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Technically Enhanced Naturally Occurring Radioactive Materials in Coal Seam Gas Production-A Review

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18/05/20 Sciences Reference this

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Abstract

Coal seam gas (CSG) is an unconventional gas production process as global demand for cleaner energy. It is important to understand the inherent radionuclides in CSG and their enhancing mechanisms for better naturally occurring radioactive materials (NORM) management. The enrichment of such radionuclides is a potential risk to health and environmental exposure.  This review endeavours to explore the sources of NORM, as well as potential technical enhancement routes for these radionuclides and their progenies in CSG production.

 

1         Introduction

It is well known the presence of trace amount of naturally occurred radioactive materials (NORM) in coal and overburden. The NORM from coal can be further concentrated during coal mining and its further use. For example, fly ash from coal combustion is a typical route for technically enhanced naturally occurring radioactive materials (TENORM) of coal (Boukhair, et al., 2016). The management strategy and regulation for the coal industry are well established (Government of Western Australia, 2010) (Australian Radiation Protection and Nuclear Safety Agency, 2005). Different to the coal mining process, coal seam gas (CSG) is a non-conventional gas extraction process, in which CSG (mostly methane) and formation water are extracted while coal and overburden largely remain undisturbed in the strata. 

The management of conventional oil and gas production was well established in the 1980s (International Association of Oil & Gas Producers, 2008) (UK Department of Energy & Climate Change, 2014).  In the United States, it is reported that about 5 million cubic feet (141,000 cubic meters) of accumulated sediment with NORM concern each year (Thompson, et al., 2015). The NORMS, which are inherent in the reservoir and strata rock, are brought to surface with gas, water and liquid hydrocarbon  (Garner, 2017) (Bou-Rabee, Al-Zamel, Al-Fares, & Bem, 2009). They are further concentrated in the waste stream, which poses greater environmental and personal health risks (Mously & Cowie) (URS, 2005).

As an evolving unconventional gas process, the risk of the NORM in CSG production is not well understood. The author believes that the experience and lesson learnt in conventional oil and gas industry are invaluable for CSG (Smith, 1992).  However, natural radionuclides and their decaying progenies are present and their enhancing mechanisms in CSG need further exploring (Khan & Kordek, 2014), which may lead to potential excessive personal exposure and accumulation of regulated radioactive waste (CSIRO, 2017).  It is critical to understand the origin of radionuclides and their enhancing mechanisms for better  NORM management.

2         Radionuclides in coal and CSG

It is reasonable to conclude that the NORM in CSG comes from coal and overburden’s radionuclides and their progenies.  Coal normally contains trace amounts of 238U,  235U, 232Th  and 40K. These radionuclides are associated with minerals in the coal, such as sulphides or occur in the minerals making up the coal formation (McOrist & Brown, 2009). The average concentration of U and Th is studied in coal from the Bowen Basin, Queensland Australia(Boyd, 2004). Apparently, the concentration depends on the type of coal, geological formation, etc. The trace elements of Th and U in coal are listed in Table 1 (PARAMI, 2008).

Table 1 Th and U concentration in coal

Element

World Coal Average

Crustal Average

Bowen No.2

Wtd Ave Conc

vs world coal

vs crustal concentration

Thorium

0.5 – 10

5.8

5.73

Average

1.0

Uranium

0.5 – 10

1.6

1.16

Average

0.7

The  IAEA report identified 238U, 232Th, 210Pb, 210Po, 228Ra, 226Ra  and 40K  as major radionuclides in the Australian coal. The concentration range is coincident to the CSIRO report (CSIRO, 2017)  

2.1        Uranium

238U is one major NORM source in the coal. Uranium minerals usually have very low solubility in water and hydrocarbon liquid. Therefore, it will only be present in drill cuttings and sometimes found in sludges and sediment. However, uranium is usually the parent of major coal NORM progenies.  (Thompson, et al., 2015).

2.2        Thorium

232Th is another NORM source of coal. Similar to uranium, it also has low solubility in both water and liquid hydrocarbon. It will be present in drill cuttings and sludges and sediments. However, the concentration is rather low (Thompson, et al., 2015).

2.3        Radium

228Ra and226Ra are the decay products of 232Th and 238U individually. Radium mineral has high solubility in formation water and less solubility in liquid hydrocarbon. They are preferentially present in all produced water stream (Leopold, 2007). Radium is a source of external radiation dosage. Radium in water may drop out and co-deposit as scale in equipment (Chambers, 2013).

2.4        Radon

Radon is a gaseous component. It presents in coal and overburden. Radon is a decay progeny of radium and parent of progeny in further reactions (Thompson, et al., 2015). Radon may pass multiple process barrier and covert to 210Pb, 210Bi, and 210Po as particulate matters. Radon is highly soluble in liquid hydrocarbon as well (Leopold, 2007). Since little hydrocarbon condensate is present in CSG, most of the radon is either in gas stream or in the produced water stream.

Radon also has significance in the CSG liquification process. Its boiling point (-61.8oC) lies between propane(-42.1oC) and ethane(-88.6oC). Consequently, Rn is often found enriched in propane stream (Leopold, 2007).

2.5        Lead, Bismuth and Polonium

Radioactive Pb, Bi and Po are progenies of radon and thoron decay. They are often plated out in tubing, piping and downstream equipment. In the natural gas pipeline, it is normally found in pigging trash. These radionuclides are difficult to be detected externally. The only certain operations, such as internal inspection, pipeline pigging, may lead to personal and environmental exposure.

3         Radioactivity in coal and CSG

The radioactive level of coal is the overall results of the radiochemical reactions of these radionuclides and their decay progenies. The Queensland NORM guidance indicated that 232Th (Thorium -232),  238U (Uranium-238)  and their decay progenies are the major radioactivity concerns in coal mining.  The major decay chain and their progenies of U238 and  232Th are illustrated here (McOrist & Brown, 2009).  According to the code of practice, the activity concentration of 1 Bq/g is the threshold for NORM containing uranium and thorium (Queensland Government Department of Mines and Energy, 2008). 40K is also managed for personal exposure in the same guidance, although it has less significance.

3.1        Common NORM decay chain in coal

Figure 1 232Th decay chain, sourced from (McOrist & Brown, 2009). 


Figure 2 238U decay chain, sourced from (Brown & McOrist, 2009)

Extensive radioactive level survey for coal has been performed in Australia since the 1980s. It is identified that the levels of radioactivity in Australian and international coals were similar and are considered to be low  (DALE, 2006). The average NORM level in Australian coal is 370 to 400 Bq/kg, only equivalent to  1/3 of common garden soil radioactivity. The radioactivity of the main contributing radionuclides is listed in Table 2.

Table 2 Radioactivity of radionuclides in 1981 Australian coal survey (McOrist & Brown, 2009). 

Activity, Bq/kg

238U

226Ra

210Pb

228Th

228Ra

Port Augusta, SA

60

41

60

85

87

Bunbury, WA

10

12

20

30

32

Kwinana, WA

30

7

30

15

21

Gladstone, QLD

14

9

60

18

18

Callide, QLD

24

14

38

33

33

Collinsville, QLD

27

19

46

21

44

Swanbank, QLD

20

13

21

24

25

It is evident that all reported radionuclides are from 238U and its decay progenies and there is no radon reported. This is understandable that both 235U and 232Th, as well as their progenies have a relatively short half-life and difficult to be captured and analysed in the coal sample. Australian radiation protection safety guide even states that 235U decay chain and thoron (220Rn) from Thorium (232Th) contribute little significance in the NORM  level of oil and gas, although saturated Radium(226Ra, 228 Ra and 224Ra) and Radon (220Rn) dissolved in the formation water are the primary sources of NORMS (Australian Radiation Protection and Nuclear Safety Agency, 2008)

Published IEAE data showed a similar range of radioactivity for each radionuclide in Australian coal. The results are listed in  Table 3 (CSIRO, 2017)

Table 3 IEAE radioactivity of each radionuclide in Australian coal

Bq/kg

238U

226Ra

210Pb

210Po

228Th

228Ra

40K

Total activity

Australian Coal

8.5~47

11~64

20~ 33

16~28

11~169

19~24

23~140

43-1025

4         Radioactivity and potential technical enhancement in CSG production

CSG production is an unconventional gas production process. The process is illustrated in the following diagram (Figure 3). The mobility of radionuclides in CSG production is depending on the fluid chemistry in each production stage, which could be varied significantly. It is apparent that its technically enhancing path is different in each production stage.

Figure 3 CSG production: process illustration

4.1        Drilling

CSG well is well-known for its low yield rate, ranging from 0.5~ 5 TJ/day. Therefore, a large quantity of well needs to be drilled to sustain production capacity.  During the drilling stage, all radionuclides in the borehole, including coal and overburden will be brought over to surface. Normally immobilized uranium and thorium isotopes will inevitably be carried with drilling cuts. Some radionuclides are likely enriched in the mud. As drilling fluid is continuously cycled down the drill string and back to the surface and reused from well to well, it gradually takes on the salinity and the radioactivity of the formation water. Moreover, as the drilling fluid saturates and coats the drill cuttings, the characteristics of the rock will be dominated by that of the drilling fluid and brine water (Thompson, et al., 2015). It is reported that 226Ra is preferentially dissolved in brine with high chloride under temperature and pressure.

It was reported that the radionuclide concentrations were higher in horizontally drilled wells than those typically seen in conventional vertical bores. This was likely due to the greater surface area of exposure between the drilling fluids and the regions of the formation elevated in radionuclide concentration (Thompson, et al., 2015).

4.2        Field gathering and water treatment

Since gas is tightly bound to the coal and surrounding strata by hydraulic pressure and adsorption, de-watering is usually the critical step for the CSG production. The formation water and gas produced in each well are collected and transported through a field gathering system for further treatment. At this stage, U and Th, which have low solubility, are usually retained in the coal and overburden.  Only high solubility radionuclides, mainly 226Ra and its decay progenies, are carried by CSG formation water and gas as the primary source of NORMS (Varskog & Gellermann, 2012). Similar to conventional oil and gas production (222Rn), a non-polar noble gas, may dissolve in produced water (Hylland & Eriksen, 2013). Rn is also carried in the gas stream.

The NORM in the coal formation water is well recognized (Skubacz, Michalik, & Wysocka, 2011). In CSG water stream, the scale is a major NORM enhance mechanism, it may form due to change of pH, temperature and pressure. The radionuclides including Radon decay products (210Pb and 210Po), along with sulphate and carbonate, precipitate as scale or sludge in pipes and related equipment (EL-Kameesy, Diab, Ramadan, & Megahid, 2017).

Accumulated Sediments in tank bottoms and sludges is normally considered as high potential for NORM, although there are no data available on CSG production. However, in conventional oil production, it is estimated that 5 million cubic feet (141,000 cubic meters) of accumulated sediment with NORM concern each year (Thompson, et al., 2015).

RO brine is another route of NORM enhancement. In Australia, stringent environmental regulation requires CSG produced water to be further treated before its discharge and/or beneficial utilization (Khan & Kordek, 2014). Reverse osmosis (RO) technology is the current industry standard practice of CSG produced water treatment (GasFields Commission Queensland, 2014). The NORM has the potential to be enhanced in the RO’s brine stream.

4.3        Gas dehydration and transportation pipeline

CSG is dehydrated and transported through a pipeline system. The only potential NORM accumulation in this stage is Radon and its progenies. Although both 222Rn and 220Rn are potential presents, 220Rn has a reasonably short half-life of 56s and of little significance in pipeline NORM. As a gaseous component, once 222Rn passes through the compression station, its progenies can be settled and accumulated in the preferred location as particulate matters. This is the common reason that accumulation of  210Pb and 210Po in downstream piping and equipment. They may plate on gas valves, pipeline and accumulate at the particulate filter.  In the aged pipeline, pigging trash is highly susceptible to high NORM activity due to the presence of 210Pb and 210Po (Varskog & Gellermann, 2012).

The overall presence of radioactive elements in different stages of CSG production  is summarised in Table 4

Table 4  Potential presence of 238U and its progenies in CSG production

Potential presence of 238U and its progenies in CSG production

Coal and Overburden

238U; 226Ra; 210Pb 

Drilling muds

238U; 226Ra; 210Pb, 222Rn

Drilling equipment

226Ra; 210Pb,

Field gathering Pipeline

226Ra; 210Po, 210Pb 

Produced water

226Ra; 210Pb, 222Rn

Gas facility

210Po, 210Pb, 222Rn

Gas pipeline

210Po, 210Pb, 222Rn

5         Conclusion

The source of NORM in CSG and its technical enhancing mechanisms are explored. It is found that major contributors are 238U and its decay progenies while 235U and 232 Th have little contribution. The U and Th are only likely to present in drilling cuts and drilling mud, due to their low solubility. Radium and Radon are widely existing in the produced water stream. Radon has particular significance in CSG production: passing compressor station, the gaseous radionuclides can further decay into 210Pb and 210Po. These particulate matters are plated out in downstream pipeline and equipment. 222Rn may enrich in the propane stream of the liquification plant due to the similar boiling point to that of propane and ethane.

6         Reference

[1]   Australian Radiation Protection and Nuclear Safety Agency. (2008). Safety Guide Radiation protection series No.15 Management of Naturally Occurring Radioactive Material (NORM).

[2]    Australian Radiation Protection and Nuclear Safety Agency. (2005). Radiation Protection Series Publication No. 9 Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing.

[3]    Boukhair, A., Belahbib, L., Azkour, K., Nebdi, H., Benjelloun, M., & Nourreddine, A. (2016). Measurement of Natural Radioactivity and Radon Exhalation Rate in Coal Ash Samples from a Thermal Power Plant. World Journal of Nuclear Science and Technology, 153-160.

[4]    Bou-Rabee, F., Al-Zamel, A. Z., Al-Fares, R. A., & Bem, H. (2009). Technologically enhanced naturally occurring radioactive materials in the oil industry (TENORM). A review. NUKLEONIKA , 3−9.

[5]    Boyd, R. J. (2004). Trace elements in coal from Colinsville, Bowen basin, Australia : in-ground mode of occurrence and behaviour during utilisation. Townsville, Queensland: Jame Cook University.

[6]    Brown, G. M. (2009). TECHNICAL MEMORANDUM: AM_TN_2009_03_06 Assessment of Naturally Occurring Radionuclides. ANSTO Minerals.

[7]    Chambers, D. (2013). 2nd International Symposium on the System of Radiological Protection. 2nd International Symposium on the System of Radiological Protection.

[8]    CSIRO. (2017). National assessment of chemicals associated with coal seam gas extraction in Australia Technical report 5 literature review : Geogenic contaminants associated with coal seam gas operation. CSIRO.

[9]    DALE, L. (2006). ACARP No 2 TRACE ELEMENTS IN COAL. Australian Coal Research Program.

[10] EL-Kameesy, S., Diab, H., Ramadan, A., & Megahid, O. (2017). Determination of Naturally Occurring Radioactive Material in the Egyptian Oil. Arab Journal of Nuclear Sciences and Applications, 50(4), 50-62.

[11] Garner, J. (2017). Industrial radioactivity in the UK’s onshore oil and gas industry.

[12] GasFields Commission Queensland. (2014). CSG Water Treatment and Beneficial Use in Queensland Australia.

[13] Government of Western Australia. (2010). Managing naturally occurring radioactive material (NORM) in mining and mineral processing-guideline.

[14] Hylland, K., & Dag Øistein Eriksen. (2013). Naturally Occurring Radioactive Materials (NORM) in North Sea Produced Water: environmental Consequence.

[15] International Association of Oil & Gas Producers. (2008). OGP Guidelines for the management of Natually Occurring Radioactive Material (NORM) in the oil & gas industry.

[16] Khan, S., & Kordek, G. (2014). Coal Seam Gas: Produced Water and Solids: Prepared for the Office of the NSW Chief Scientist and Engineer.

[17] Lafortune, S., & Pinte, J.-C. (2010). NORM in the petroleum and geothermal industries : evolution. Helsinki: European IRPA Congress.

[18] Leopold, K. (2007). Chemical types of bounding of natural radionuclides in TENORM. Wuppertal-Heckinghausen: University Duisburg-Essen.

[19] McOrist, G., & Brown, S. (2009). ANSTO Minerals Technical Memorandum: AM_TM_2009_03_06 Naturally Occurring Radionuclides in CCP. ANSTO Minerals.

[20] Mously, K., & Cowie, M. (n.d.). Overview & Management of NORM in Saudi Aramco.

[21] PARAM, V. K. (2008). DETERMINATION OF TECHNOLOGICALLY ENHANCED NATURALLY OCCURRING RADIOACTIVE MATERIAL (TENORM) IN ASHES FROM COAL-FIRED THERMAL POWER PLANTS IN THE PHILIPPINES. Quezon City: University of the Philippines Diliman.

[22] Queensland Government Department of Mines and Energy. (2008). Guidance Note QGN12 Radiation protection from naturally occurring radioactive materials (NORM) during exploration.

[23] Skubacz, K., Michalik, B., & Wysocka, M. (2011). Occupational radiation risk caused by NORM in coal mining industry. Radioprotection, S669–S674.

[24] Smith, K. (1992). An Overview of Naturally Occurring Radioactive Materials (NORM) in the Petroleum Industry.

[25] Thompson, J., Costello, F., Fahd, Z., Forsee, G., Gavitt, S., Goretzki, B., & Patrick, D. (2015). E-42 TASK FORCE REPORT REVIEW OF TENORM IN THE OIL & GAS INDUSTRY. Conference of Radiation Control Program Directors.

[26] UK Department of Energy & Climate Change. (2014). Strategy for the management of Naturally Occurring Radioactive Material (NORM) waste in the United Kingdom.

[27] URS. (2005). Radioactive Waste Store Feasibility Study – Stage Two.

[28] Varskog, P., & Gellermann, R. (2012). NORM Waste Streams in the Norwegian petroleum Industry. Dresden.

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