Ukrainian oil field NORM contamination examined

Nikolay E. Zhuravel
SVNC Intellect-Service Ltd.
Kharkov, Ukraine

A water-supply well was found to be a significant contributor to some of the high radiation levels of mineral scales found in oil well tubulars in northeastern Ukraine.

In other cases the radiation was linked to lithological and geochemical properties of the aquifers in this area.

The aquifer rocks are the source for the chemicals that form mineral deposits. In particular, the presence of uranium-bituminous aggregates determines the radiation levels and isotopic composition of mineral scale inside pipe.

In the Dnieper-Donets basin, uranium-bituminous aggregates are related predominantly to zones with disjunctive faults. Therefore, wells within these areas are more likely to encounter radioactive nuclides.

Radioactively contaminated equipment is observed mostly in oil fields that have produced for a long time. The radiation is found in high water cut wells during the later stages of a field's life, as well as in waterfloods.

Radiation

The problem of natural radioactive nuclides carried by produced water and oil back to the surface has been discussed for several decades, especially in the U.S.

Studies have found that radium and thorium salts produced with oil, over decades, have contaminated some oil fields in Louisiana, California, Pennsylvania, and Alaska. Radium salts have been detected in scale on the inside of tubulars, pumps, and storage tanks.¹

The problem is important because radioactively contaminated equipment is a hazard for personnel.²

Radioactive contamination, from naturally occurring radioactive materials (NORM), has been measured in Northeastern Ukraine oil and gas field production equipment.³ Also, radioactive anomalies have been detected in surface soils where produced water was discharged.⁴

Dnieper-Donets depression

The oil and gas fields investigated for NORM contamination are in the Dnieper-Donets depression, one of the main tectonic features of an ancient East European platform.

The depression was formed in a paleorift, which determined the depression's structure and the composition of the deposition. The depression is believed to have distinct axial and edge zones.

Alongside the axial zone, the crystalline basement lies at a depth of 1.5 km in the northwest and at up to 20 km in the east. The sedimentary cover is formed by Devonian, Carboniferous, lower Permian, Triassic, Jurassic, Cretaceous, and Tertiary deposits of different facies. The largest volume of oil/gas pools is contained in Carboniferous strata.

Radiation in equipment was measured in more than 30 oil and oil/gas fields with an SRP-88 type geologic radiometer.

In the Kachanovkoye oil field, tubing string samples with elevated radiation underwent spectrometer analysis with a gamma spectrometer, based on Nokia semiconductor detectors.

The samples scraped from inside of tubulars were subjected to mineralogical analysis with an ISM-1C845 scanning electron microscope. Some samples were also studied with X-ray quantitative phase analysis.

Scale mineralogy

Radiation in oil field equipment is related to mineral scale. During production, a thin brown scale with reddish or yellowish tint, 1.5-2.0 mm thickness, forms on the inside of piping and other equipment. ⁵ Near the surface, where tubing strings display low radiation levels of 150-200 microroentgens/hr (mR/hr), the mineral scale is formed predominantly by Fe-hydroxides with a small amount of barite, chalcedony, calcite, and insignificant impurities of gypsum, galena, sphalerite, and halite. But in tubulars pulled from greater depths, the mineral scale has a more complex composition. In cross-section, this scale has three distinct layers. In the Anastasievskoye oil field, tubing strings with a radiation level of 2,000 mR/hr typically displayed a three-layer structure.The first layer inside the tubing is formed by galena (PbS) containing fine round-shaped inclusions of pyrite (FeS₂), quartz (SiO₂), native nickel, colloform deposits of Fe-hydroxide, and infrequently calcite (CaCO₃). Besides these minerals, the presence of barite (BaSO₄) and ammonium chloride (NH₄C₁) is possible.⁶

Most often, galena has a feather-structure aggregate, but a more rare form has square-section crystals. Calcite is formed as extremely fine deposits associating with acicular crystals of goethite.

The second, intermediate, mineral scale layer has the appearance of an amorphous substance. A spectrogram indicated a predominance of Cl, S, and Si and distinct expressions of Ba, Fe, and Ni. Sometimes Pb and P were encountered.

The composition of the second layer is not homogeneous. As a rule, the lower part contains more Pb and Si and the upper one contains Ba and P. This points out that in the lower part there is a presence of galena, quartz, and probably pyrite and nickel compound inclusions.

The upper part contains barite, quartz, phosphates, pyrite, and nickel compounds.

The third (upper) scale layer is formed by crystalline radiobarite, with crystals less than 1 mm in size. The surface is covered with colloform brown Fe-hydroxides (hydrogoethite). Radiobarite has the form of flattened pyramidal-shaped crystals. Together with barium, sulfur, and oxygen, it contains strontium and radium that substitutes barium isomorphically. Besides barite, the third layer contains quartz and pyrite.

Based on this description, mineral scaling results from the contact of high-mineralized thermal waters with tubular surfaces against a background of varying physical and chemical properties. The most important factors determining the mineral composition of scale are thermodynamic conditions and water aeration.

The system is influenced by the pressure gradient. Pressure decreases control the mineral scaling.

As pressure decreases, many complex ions decay, and as a result, a deposition of impurity components from a sulfide solution, particularly galena, is possible.

For higher-pressure gradients, sulfide-type mineral scaling is replaced with sulfate type. In this case, barium plays the role of the cation.

Barium is one of the most widespread impurity components of chloride waters. The zones in the scale structure, such as the transition from galena to barite accumulations, reflect the dynamic conditions in the system.

The described effect resembles geochemical zones of ore-bearing hydrothermal systems. In such systems, barite accumulations are in an outer layer and galena accumulations in a sub-layer.

If there is a constant relationship between the ore-forming solution and thermodynamic condition, the deposition of certain mineral substances depends on the temperature and pressure at a specific point.

As distance increases from the substance source, the point will have a lower temperature and pressure. And in case of tubulars, the same role is played by time due to high-rate fluid flow. Flow dynamics change as the field is developed. Mineral zones are formed not in a vertical orientation, collinear with fluid flow, but in a horizontal orientation in the form of scale on the inside of pipe. Temperature, in this case, is not an important consideration.

Radionuclide composition

First of all, mineral scale radiation inside tubulars is related with thorium and radium. Thorium is always present in hydrogoethite, whereas calcite deposits do not contain thorium. The greatest amount of thorium is measured in colloform goethite.

The more crystallized lentil-shaped deposits of Fe-hydroxide contain less thorium. Thorium is practically absent in flocculent and acicular aggregates of hydroxides.

In high-radioactive mineral scale, ²²⁶Ra is found in radiobarite. In low-active scale ²²⁶Ra is found in galena and in RaCl₂.⁶ Radium impurity is not identified in low-active sulfate mineral scale of barite and gypsum. Also, radium was not detected in Fe-hydroxides.

On average, about 70% of the total radiation is related with the ²³⁸U (²²⁶Ra, etc.) decay series and the remainder with the ²³²Th decay series.

Local anomalies

Almost all production equipment in the oil and gas fields on the northern edge of the Dnieper-Donets depression have been tested for radiation. Radioactive contamination was found in localized areas that can be correlated with geologic or man-made causes ( Fig. 1 [132,629 bytes]).

Radioactive anomalies on the north edge of the Dnieper-Donets depression correlate with geological features. In this area, the important structural feature is the abyssal boundary fault that is a kind of seam, limited by parallel fractures. In the boundary fault zone, six fractures were identified.⁷

In the zone, boundary fault displacement varies from 0.75 to 4.9 km. The boundary fault zone influences sedimentary cover, as well. The sedimentary cover has 0.2-2.0 km long fractures in Devonian and Carboniferous deposits. In younger complexes these fractures are expressed as dislocation zones and small-amplitude fault zones.

Man-made causes also control radioactive contamination. The highest radiation from production equipment was measured in the oil fields that have been producing for a long time. These include the Glinsko-Rozbyshevskoye, Kach-anovskoye, and Rybalskoye fields.

The average radiation measured in production equipment in these fields is about 60-150 mR/hr, but localized rates can be much higher, such as the 6,000 mR/hr measured in the Kachanovkoye, Rybalskoye, Artyukhovskoye, and Anastasievskoye fields.

These higher radiation levels may be related to the mineral content in produced water injected for reservoir repressurization.

Radioactively contaminated equipment, in the central graben of the Dnieper-Donets depression oil fields, may also be caused by the infrastructures for transporting, gathering, and treating the production from the Anastasievskoye, Artyukhovskoye, and other oil fields.

Because this infrastructure ties together a number of fields, it may explain the high level, up to 1,000 mR/hr, of radiation measured in the Glinsko-Rozbyshevskoye, Gnedintsevskoye, and Talalaevskoye fields.

The distribution of radioactive contamination within an oil field can be illustrated by the Bugrevatovskoye oil/gas field. This field and other fields producing for a long time in the Dnieper-Donets depression are being repressured with water from a Carboniferous formation. The water contains various minerals.

In general, the Bugrevatovskoye field's 8-14 mR/hr natural background radiation is only slightly different from other areas, but the field does have localized radioactive contamination that is much higher. The highest readings, up to 390 mR/hr, were measured in the pipe transporting produced water from water-supply wells to the pumping station. Elevated radiation levels were also measured on wellhead equipment of the 7 water supply wells and 14 injection wells in the study.

Elevated radiation levels were found more on injection-well equipment rather than on water-well equipment. Also, the location of the injection wells did not matter because the water is supplied from a pumping station that has equipment contaminated with natural radionuclides. Water-supply wells are the only possible source for the radionuclides.

Well N151 was the only water supply well found to have wellhead equipment with a high level of radiation contamination (Fig. 2 [112,202 bytes]). This well differs from the other water-supply wells in that it is drilled into the major north-east fault that forms a limit to the tectonic blocks forming the Bugrevatovskaya anticline.

Radiation anomalies also appear in wells drilled in major faults of other oil fields.

Radionuclide sources

Radiation anomalies of various intensities have been detected while drilling in the Paleozoic and Mesozoic section of the central part of Dnieper-Donets depression. Several cores, from depths up to 2,500 m, have contained uranium-bituminous mineralization.

Uranium-bituminous mineralization is found in the Triassic, upper and lower Permian, and upper and part of the mid-Carboniferous.⁸

The highest radiation is related to upper Carboniferous sandstone. Most of the radiation anomalies are in the blocks separated with tectonic faults. Probably, these disjunctive regions control uranium mineralization. Also, elevated concentrations of zinc, barium, and strontium were detected in ore-type rocks.

Based on the wells in the Bugrevatovskoye field, we can assume that the main source of radionuclides contaminating production equipment is mineralized water extracted from upper Carboniferous zones that have a tectonic weakness.

Acknowledgment

This article is based on a study done by the author: V. Shumlyansky, and M. Bezuglaya that was sponsored by NGDU The author thanks Nikolay Lylak, NGDU Okhtyrkanaftogaz, and A. Vasilyev, State University of Kharkov, for discussing their research and special thanks also to A. Semenyutin, Ukrniiskhr, Kiev, and Alexander Khoroshun, NG DU Okhtyrkanaftogaz, for their critique of this article.

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The Author