–6.7 5.7–8.4
1118–1781 69.9–119.2 6.2–7.1
Table 6. Ranges of mean, standard deviation and relative standard deviation of the reproducibility measurements for 226 Ra, 228 Ra and 210 Pb with reference to the counting levels.
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Sample–detector distance
0 cm
5 cm
10 cm
Nuclide
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
226 Ra
228 Ra
210 Pb
MeanR (Bq g−1 ) SR (Bq g−1 ) RSDR
18–31 0.2–0.5 1.2–1.7
3–4 0.2 5.5–6.8
25–300 0.6–4.5 1.5–2.4
379–1424 2.8–9.2 0.6–1.4
33–100 1.6–3.5 2.2–9.0
302–1126 12.3–26.9 2.4–4.2
2319–2887 9.4–20.8 0.4–0.7
70–96 6.5–7.7 6.7–9.9
1015–1738 56.8–97.3 5.3–5.6
Table 7. Percentage of the contribution of the studied components to the total uncertainty, for the determination of the activity concentrations of 226 Ra, 228 Ra and 210 Pb in the scale samples. Radionuclide 226 Ra 228 Ra 210 Pb
Net count
γ line-intensity
Efficiency
Sample mass
Counting time
99.60 99.51 99.73
0.0007 0.0069 0.0004
0.36 0.31 0.24
0.03 0.10 0.02
0.02 0.08 0.01
were counted further away from the detector, which could be a consequence of increasing the attenuation of its peak energy. Reproducibility RSDR for the radionuclides of interest showed comparable trends to those reported for RSDr . For example, RSDr and RSDR for 226 Ra were 0.4–0.6 and 0.4–0.7 at the 10 cm sample–detector distance, so little variation could be seen within the parameters studied (analyst and time scale). This did not include 228 Ra, which was counted at the face of the detector because of its relatively low activity concentration (3–4 Bq g−1 ). Finally, the RSDr and RSDR obtained herein (at 10 cm counting level) were comparable to the values interpolated from the literature [34,35] for soil samples measured by gamma spectroscopy at the AECS, i.e. 0.6 and 0.7 for 226 Ra (at the detector face). The authors reported higher RSDr and RSDR for 210 Pb (2.2 and 2.6, respectively), for larger sample mass (ca. 30–40 g) than those used in this study (i.e. 5–10 g) giving maximal values of 2.0 and 2.4 (at the detector face), respectively. Uncertainty of gamma measurements Total uncertainty of gamma measurement of the studied radionuclides was calculated using Equation (2). Sample mass, counting time, gamma peak intensity, and efficiency and count rate were the main components of total uncertainty. Contribution of these components to
the total uncertainty was calculated using the Spreadsheet method.[36] The total uncertainty for 2σ of the activity concentrations 373, 51.5 and 193 Bq g−1 of 226 Ra, 228 Ra and 210 Pb, respectively, was found to be 7%, 4% and 9%, respectively. The uncertainty due to net counting was the highest contributor to the total uncertainty for the radionuclides studied, which exceeded 99% (see Table 7). Uncertainly of counting efficiency was the next contributor, ranging between 0.24 and 0.36%, whereas uncertainty related to sample mass was ≤ 0.1%. In consistence with the literature,[34] it was found that the relative uncertainty by gamma spectrometry increased linearly with decreasing activity concentration of the radionuclides studied. Linear coefficients were found, herein, to be 0.996, 0.999 and 0.616 for 226 Ra, 228 Ra and 210 Pb, respectively.
Conclusion The present work represents a preliminary investigation of the oilfield scales using a gamma spectrometry, to pave the way for establishing a suitable radioactive waste management plan. The maximum activity concentrations of 226 Ra, 228 Ra and 210 Pb were determined using HPGe detector and found to be 2922, 254 and 1794 Bq g−1 for 226 Ra, 228 Ra and 210 Pb, respectively. These results were within the
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literature stated values. Application of Box plots, ANOVA and PCA as statistical approaches was made, in which the medians were 393, 56 and 503 for 226 Ra, 228 Ra and 210 Pb, respectively. 226 Ra and cps/density showed the best correlation (with correlation coefficient of 0.983), while less correlation was observed for the other radionuclides. Since scale samples are known to have high densities, complex chemical compositions and high activities of radionuclides, method validation of gamma measurements was carried out, in which MDA, intermediate precision, repeatability and assessment of uncertainty were the parameters investigated. The MDAs were comparable to those reported in the gamma-ray spectroscopy laboratory at the AECS for soil samples. RSDr and RSDR were improved by increasing the sample–detector distance for 226 Ra, whereas an opposite effect was noted for 210 Pb. Finally, the approach used in this study could be applied for accurate radiochemical characterization of scale samples. Acknowledgements The authors thank Mr. W. Ahmad and the administrative team at the NDF for the assistance in sampling. The Director General of the Atomic Energy Commission of Syria is highly appreciated for his support throughout the work. Disclosure statement No potential conflict of interest was reported by the author(s). References [1] Gazineu MHP, Hazin CA. Radium and potassium-40 in solid wastes from the oil industry. Appl Radiat Isotopes. 2008;66:90–94. [2] Gazineu MHP, de Araújo AA, Brandão YB, Hazin CA, de Godoy OJM. Radioactivity concentration in liquid and solid phases of scale and sludge generated in the petroleum industry. J Environ Radioactiv. 2005;81:47–54. [3] Hamlat MS, Djeffal S, Kadi H. Assessment of radiation exposures from naturally occurring radioactive materials in the oil and gas industry. Appl Radiat Isotopes. 2001;55:141– 146. [4] OGP. Guidelines for the management of naturally occurring radioactive material (NORM) in the oil & gas industry – OGP. England and Wales: The International Association of Oil & Gas Publications; 2008. [5] Shawky S, Amer H, Nada AA, Abd El-Maksoud TM, Ibrahiem NM. Characteristics of NORM in the oil industry from Eastern and Western deserts of Egypt. Appl Radiat Isotopes. 2001;55:135–139. [6] IAEA. Extent of environmental contamination by naturally occurring radioactive material (NORM) and technological options for mitigation. Technical Reports Series No. 419. IAEA, Vienna, 2003. [7] Smith KP. An overview of naturally occurring radioactive materials (NORM) in the petroleum industry. Illinois: Department of Energy; 2008. [8] IAEA. Radiation protection and the management of radioactive waste in the oil and gas industry. Safety Reports Series, No. 49. Vienna, Austria: IAEA; 2004.
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