Gamage et al/Journal of Tropical Forestry and Environment Vol. 8, No. 01 (2018)/ 55-63 55 Radioactive and Non-Radioactive Element Analysis of Dorado Gas Discovery of Sri Lanka and Their Influence on Natural Environment S.S.N. Gamage1*, R.M.T.S.Ratnayake1, A.M.A.D.M. Senadhira2, D.A. Weerasinghe2, V.A. Waduge3 1 Department of Physics, University of Sri Jayewardenepura, Sri Lanka 2 Petroleum Resources Development Secretariat, Sri Lanka 3 Atomic Energy Board, Sri Lanka Date Received: 2017-12-10 Date Accepted: 2018-04-29 Abstract Naturally-occurring radionuclides deposited beneath the earth, which are referred to as "NORM" and other toxicnon-radioactive elements transported to the earth surface with the oil and gas production. Hence, knowledge of the prevailing background levels of these elements in the subsurface reservoir formations is valuable to all stakeholders, most notably to regulatory authorities of the country. The drill cuttings obtained within depth range 3025m to 3095m of reservoir sand section in the deep water exploratory well (CLPL- Dorado 91 H/1z) drilled in the Mannar Basin offshore Sri Lanka were subjected to high-resolution Gamma-ray spectrometry and X-ray fluorescence (XRF) spectrometry. As test results revealed activity concentration of 40K varies from 0.338 Bq/g to 0.514 Bq/g, 210Pb from 0.007 Bq/g to 0.015 Bq/g, 226Ra from 0.012 Bq/g to 0.0145 Bq/g while 232Th levels are between 0.030 Bq/g to 0.040 Bq/g. According to the XRF testing levels of significantly hazardous non-radioactive elements are considerably lower, except for the level of the Barium. The Pb level varies between the 48 ppm to 22 ppm. The Thorium level varies between 9.6 ppm to 10.1 ppm. Manganese has a range of 5,173ppm to 653ppm.The barium levels are between 118,666 ppm to 24,400 ppm. NORM concentration of the tested section were on the lower side when results matched with the IAEA published data on NORM concentration in oil, gas and there byproducts and therefore there will be low level of NORM contaminations when the Dorado gas discovery proceeds to the production stage. Further there is no harmful public exposure from NORM by disposing these drill cuttings to environment or storing at any site location. But the disposal of the drilling mud and handling of the drilling mud should be conducted with cautious since extremely high Ba levels can potentially cause health problems. Keywords: NORM, drill-cuttings, oil, gas, Sri Lanka, XRF 1. Introduction 1.1 Background Sri Lankan upstream petroleum industry is still in the early phases of exploration in which wells are drilled to discover and assess hydrocarbon and to derive fundamental strategies for future development stages. It had been started in 1960’s and during 1972-1975 first three exploration wells were drilled Pesalai-1, Pesalai-2 and Pesalai-3 in the Cauvery Basin of Sri Lanka. Later, Palk Bay-1 and Delft- in 1976 and Pedro-1 and Pearl -1 in 1981were drilled but there have not been any significant hydrocarbon discoveries except small discovery in Pesalai-1. * Correspondence: shanthagamage@sci.sjp.ac.lk ISSN 2235-9370 Print/ISSN 2235-9362 Online © University of Sri Jayewardenepura mailto:shanthagamage@sci.sjp.ac.lk Daham DOI: 10.31357/jtfe.v8i1.3483 56 Then in 2001, 2005, 2009 and 2012 seismic surveys has been carried out to acquire vital data. (PRDS-Sri Lanka, 2017) Most recently starting from 2011 four new wells Dorado, Dorado-north, Barracuda and Wallago have been drilled and discovered natural gas from Dorado and Barracuda wells (Ratnayake et al., 2017). This study was primarily focused on obtaining the NORM concentration levels of the drill cuttings from the gas reservoir sand section of the Dorado gas discovery off-shore Mannar Basin, compare those obtained levels with the global NORM levels, and correlate the results with well log data. Moreover, XRF testing results was conducted to determine non-radioactive hazardous elements. The subsurface formations which are rich in hydrocarbons are also comprise naturally- occurring radionuclides; Uranium, Thorium, Potassium, Radium and Lead which are referred to as "NORM" (Paranhouz, 2005) plus non-radioactive materials which are toxic to natural environment. Industrial processes involved in oil and gas production such as treating and refining activities direct all these elements which are trapped inside rock layers of the earth to flow to the surface and contaminate natural environment (Gray, 1993, Godoy, 2003), as such radionuclides along with the other minerals which are dissolved in the salty water, precipitate out and forming various wastes at the surface during the different stages of the oil and gas production. Some of these NORM contaminated waste associate with the production of the petroleum industry are, hard mineral scales formed inside the pipes, sludge disposed after treating water and hydrocarbons, contaminated equipment and components and produced water. IAEA safety report (2003), IAEA training course report (2010). Since the petroleum production processes are collecting and concentrating these NORM associated waste and other toxic materials, there is a prospective of exposing them to the natural environment. Therefore identification of background level of NORM and non-radioactive hazards materials in the subsurface hydrocarbon bearing formations will be beneficial in predicting contamination levels during the oil and gas production. The Department of Physics, University of Sri Jayewardenepura collaboratively conducted this study with the Petroleum Resources Development Secretariat and Atomic Energy Board of Sri Lanka and this was the first study which has been conducted regarding the NORM background levels of discovered reservoir sand sections in the drilled exploratory wells in the Mannar Basin offshore Sri Lanka. 1.2 Location of the study The Mannar basin is located at approximately 60o-90o North latitude by 78o-80o East longitude, and bounded from south west to north west Sri Lanka, south east of India and south of the Cauvery basin. In terms of size, the SL side of the Mannar basin approaches in an area of approximately 42,000 km2with a sediment accumulation of possibly up to 10kms in the deeper water areas of the basin. The Dorado well where drill cuttings were obtained is located in M2 block of Mannar Basin as shown in Figure 1 (PRDS-Sri Lanka, 2013). CLPL-Dorado-91H/1z (Dorado), well was drilled in a water depth of 1354 m, which penetrated a hydrocarbon rich sandstone between the depths of 3,044-3,069 m, measured depth (MD). Total depth of the well was 3,288 m, MD. It is the first exploration well to discover hydrocarbons in the Gulf of Mannar. Gamage et al/Journal of Tropical Forestry and Environment Vol. 8, No. 01 (2018)/ 55-63 57 2. Methodology 2.1 Sample selection & preparation Suitable depth range was decided for the samples selection using seismic and well log data. Then 15 samples each weighing about 250 g were taken from 3,025 m to 3,095 m depth range with five meter intervals. Then the drill cutting samples were air-dried. After that samples were oven dried at 100o C temperature for 10 hours to get rid of all the moisture and packed into sealed polyethylene bags. 2.2 Gamma Ray Spectroscopy The dried drill cutting samples were grinded and were sieved through a sieve of 2 mm size. Then, fine powdered samples were packed and sealed in an airtight G1 geometry container for gamma spectrometry testing and labeled according to their respective depths. Then they were stored for 21 days before its testing to achieve the secular equilibrium between elements. Figure 1. Location of the Gas discoveries and other exploration wells (Ratnayake et al., (2017). 58 This test was carried out at the Atomic Energy Board of Sri Lanka, using a high-resolution gamma spectrometer. The gamma spectrometry system was equipped with a coaxial n-type high purity germanium (HPGe) detector connected through amplifiers and multi-channel analyzer driven by a computer based operating system. The detector had a coaxial closed facing geometry with the following specifications. Detector mode GX3020 with a resolution of full width at half maximum (FWHIM) at 122 Kev of Co-57. The detector was shielded by a cylindrical lead shield, which had average thickness of 10 cm to reduce the background radiation. Genie 2000 software package was used for data acquisition. HPGe gamma spectrometry system was calibrated using the point sources of 137 Cs, 60 Co and 241Am and calibration was verified using the IAEA reference materials. The efficiency calibration was done by Geometric Composure method (Lab SOCS, Canberra) and the test method was validated by analyzing standard reference material, IAEA-134 and IAEA-414 (Ratnayake et al., 2017). 2.3 Total activity concentration Total Activity Concentration for Uranium, Thorium series and for potassium was calculated using the following Equation 1 (Omar et al 2008) TAC =40K+6x226Ra+3x210Pb+232Th (1) 2.4 Correlation with well logs Test results obtained from the testing were statistically analyzed with the well log data (Gamma ray log) to validate the test results. Minitab software was used for the analysis. Since the well logs were in different units (GAPI) the two data sets were standardized. Then 2k factorial design test was applied and p value was obtained. 2.5 XRF testing Five samples were selected to represent the total depth covering the reservoir sand section. From each samples 2 g quantity was separated at first .Then using non contamination hand grinder the samples were grinded. Then it was sieved by a sieve size of 63 mm and obtained 0.5 g. After that pressed powder method was used to prepare pellets. In the process of making pellets, 0.05 g of cellulose was added to 0.15 g of drill cutting powders. Cellulose was used as a bind in making of pellets in order to pellets unbroken (IAEA 1983), (Demir et al., 2006). 3. Results and discussion 3.1 Gamma ray spectroscopy results When results of the tested samples between depth the interval considered 40K has the highest activity concentration among the elements which has highest level of 0.514 Bq/g and a minimum value of 0.338 Bq/g. The activity concentration levels of 210Pb varied from 0.007 Bq/g to 0.015 Bq/g, whereas 232Th varied from 0.030 Bq/g to 0.040 Bq/g while 226Ra showed an activity concentration range of 0.0145 Bq/g to 0.012 Bq/g. According to calculated values of total activity concentration with depth, the total activity concentration has a maximum value of 0.64 Bq/g and minimum value of 0.51 Bq/g within the tested depth interval and moderately consistence throughout the 3020-3095 depth interval Gamage et al/Journal of Tropical Forestry and Environment Vol. 8, No. 01 (2018)/ 55-63 59 Table 1: Table of Activity concentration level with error for tested depths. Table 2 consist of NORM concentration levels in oil, gas and their byproducts from IAEA safety report (2003), hence when compared that levels with NORM levels of reservoir section it implies that the activity concentration result levels in a low range. Moreover, the whole amount of the NORM in the reservoir are not mobilizing to the surface therefore the levels in the productions should be in much lower levels. According to the output of the 2k factorial design which was applied to check whether there is a significant difference test results and gamma ray log, the obtained p-value is 0.941.This p value is greater than the significance level of 5% which means there are no evidence for us to reject the null hypothesis which states that test results are equal to the gamma ray log data. Therefore we could conclude that there is no significance difference between the test results and gamma ray log. Hence this statistical analysis validate the gamma ray spectrometry test results to a certain extent, but the number of samples tested was not sufficient to conduct advanced analysis between results and gamma ray log hence the accuracy of the analysis will not be in a higher percentage. Table 2: Concentration levels of NORM in oil, gas and by products IAEA safety report (2003). Radio- nuclide Crude oil Bq/g Natural gas Bq/m 3 Produced water Bq/L Hard scale Bq/g Sludge Bq/g 238 U <0.01 — 0.0003–0.1 0.001–0.5 0.005–0.01 226 Ra 0.000–0.04 — 0.002–1200 0.1–15 000 0.05–800 210 Po 0–0.01 0.002–0.08 — 0.02–1.5 0.004–160 210 Pb — 0.005–0.02 0.05–190 0.02–75 0.1–1300 222 Rn — 5–200 000 — — — 232 Th 0.000 03–0.002 — 0.0003–0.001 0.001–0.002 0.002–0.01 228 Ra — — 0.3–180 0.0 –2800 0.5–50 224 Ra — — 0.5–40 — — Depth interval(m) Average depth K-40 (Bq/g) Pb-210 (Bq/g) Ra-226 (Bq/g) Th-232 (Bq/g) 3020-3025 3022.5 0.388±0.016 0.007±0.001 0.012±0.001 0.030±0.002 3025-3030 3027.5 0.410±0.028 0.011±0.002 0.013±0.001 0.031±0.003 3030-3035 3032.5 0.430±0.030 0.009±0.002 0.014±0.001 0.032±0.003 3035-3040 3037.5 0.401±0.028 0.015±0.002 0.013±0.001 0.031±0.003 3040-3045 3042.5 0.421±0.018 0.008±0.002 0.014±0.001 0.035±0.003 3045-3050 3047.5 0.413±0.028 0.009±0.002 0.014±0.002 0.035±0.003 3050-3055 3052.5 0.413±0.027 0.007±0.002 0.013±0.001 0.036±0.004 3055-3060 3057.5 0.420±0.028 0.0004±0.0001 0.014±0.001 0.035±0.004 3060-3065 3062.5 0.422±0.028 0.009±0.002 0.014±0.001 0.037±0.004 3065-3070 3067.5 0.406±0.029 0.009±0.002 0.014±0.001 0.036±0.004 3070-3075 3072.5 0.422±0.029 0.011±0.002 0.014±0.001 0.038±0.004 3075-3080 3077.5 0.435±0.030 0.011±0.002 0.015±0.001 0.040±0.004 3080-3085 3082.5 0.426±0.030 0.012±0.002 0.014±0.001 0.035±0.002 3085-3090 3087.5 0.514±0.036 0.009±0.002 0.012±0.001 0.031±0.003 3090-3095 3092.5 0.419±0.029 0.008±0.002 0.012±0.001 0.035±0.004 60 3.2 XRF test results According to the XRF test results levels of significantly hazardous elements are considerably in the lower range, except the level of the Barium (Ba). Pb varies between the 48 ppm to 22.2 ppm. The Thorium level varies between 9.6 ppm to 10.1 ppm. Manganese has a range of 5,173.333 ppm to 653.3333 ppm. The barium levels are between 118,666.7 ppm to 24,400 ppm. Element 3,090-3,095 (m) 3,075-3,080 (m) 3,060-3,065 (m) 3,045-3,050 (m) 3,030-3,035 (m) K 19,466.670 20,400.000 17,600.000 15,200.000 13,600.000 Ca 190,666.700 165,333.300 148,000.000 137,333.300 137,333.300 Ti 5,773.333 3,186.667 3,000.000 0.000 0.000 BA 24,400.000 106,933.300 98,133.330 112,666.700 118,666.700 V 570.667 1,973.333 1,866.667 2,826.667 3,413.333 CR 0.000 0.000 0.000 0.000 241.333 CE 0.000 0.000 2,693.333 0.000 0.000 MN 653.333 654.667 5,173.333 4,386.667 4,093.333 FE 51,866.670 53,600.000 38,666.670 31,600.000 31,466.670 CO 181.333 115.733 78.000 96.933 0.000 NI 156.000 82.667 102.533 64.533 54.533 CU 50.533 73.467 326.667 314.667 254.667 ZN 77.467 165.333 961.333 849.333 801.333 GA 32.267 24.800 116.667 105.600 95.333 AS 41.067 0.000 0.000 0.000 0.000 PB 22.267 39.467 48.000 41.333 46.000 BR 13.600 12.933 4.640 5.187 6.493 RB 68.000 67.733 43.067 36.267 35.867 SR 540.000 546.667 458.667 418.667 458.667 TH 9.653 10.133 0.000 0.000 0.000 Y 11.973 22.000 9.733 8.000 8.493 As clearly visible in Figure 2 Ca, Ba, Fe has considerable levels of element concentrations in tested samples. Only the Barium levels are considerably higher than expected international levels. This can be due to the effect of drilling mud mixed with the drill cuttings as the samples were only dried without washing. When preparing drilling mud, BaSO4 is added to the fluid mixture to increase the density hence this indicates that drilling mud contains very high amount of Ba. Table 3: Element concentrations in ppm according to the depth. Gamage et al/Journal of Tropical Forestry and Environment Vol. 8, No. 01 (2018)/ 55-63 61 Figure 2: Element concentrations in ppm with depth. 4. Conclusion When the Gamma-ray spectrometry taken in to the consideration, the activity concentrations levels of 40K are most prominent and considerably higher than the other NORM elements. 210Pb, 226Ra and 232Th levels do not vary considerably throughout the depth range tested. Further, the accuracy of the test results of Gamma ray spectrometry were confirmed by the final result of the statistical comparison with gamma ray log data, as P value is higher than 5% it indicates these is no significant difference between gamma ray log and experimental radiation levels. Since the tested sample is small. The results indicates the NORM levels of the Dorado gas reservoir sand section is low reasonably to the sedimentary rocks found around the world, hence the mobilizing amount of NORM levels to the earth surface have to be lower than the levels of the reservoir which are significantly in the lower side when compared to the IAEA levels, Hence if appropriate production and waste disposal procedures can be implemented such as treatment of produced water before disposal or reusing in well operations, procedure for disposal of sludge and solid waste with minimum environmental impact, health and safety regulations for workers in the oil and gas fields according to the American Petroleum Institute and International Atomic Energy Agency standards (Environmental Protection for Onshore Oil and Gas Production Operations and Leases, 2009), (Overview of exploration and production waste volumes and waste management practices in the united states, 2000), (Guidelines for Commercial Exploration and Production Waste Management Facilities,2001) and IAEA (2010), (2003) then the amount of contaminations from NORM can further be minimized. (Ratnayake et al., 2017). Moreover, when considering the XRF results, apart from NORMs the attention should be given to the other non-radioactive hazardous element concentrations as well since the long term accumulations can pose a threat in the future. As clearly visible, only Ca, Ba, Fe has considerable levels of element concentrations in tested samples and apart from that the Barium levels are 62 considerably higher than expected international levels. This can be due to the effect of drilling mud mixed with the drill cuttings as the samples were only dried without washing. When preparing drilling mud, BaSO4 is added to the fluid mixture to increase the density. Therefore the disposal of the drilling mud and handling of the drilling mud should be conducted in a careful manner since extremely high Ba levels can potentially cause health problems. Accepted barium levels are varied between 15 ppm to 3,500 ppm (Tox Guide TM for Barium, 2007). The acceptable levels can be varied around the world according to the geographical conditions and environmental sensitivity. Therefore conducting the both of these testing for drill cutting samples obtained from the other remaining wells to get a generalized value range for sedimentary rocks in the Mannar basin, Sri Lanka would be vital for the future proceedings. Acknowledgement Special gratitude to petroleum resources development secretariat for providing required data and financial funds to conduct the research successfully. References Canadian Association of Petroleum Producers (2000) Naturally Occurring Radioactive Material (NORM) guide. Canada, pp.9. Demir, F., Budak, G., Baydaş, E. and Şahin, Y. (2006). Standard deviations of the error effects in preparing pellet samples for WDXRF spectroscopy. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 243(2), pp.423-428. Environmental Protection for Onshore Oil and Gas Production Operations and Leases (2009) Available at: http://www.api.org/~/media/files/policy/exploration/api_rp_51r.ashx. Godoy, J.M. and Petinatti da Cruz, R. (2003) ‘226Ra and 228Ra in scale and sludge samples and their correlation with the chemical composition’, Journal of Environmental Radioactivity, 70(3), pp. 199–206. Gray, P.R. (1993) ‘NORM contamination in the petroleum industry’, Journal of Petroleum Technology, 45(01), pp. 12–16. Guidelines for the management of Naturally Occurring Radioactive Material (NORM) in the oil & gas industry Report No: 412 (2008). Available at: ftp://ftp.consrv.ca.gov/pub/oil/SB4DEIR/docs/HAZ_IAOGP_2008.pdf. Guidelines for Commercial Exploration and Production Waste Management Facilities (2001) Available at: http://www.api.org/~/media/Files/EHS/ Environmental_Performance/E_P_Waste_Guidelines.pdf. IAEA (2003) Radiation Protection and the Management of Radioactive Waste in the Oil and Gas Industry. Safety report series. IAEA (2010) Radiation protection and the management of radioactive waste in the oil and gas industry, Training course Series No.40. Available at: http://www-pub.iaea.org/MTCD/publications/PDF/TCS-40_web.pdf IAEA (1983) Sample preparation techniques in trace element analysis by X-ray emission spectroscopy, Vienna: pp.112-113.Available at: http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/15/022/15022526.pdf [Accessed 23 Apr. 2018]. Omar, M., Hamzah, M.S. and Wood, A.K. (2008) ‘Radioactive disequilibrium and total activity concentration of NORM waste’, J NUCL. & Rel. TECH, Volume 5(2), pp. 47–56. Overview of Exploration and Production Waste Volumes and WasteManagement Practices in the United States (2000) Available at: http:// www.api.org/~/media/files/ehs/environmental_performance/icf-wastesurvey-of-eandp-wastes- 2000.pdf?la=en. Prds-srilanka.com. (2017). PRDS Sri Lanka - Origins. Available at: http://www.prds- srilanka.com/exploration/origins.faces PRDS (2013) PRDS Sri Lanka - regional geology. Available at: http://www.prds- srilanka.com/exploration/mannarBasin.faces;jsessionid=D7E49C8B71958661FA9A0E253AC A7%20D1C.jvm1. Gamage et al/Journal of Tropical Forestry and Environment Vol. 8, No. 01 (2018)/ 55-63 63 Ratnayake, R. M. T. S., Gamage, S. S. N., Senadhira, A. M. A. D. M., Weerasinghe, D. A. and Waduge, V. A.(2017). NORM analysis of the reservoir sand section in the Dorado natural gas discovery, Mannar basin offshore Sri Lanka. Journal of the Geological Society of India, 89(6), pp.683-688. ToxGuide TM for Barium. (2007). [ebook] Atlanta, GA: Agency for Toxic Substances and Disease Registry. Available at: https://www.atsdr.cdc.gov/toxguides/toxguide-24.pdf [Accessed 9 Sep. 2017].