Characterization of Naturally Occurring Radioactive Material (NORM) Generated from the Lower Cretaceous Carbonate Formations in the Arabian Peninsula and Gulf

For most of the petroleum reservoirs in the world, the impact of the mother nuclides such as uranium-238 on the deposition of radioactive materials a.k.a. NORM from the formation and particularly as mineral scales adherent to tubulars and other equipment is considered insignificant. Uranium is regarded as insoluble in the reservoir formation and the prevailing understanding is that dissolved radium is not generated necessarily by U and Th in solution. Consequently, “uranium scale”, as a technical term, seems to be non-existent in the oil and gas industry. However, gamma spectroscopy and chemical analysis of NORM scale samples of carbonate origin, performed in the course of this research, has proven that they actually can contain uranium. A fundamental relationship between uranium and vanadium in scales is presumed. The highest specific activity of 238U in scale analyzed with gamma spectroscopy was at 11.4 [Bq/g] and supplementary chemical analysis has shown concentrations of up to 740 ppm total uranium and up to 260 ppm vanadium.


Introduction
Naturally Occurring Radioactive Material (NORM), as a characteristic oil and gas waste, is usually generated in the stage of production, but occasionally also during drilling of exploration wells. The presence of NORM in the oil and gas industry has been reported earlier by various authors [1][2][3][4][5]. The source of NORM are radionuclides and particularly their water-soluble radiochemical compounds which exist naturally in the geological formation of the hydrocarbon reservoir. The primary radioisotopes of concern are of the uranium-radium decay series ( 238 U), the thorium ( 232 Th) decay series and potassium ( 40 K) which occur freely in the earth's crust [6,7]. Not frequently reflected for the evaluation of NORM is the actinium ( 235 U) decay series [8]. Radioisotopes would naturally be in secular equilibrium if remaining undisturbed for a long enough timeline, but owing to the possible migration of some radionuclides into a mineral matrix and getting deposited at different locations, these radionuclides may become "unsupported" , and hence have an activity different from their parents in the decay chain [9]. The concentrations of uranium and thorium in the earth's crust is 4.2 and 12.5 ppm, respectively [10]. Both won't appear, as per the prevailing opinion, in significant quantities in NORM samples, because they are commonly considered relatively stable, which means that they mainly occur in form of water-insoluble minerals [11].
Water-soluble compounds of naturally occurring radionuclides, particularly radium compounds, throughout their passage via naturally radioactive geological stratigraphic horizons such as bedrock and shale will leach into the formation water [12]. This is particularly the case in the Middle East, where carbonates and sandstone prevail. Water comes to the surface along with the pumped crude as an oil/produced water mixture, where the dissolved mineral salts precipitate and form less soluble minerals in scales but also in sedimentation areas as sludge [13], as shown in Fig. 1.
The main types of NORM from the oil and gas industry as shown in Fig. 1 are, therefore: Mineral scales, deposits and thin films (radon progeny) accumulate in plant equipment and particularly adherent to tubular (drill pipes, casings and tubulars); and Petroleum hydrocarbon (PHC)-sludge and scrapings from cleaning and pigging activities, which contain radioactive mineral solids.

Mineral Scale
Scale appears as a mineral layer accumulated inside pipes, separators, filters, pumps, wellheads (aka. Christmas trees) and other equipment, as a result of precipitating newly formed (formerly soluble) radionuclide compounds in the produced water. Precipitation is mainly due to changes in pressure and temperature [7], but mechanical impacts also play a role and the amount of the precipitate depends on the physical and chemical properties of the produced water (brine) [15] such as salinity, or pH level [16]. The highest concentrations of NORM are typically found in scale deposits that form under certain physical and chemical conditions when dissolved radionuclide compounds co-precipitate together with barite and celestite (BaSO4 and SrSO4, respectively) and/or aragonite (CaCO3). These sulfates and carbonate, after generation from the minerals dissolved in the brine/produced water, form insoluble scales but also deposits of high density (often confused with mineral hardness), on the inside of tubulars and production or exploration equipment. Just like calcium, strontium and barium, radium also belongs to the alkaline earth metals (2nd group of the PSE) and hence radium has geochemical affinities to similar elements. It co-precipitates with them forming radium sulfate (RaSO4) and particularly mixed sulfates within the Ra-Ba, Ra-Sr and the Ra-Ba-Sr system. These sulfate scales commonly do not contain uranium. However, under alkaline conditions radium carbonate (RaCO3) in particular, but also radium silicate (RaSiO3) may be generated [17][18][19]. Under these correspondingly alkaline conditions the formation of "uranium scale" occurs as shown as a result of this research.

NORM Sludge
Sludge is an oily residue that accumulates at the bottom of storage tanks and other production equipment, particularly in separators. Sludge is made of deposits from heavy hydrocarbons, tight emulsions, produced formation sand and small quantities of corrosion and flaking debris that settle out of suspension in some field equipment [13]. Sludge from pipeline pigging is also addressed as scrapings. NORM in sludge accumulates when radium coprecipitates with silicates and carbonates [9,20,21]. The specific activity of radium-226 in sludge can reach values of up to 3500 Bq/g [22].

Lead Films
Films, as shown in Fig. 1, are very thin layers that contain mostly the daughter nuclides of radon ( 222 Rn; t1/2 = 3.8 days) such as bismuth ( 214 Bi; t1/2 = 19.9 min), lead ( 210 Pb; t1/2 = 22.2 years) and polonium ( 210 Po; t1/2 = 138.4 days). Highly variable ranges of specific activities were measured for scales and can be as high as 15,000 Bq/g for 226 Ra [22]. 210 Pb and 210 Po are also found in films inside gas pipes, gas/oil separators, liquid natural gas storage tanks, in sludge accumulating in tank bottoms, dehydration vessels, and in waste pits in addition to crude oil pipeline scrapings [11].

Black Dust
Black dust (black powder), as a corrosion product of pipe material together with possibly adhered lead films in gas transmission pipelines, commonly contains 210 Pb but also mercury, which is associated to thiols (mercaptans) in the gas reservoir or condensate. These waste types are not subject of this paper which focusses in particular on mineral scales from oil production and exploration.

Hazards of NORM
Radioactive wastes associated with the exploration and production of oil and gas are considered carcinogenic, mutagenic and teratogenic; they are harmful to humans, animals and the environment [23]. These NORM wastes shall be managed in such a way that they do not affect human health and the environment. More recent technical options for treatment, removal and decontamination of NORM-and TENORM-contaminated equipment and debris were published earlier [17,[24][25][26]. TENORM, as a technical term, describes in particular technologically enhanced NORM, which is rather generated in specific downstream operations such as ethylene cracking etc. than in offshore operations.

Objective of this Paper
The objective of this paper is the characterization of NORM scales from carbonate-rich limestone dominated geological formations which consequently will have a different radiochemical and chemical composition, in particular, the formation of 'uranium scale', in contrast to NORM scales generated from other facies, as mentioned above.

Existence and Origin of Soluble Uranyl Minerals in Carbonate Formation Rocks
The Lower Cretaceous Formations in the Arabian Peninsula and Gulf appear to a high extent to be of carbonate origin [27][28][29][30]. From the relevant literature concerning the appearance of NORM-scales, it corresponds to the general view, that usually only the daughter nuclides of the uranium ( 238 U), thorium ( 232 Th) and the actinium ( 235 U) decay series are present. Owing to the low solubility of uranium compounds in water such as tri-uranium octoxide (U3O8) in the formation concerned [7,31] and the current understanding, that dissolved radium is not generated by U and Th in solution [32]. Consequently, the characterization as "radium-rich scale" , a.k.a. "radium scale" [6,33] is widely used, whilst the term 'uranium scale' seems to be fundamentally unknown in the oil and gas industry. Uranium in its compounds usually exists in valences of +2, +3, +4, +5, and +6. In aqueous solution, only ions having a valence of +4 and +6, exist. The redox behaviour of uranium allows to transfer tetravalent into hexavalent uranium, simply by oxidation in an aqueous solution: U 6+ is the most stable species of uranium and exists predominantly as uranyl cation (UO2) 2+ , whilst in sodic alkaline carbonate enriched solutions, U 6+ is available as uranyl tricarbonate complex [UO2 (CO3) 3] 4- (Fig. 2).
The most plausible reason for the migration of commonly stable uranium oxides into the environment and the generation of uranium-bearing NORM scales is the formation of uranyl carbonate and hydroxide minerals and their ability to be more water-soluble as mixed complex ions such as (UO2)2CO3(OH)3 - [35,36].
Limestone, dolomites and other sedimentary carbonate rocks have been considered earlier as non-uraniferous [37]. However, carbonate-cemented calcretized sediments host large surficial uranium deposits [37][38][39][40], where in particular the formation of 'uranovanadates' is considered a major criterion for the genesis of such deposits [41,42]. Relevant members of the carnotite group and other double salts of the uranyl cation are listed in Table 1.
More recent articles concerning the appearance of uranyl vanadates in carbonates and calcretes, particularly in countries of the Middle East, were published for a study area in Central Jordan by Koury et al. [46]. Pracejus and his co-workers [47] also published a paper concerning tertiary limestone and calcretes in Southern Oman. Consequently, secondary formed uranovanadate minerals may act as a source of soluble uranium in formation water, particularly in a carbonate environment. The solubility of carnotite in carbonated water is about three orders higher than in pure water [48]. Also, the existence of uranyl selenites may play a role in the generation of uranium scales.

Gamma-ray Spectroscopy
Gamma-ray spectroscopy (gamma spectroscopy) is a key technology for the quantitative study and analysis of the energy spectra of gamma-ray emitters. The measuring arrangement typically consists of a scintillator that emits   The absolute efficiency of the detector was calculated based on the activity of 40 K, using a potassium chloride sample [52], and found to be 5.024 ± 0.014% at 1460.82 keV. The efficiency curve for the NaI(Tl) detector was produced using GEANT4 simulation [53] and then normalized to match the measured value for 40 K.
The detector was calibrated by using a low activity sample, prepared in the lab using "Gas Light Lamp Mantle" containing thorium ( 232 Th), mixed with organic brown sugar, which was also used for recording the background radiation. It also showed a quite similar geometry to that of the NORM samples. The detector's resolution at 609 keV was 7.90 ± 0.05%. The radionuclides studied in this work were 226 Ra (1600 y), 214 Pb (26.8 min), 214 Bi (19.9 min), 234 Th (24.1 days) and 235 U (7.038 × 10 8 y). The spectra produced by the NaI detector indicated no evidence of 228 Th or 40 K activity beyond the critical limit.
From 26 samples analyzed, 9 samples had no detectable activity, and hence the activity of any possibly present radioisotope was below detection limits. Ten samples contained 226 Ra and its progeny, and seven of the samples contained 235 U and 238 U but not 226 Ra, 228 Ra or 232 Th and their progenies.

Survey of 226-Radium and Its Progeny For this survey, only 226
Ra and its progeny were examined, using the NaI(Tl) detector. The lines of 226 Ra at 186.211 keV (3.64%), 214 Pb at 351.9321 keV (35.6%) and 214 Bi at 609.32 keV (45.49%) were analyzed for specific activity measurement. For closed isotopic systems, the secular equilibrium between the parentdaughter radionuclide can be attained after a period equivalent to approx. 6 times 10 half-lives of the daughter nuclide, where no parent decay occurs [54] (~20h in the case of 214 Bi). Comparable activities were noted for 214 Bi and 214 Pb for the higher activity samples (5 to 10 times higher). These samples had no measurable activity for the 228 Ra progeny (daughters of 232 Th) and 40 K. Uncertainties, also shown in Table 2 (and later on, Table 3), are obtained by combining components from peak fitting and the sample's mass measurement.
Previous work studying the equilibrium in NORM samples has shown that 226 Ra activity was almost always inferred from 214 Pb and 214 Bi [55,56]. Consequently, a possible interpretation for the higher values obtained could be the interference from 235 U line at 185.715 keV (57%). (Table 2).
In Fig. 3 the 226 Ra spectrum is shown with the three lines used for activity calculation, using the example of sample SCALE-B-42.

Survey of 235/238-Uranium and Its Progeny
No significant activity is present for 226 Ra or 232 Th and their progenies in these samples. The evaluation of the 238 U activity has been done using its daughter 234 Th (24.10 d), a peak that is a combination of the lines 92.38 and 92.80 keV with intensities 2.13 and 2.1%, respectively. It was not possible to use the 1001.03 keV line from 234m Pa which is highly favoured for uranium measurement [57] due to detector resolution and its low intensity (0.842%). 235 U activity was evaluated using the line at 185.715 keV (57%). Such a spectrum is usually obtained in water treatment/purification facilities. A similar spectrum was reported for IONEX resin, a material used in filters which is 'highly selective for uranium' ( [58]; Table 3).
In Fig. 4 the 238/235 U spectrum is shown with the two lines used for activity calculation, using the example of sample SCALE-B-34.

Gamma Spectroscopy Applying a High Purity Germanium (HPGe) Radiation Detector
Owing to the limited energy resolution of NaI(Tl) detectors, the analysis can be quite difficult and hampered by the  natural limitations of the process, especially for multiplet structures where multiple gamma lines sometimes interfere with X-rays in certain peaks. Three of the already measured (with the NaI(Tl) detector) scale samples (A-67, A-68 and A-72) were re-measured at the 'Laboratori Nazionali del Gran Sasso (LNGS)', using HPGe (high purity germanium) GeDSG detector for an acquisition time of more than 24 h (Tables 4 and 5). The activity measured with the NaI (Tl) detector was close to the results of the HPGe measurement and several  [58], but also to some discrepancies in other cases [59]. Fig. 5 shows the spectra of samples measured with both the NaI(Tl) and the HPGe detector.

Chemical Analysis and Mineralogical Interpretation
In addition to a radiological examination, a chemical analysis of all relevant metals is essential for complete characterization.

Visual Pre-examination of Selected Scale Samples
The visual appearance of the same three samples analyzed by HPGe (A-67, A-68 and A-72) is shown in Fig. 6. Iron, mainly from the corrosion of steel, is the dominating metal, which is also reflected in the analysis results ( Table 7). The matrix of the scale is carbonate, mainly ferrous carbonate such as antiferromagnetic siderite FeCO3. A minor fraction appears to be goethite (α-FeOOH) which was not fully migrated into the existing carbonate matrix. Radium exists mainly as radium carbonate, namely as Ra-Ba-carbonate, and to a minor extent also Ra-Ba-sulfate which is overgrown/embedded within the mentioned Iron(II)-carbonate matrix.
With regard to the quantitative analysis, the instrument was calibrated using a certified standard solution containing 10 ppb of Li, Y, Ce, Tl. The uncertainty in this working mode (semi-quantitative analysis) is about 30% of the given concentration values. A reagent blank was also measured and its signal was subtracted from the sample signals. The results are shown in Table 7. The concentration values refer to solid dry samples.
The clearly demonstrated existence of transition metals such as copper, lead and vanadium in the analyzed scale (Table 7), allows conclusions to be drawn towards uranovanadates (see Table 1) as being a possible source. Table 9 shows estimates of the order of magnitude of relevant uranium-transition metal ratios calculated from Tables 7 and 8 and relates to the entire uranium analyzed.
An exact assignment to individual transition metals or minerals is not directly possible. However, this comparison indicates a possible trend and shows an apparently good match for sengierite, whilst carnotite seems not to be the only possible source for vanadium and the appearance of lead can be allocated to curienite, because the lead-uranium ratio of the mineral is fitting into the lead-uranium ratio of the scale samples.

Uranium Isotopic Ratio Measurements
To perform U isotopic measurements, the same solution had been analyzed. The instrumental mass bias was corrected by 50 ppb of natural uranium isotopic reference material (IRMM-3184). (Table 10). The abundance ratio of the isotopes ( 235 U/ 238 U) as a natural rule is 7.257E-03 [60]. The resulting ratio for the reference material in Table 10 of 7.262E-03 demonstrates good agreement and acceptable accuracy. The concentration values refer to the solid samples the uncertainty is about 30% of the given values Uranium-transition metal ratios calculated from Tables 7 and 8 Vanadium

Summary and Outlook
In the course of this study, it was proven several times by radiological and chemical analysis that uranium definitely exists in NORM-bearing mineral scales. The highest specific activity of 238 U analyzed in the subject scale samples with gamma spectroscopy was 11.4 [Bq/g] and supplementary chemical analysis has shown concentrations of total uranium of up to 740 ppm, which is still far below the exploitable limit of uranium mines. However, the technical term 'uranium scale' is justified. The very likely existence of uranovanadates such as carnotite K2(UO2)2(VO4)2·2-3H2O and other minerals of the carnotite group and related species as a typical appearance of calcrete hosted uranium deposits and their increased aqueous solubility in carbonate environments are considered a possible source for the generation of 'uranium scale' in oil production equipment from the Lower Cretaceous Carbonate Formations in the Middle East.
Based on the results of this study, scale removal from tubulars and equipment (descaling), which is a relevant issue in the oil and gas industry, is not possible with chemical solvents or acids owing to the insoluble siderite matrix of the scale. Consequently, wet-mechanic pipe cleaning technologies may be applied such as high-pressure jet cleaning either with or without supportive abrasives. Processing of NORM scale after removal from the equipment with the typical technologies known from ore processing such as milling and flotation [61] is conceivable, but currently fails due to low available quantities and economic viability as well as the lack of recycling and disposal options for the generated waste streams.

Funding. Open access funding provided by Montanuniversität Leoben.
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