Naturally occurring radioactive materials and related hazard indices in Ahdeb oil field

In this work, measurements of activity concentration of naturally occurring radioactive materials (NORM) isotopes and their related hazard indices for several materials such as crude oil, sludge and water in Ahdeb oil fields in Waste governorate using high pure germanium coaxial detection technique. The average values for crude oil samples were174.72Bq/l, 43.46Bq/l, 355.07Bq/l, 264.21Bq/l, 122.52nGy/h, 0.7138, 1.1861, 0.601 mSv/y, 0.1503mSv/y and 1.8361 for Ra-226, Ac-228, K-40, Ra eq, D, H-external and H-internal respectively. According to the results; the ratio between 238U to 232Th was 4, which represents the natural ratio in the crust earth; therefore, one can be strongly suggested that the geo-stricture of the Ahdeb oil fields dose not contents any kind of rocks. Although the results indicate the rising in the activity concentration of NORM isotopes, the national and international comparisons proved that it is still in the world range limits.


Introduction
Naturally occurring radio nuclides are present at varying concentrations in the earth's crust and can be concentrated and enhanced by processes associated with the recovery of oil and gas. This "enhanced'' NORM, often known as TENORM (Technologically-Enhanced Naturally Occurring Radioactive Materials) can be created when industrial activity increases the concentrations of radioactive materials or when the material is redistributed as a result of human intervention or some industrial processes, TENORM also can be the by-product or waste product of oil, gas and geothermal energy production. Sludge, drilling mud, and pipe scales are examples of materials that can contain elevated levels of NORM, and the radioactive materials may be moved from site to site as equipment and materials are reused [1]. In addition, the oil can be classified as useful and good if it contained high concentration of natural gas, if not it called the dead oil. Further, there are many evidences that high concentrations of radon gas can be associated with the natural gas. Therefore, the measurements of the concentration of radon gas in the oil fields are important if one needs to evaluate the overall NORM and TENORM [1]. The Kut City (the capital of Wasit province) is located in eastern Iraq, on the left bank of the Tigris river, about 160 kilometers south east of Baghdad, between 32˚30'20"N latitude and of 45˚49'29"E longitude, with a population of approximately 374,000. Kut city depends on the oil fields, which are: the first of two oilfields Ahdab field and Badra and this study was limited to the Ahdab oil field that is an oil field located in the hand -Ahrar 27 km west of the Kut city. Estimated oil reserves of the field by more than a billion barrels. It started producing oil from the Ahdeb field in the July 1, 2011, and operated by China National Petroleum card production to 60 thousand barrels per day, and will increase production capacity to 120 thousand barrels per day within six months, and will be exported most of the production Ahdab field, but some of it will be used as fuel for power plants nearby to ease the shortage of electric power. However, this paper dedicated to answer some important questions;  What about the NORM and TENORM concentrations in Ahdeb oil fields?  What about the values of radiological hazard indices and are these values within the allowed international limit?
All the results of the NORM measurements, which were measured in this work, can be categorized into 226 Ra, 228 Ac, and 40 K isotopes. Specific activity (SA) and its radiation hazard indices, which contented the gamma dose rate (D), the radium equivalent (Ra eq ), the external and internal hazard indices (H ex , H in ), the representative level index, (I γr ), and the annual effective dose equivalent (AEDE), were estimated.
In order to get accurate and confident results, measurements for each parameter such as S.A (for each isotope such as 226 Ra) have been done for three different samples, then the average of these measurements have been taken as a final result.

Collection of samples
A number of 26 samples of crude oil, sludge, corrosion, and ground water samples were collected form Ahdeb oil field. These samples were exposed to sunlight for an extended period of time were quite dry, while those that were recently removed from separation tanks or stored in barrels, tended to be more oily following collection.

1-The crude oil samples
The overall results of the specifications for the investigated samples (the crude oil samples) have been presented in Table 1 is an important observation from the NORM results of these samples, which is the ratio between 226 Ra (which belongs to 238 U chain) and 228 Ac (which belongs to 232 Th chain). This ratio approximately have value near to the natural value, about 4, and this can be attributed to the nature of the Ahdeb land, which have a plane surface and do not have any kind of rocks. However, the differences in the activity concentrations of NORM isotopes of the crude oil samples can be attributed to the differences in the depths of oil wells. If the oil well has high depth, then the activity concentrations of NORM isotopes have large values. The average value of radium equivalent activities (Raeq) was 264.21Bq⁄l. This value is also considered to be normal [2], while the highest Raeq was 311.01 Bq/l for O13 sample (taken from the Rumaila layer of the first well) and the minimum Raeq was 187.82 Bq⁄l for O1 sample. The overall average value of gamma dose rate (D) for the full sample set was found to be 122.52 nGy⁄h which was 144.51 nGy⁄h for O13 sample that was higher than the permissible value and the minimum value was 86.97 nGy⁄h for O1 sample. The overall average values of the H-external and H-internal hazard indices (Hex and Hin), were 0.7138 and 1.1861 for Hex and Hin, respectively. The highest values of Hex and Hin were 0.8403 and 1.3861 for the O13 sample and the minimum values were 0.5075 and 0.8403 for O1 sample, respectively. The values of the Hex and Hin indices must be less than unity (≤1) for the radiation hazard to be negligible, [3]. So, it is noted that all values of Hex and Hin had difference values, and Hin had high values, which were more than the allowable limits (≥1), while Hex had normal values.

2-Results of the sludge samples
Six samples of the sludge were collected from different Ahdeb oil field and the activity concentration has been measured. The overall results for the activity concentration and the hazard indices are presented in Table 2. The results of these samples can be classified as TENORM, because some of it deduced from refinery stations and the others precipitated when oil extracted.  (≥1) and, therefore, authorized person must be followed the radiation safety requirements.

Annual effective dose ( ) ⁄ :
The average values of the annual effective dose equivalent (AEDE) due to indoors (E ied ) and outdoors (E oed ) terrestrial gamma radiation were calculated to be 1.148 and 0.287mSv y ⁄ . The maximum values of E ied and E oed were 1.903 and 0.476mSv y ⁄ for SL2 sample, and the minimum values were 0.626 and 0.156 for SL6 sample.

Representative level index, (I γr ):
The overall average value of I γr was 0.289, whereas the maximum value was 5.802 for SL2 sample and the minimum value was 1.914 for SL6 sample. In order to establish the works of sludge samples, corrosion sample was taken from the tubes of oil and investigated against NORM isotopes (we do not illustrate the result of this sample with the sludge samples because it resulted from the chemical effects of oil on the tubes). The overall results for the measured corrosion samples have been presented in Table3.The obtained data revealed that the overall average value of the specific activity for 226 Ra, 228 Ac and 40 K of this sample were 473.95, 124.72 and 725.14 Bq kg ⁄ . However, all the hazard indices of this sample showed high values comparing with other samples such as crude oil and sludge samples, as shown in Table3. Therefore, these results compel us to deal cautiously with the oil pipelines.

3-The water samples
As you know, that in the oil extraction process, the extracted oil was mixed with some quantities of ground water. Therefore, these waters must be refined in specific stations. However, measurements of NORM isotopes in these water samples give us some indications about the last fate of it and about the ability of how can one get rid these waters? Four samples of the extracted ground water were collected from deferent Ahdeb oil fields and the activity concentration have been measured. The overall results for the activity concentration and the hazard indices have been presented in Table 4. The obtained data revealed that the average activity concentrations for 226 Ra, 228 Ac and 40 K for these set of samples had the values; 25.05, 7.59 and 35.17Bq l, ⁄ respectively.

Annual effective dose:
The average values of the indoors and outdoors annual effective dose equivalent (AEDE) due to terrestrial gamma radiation for all samples of water samples were 0.087 and 0.022mSv y ⁄ , respectively. The maximum value ofE ied was 0.176 mSv y ⁄ for W4 and the minimum value was 0.045 mSv y ⁄ for W3. Whereas the maximum value of E oed was 0.044 mSv y ⁄ for W4 and the minimum value was 0.011 mSv y ⁄ for W3.

Representative level index, (I γr ):
The average value of I γr for all water samples set was 0.226. The maximum value was 0.539 for W4 and the minimum value was 0.138 for W3. Finally, NORM measurements in the oil fields were done in several sites in the world (one can be said in all oil fields in the world) therefore, if we compared our results with others, one can note that our results are still in the world range, as can be shown in Table5. In general, the world results showed that the ratio between the 238 U and 232 Th chains had different values ranged from the natural ratio (4) to different values, whereas our results had ratio about 4. These differences can be attributed to the nature of the ground and to the nature of the oil (if the oil has high concentration of natural gas, then the oil has high concentration of radon and, therefore, more radium, which belongs to 238 U chain).