2.7.2.3 - Radium Isotopes and Pb-210 in Scale, Deposits and Production Water
Introduction
Natural radionuclides, mainly Radium, Lead and Polonium are a minor component of rocks in the subsurface. They can be co-extracted and being subsequently trapped/accumulated whilst the life cycle of hydrocarbon production that commonly lasts for tens of years. This results that these natural radionuclides can get accumulated for a long time in form of hard scale (carbonates and sulphates) and black powder deposits (thin films of Lead), semi-liquid sludges (hydrocarbon/soli mixtures) and within the liquids (production water and hydrocarbon itselfes). Where these products accumalute on/around the equipment used during extraction (e.g. piping, storage tanks and tubing material) radiation risk can result. Problems arise by residues and sludge where such NORM often becomes concentrated during the process of extraction, transport, and storage of crude oil. Additionally, Radon is accumulated in natural gas or is co-extracted into oil as organic phase where it equilibrates with its progenies.
Thus, NORM creates a possible hazard to workers both by external radiation exposure and internal due to incorporation during intervention work, and to the environment due to waste disposal. The determination of 222Rn, 226,228Ra, 210Pb, and 210Po in the various production stages is a precondition for an efficient Radiation Protection Management in mineral oil exploitation. Liquid Scintillation has proven to provide an efficient, simple and in situ applicable tool for the measurement of NORM in the oil and gas industry [Möbius et al. 2007, 2008, 2009], see also more recently [Idoeta et al. 2017].
226,228Ra as carbonate scale may be measured after dissolution in small quantities of HNO3 and addition of a gel forming cocktail, being capable for α/β-PSD separation. Moreover, the isotopic ratio of Radium isotopes in the different scale fractions may be determined by using the modified Radium RAD Disk filter method.
The 226Ra content may be estimated directly both in solid carbonate and sulphate scale materials after grinding and Rn exhalation into an organic LS cocktail after storage. However, emanation yield with approximately 20 to 25 % is not quantitative.
Our investigations on the Rn emanation yield of scale and ore powder showed that the Rn release depends on grain size (higher for smaller grain sizes), quantity of sample material and distribution of Ra within the material. Maximum values of 30 % have been found for sample amounts lower 200 mg and 60 to 125 micrometer in grain size.
226Ra, 228Ra and 210Pb in production water can easily be determined after acidification to 2 M HNO3 and mutual separation on Radium RAD Disk filters by elution with DHC and EDTA successively using OptiPhase HiSafe III cocktail for measurement. The LS method overcomes the problems with g-spectrometry, resulting in interferences of the 186 keV 226Ra γ-line and self absorption of the low energetic 210Pb γ-emitter. However, it must be noted that production water is not commonly freshwater, but salt water or even brine. In these cases the amount of sample material needs to be adjusted (reduced) in order to overcome problems with interferences from Barium.
Materials and Equipment
Toluene Scint, MaxiLight+ (HIDEX) and BetaPlate Scint (Revvity) as organic cocktails
AquaLight+ (HIDEX) and Ultima Gold AB (Packard/Revvity) for gel samples
OptiPhase HiSafe III (Revvity) for EDTA solutions
Standard solutions of 226Ra, 210Po (209Po) and 210Pb
Multi-nuclide γ-standard (QCYB410, QSA Global)
228Ra standard (has been prepared from aged Th-nitrate)
Radium RAD Disk filters (3M EMPORE)
Grinder/mill (e.g. ball mill MM200, Retsch)
Procedures
226,228Ra as Carbonate Scale
226,228Ra as carbonate scale may be measured after dissolution in small quantities of HNO3 and addition of a gel forming cocktail like AquaLight+ or Ultima Gold AB by applying α/β-PSD spectrometry. Using the modified Radium RAD Disk filter method (procedure 2.2.1.5.), the isotopic ratio of Radium isotopes in the different scale fractions may be determined additionally.
226Ra as Sulphate Scale
Scale/ore samples for emanation measurements were prepared by weighing the grinded dry scale/ore fractions and covering it with MaxiLight+ or Toluene cocktail. Measurements were done after storage (see 2.2.1.4.) and equilibration with Rn emanation.
210Pb in Black Powder
Black Powder samples are dissolved/leached in conc. HNO3. After filtration/evaporation to dryness and dissolution in small volumes of water, they are measured by LS after addition of a gelating cocktail.
226Ra, 228Ra, and 210Pb in production water
226Ra, 228Ra and 210Pb as dissolved in production water can easily be determined after filtration from the solid residue and acidification to 2 M HNO3. The radionuclides are mutual separated on Radium RAD Disk filters by elution with 5 mL DHC and EDTA each. After addition of 16 mL OptiPhase HiSafe III cocktail, the samples are measured (see 2.2.1.5.).
The solid part after filtration can be grinded and 226Ra quantified through Rn emanation according to sulphate scale.
210Po
210Po in all samples may be determined after dissolution and extraction into a Polonium Extractive Scintillator like POLEX (3M EMPORE), if present together with other α-nuclides.
Evaluation
Data are evaluated according to the general scheme by taking into account the different measuring efficiencies (i.e. 100 % for 226Ra, 210Po, and 60 % for 228Ra and 210Pb).
Quench correction for the low energy β-emitters is advisable.
Attention should be paid to Rn-emanation from scale samples which depends on the conditions and is generally only between 20 to 30 %. Therefore, this step can only be used as estimation.
Idoeta R., Herranz M., Olondo K., Rozas S. and Legarda F. 2017: Ra-226 determination by LSC and alpha-spectrometry in by-products from oil industry; Paper ID 116, “LSC2017 Advances in Liquid Scintillation Spectrometry”, Copenhagen
Möbius R., Bartenbach M. and Veguera S. 2007: Problematik der natürlich vorkommenden Radionuklide (NORM) in der Erdöl/-gasindustrie und deren Bestimmung mittels Flüssigszintillationsmesstechnik; Proceedings LSC-Anwendertreffen, Karlsruhe 2007
Moebius S., Moebius R., Bartenbach M. and Ramamonjisoa T. 2008: Liquid Scintillation for NORM in the oil and gas industry; Paper, Proceedings IRPA12, Buenos Aires 2008
Moebius R., Bartenbach M. and Moebius S. 2009: Novel LS methods for natural radionuclides in the oil and gas industry; in: J. Eikenberg et al. “LSC 2008 Advances in Liquid Scintillation Spectrometry”, pp 397-407, Radiocarbon 2009, Tucson
