TENORM in oil and gas as industries

The human manipulation of natural occurring radioactive materials (NORMs) may lead to their concentration beyond normal background levels: TENORMs. This can result in the increased risk for human exposure. Some of the industries responsible for TENORMs release are mining, phosphate processing, metal ore processing, heavy mineral sand processing, titanium pigment production, fossil fuel extraction and combustion, manufacture of building materials, thorium compounds, aviation, and scrap metal processing. Radionuclides can be concentrated in the products, the byproducts or the waste arising from these industries.

OIL AND GAS INDUSTRIES
The industrial processes used in the oil and gas industries can accumulate the natural radionuclides of Th-232 and U-238 series in the form of scale and sludge in tubing and surface equipment⁽¹⁾. These parent radionuclides have very long half-lives and are everywhere in the earth’s crust. Analysis of the reservoir rock that contains the oil, gas and formation water showed that it does not mobilize U-238 and Th-232: uranium and thorium are relatively insoluble and remain stationary in the reservoir. The formation water contains the Group II cations calcium, strontium, barium and radium dissolved from the reservoir rock. This also includes the radioactive isotopes of radium: Ra-226 from the U-238 series and Ra-228 and Ra-224 from the Th-232 series (Figure 1). Radium is more soluble than U and Th, and may become mobilized in the produced water phase of the reservoir, while the long-lived parents remain in the reservoir. Th-228 is detected in aged

sludge and scale due to the decay of Ra-228. Drops in pressure and temperature of the produced water increase the radionuclide solubility in production fluids; this causes the precipitation of radium and the other alkaline earth metals as sulphate and carbonate scales.

Figure 1 - Th-232 (left) and U238 (right) decay series

Scales are deposited across the plant, such as on the inner walls of production tubulars, in valves, pumps and vessels (Figure 2).

Figure 2 - Oil and gas implant and sedimentation of scales in pipeline

The activity concentrations of both the sludge and the solid scales are much higher than those of the produced water, with total radium levels of 15.2 kBq/g in scale and 25.9 kBq/g in sludge. The decay of Ra-226 leads to the formation of Rn-222 that follows the dry export gases. This leads to the

accumulation of a thin film of Pb-210 adhering the inner surfaces of the gas lines. It also accumulates on the internal surfaces of the production equipment. The authorities establish a proper management for the disposal of the TENORMs rising from the oil and gas industries. Scales and sludges containing NORMs can be removed not only during the decommissioning of the plant, but also during production and for safety reasons during operation. When the accumulation of scales and sludges interferes with the rate and safety of oil and gas production, on-site decontamination can be adopted without removing and replacing the components.
There are practical measures to keep radiation doses as low as reasonably achievable and below the regulatory dose^(a) limits for workers during operation. Although external exposure^(b) due to the accumulation of gamma emitting radionuclides is unlikely to exceed the dose limits for workers (because the dose rates are usually low), in the absence of suitable control measurements, internal exposure from ingestion or inhalation could occur. Workers could be exposed to ionizing radiation during maintenance, the transport of waste and contaminated equipment, the decontamination of equipment and also during the processing and disposal of waste. These kinds of exposure sources may also arise during the decommissioning of oil and gas production facilities and their associated waste management facilities. The practical measures to limit workers exposure include the proper use of personal protective equipment and instructions for a proper behavior.

RADIATION EXPOSURE FROM NORMs AND TENORMs

In order to evaluate radiation exposure for workers in the oil and gas industry, we have to consider all the contributions people experience in everyday life⁽²⁾. Radiation has always been naturally present and all living organisms have adapted to it. Humans are always exposed to a background radiation due to the presence of NORMs in the air, ground, rock, trees, water and building materials. Even if background radiation may be different from place to place, e.g. because of the altitude, an average effective dose^(c) of 2.5 mSv ^(d) per year can be assumed. In addition, in everyday life, people are also exposed to radiation from man-made sources, leading to an annual average exposure of about 1 mSv, the main sources of this

are medical or dental X-rays. Other contributions come from non-nuclear industries, such as the burning of coal, phosphate fertilizers, luminous watches (about 0.01 mSv per year), and from nuclear sources, such as fall-out from weapons testing or the Chernobyl accident (0.002 mSv). From numerous studies the exposure rate levels for workers in the oil and gas industry are up to 300 µSv/h. Occupational doses depend on the dose rates and the working time spent during normal activities. Generally, the annual effective doses for normal activities in the oil and gas industry should be in the range of up to 2 mSv/year. The International Commission on Radiological Protection (ICRP Publication No. 103) established that the annual incremental effective dose to persons exposed to NORMs as the result of a work

practice be limited to the value of 20 mSv/year, while the public exposure limit is 5 mSv/year.

Role of radiochemists in managing TENORMs from oil and gas industries
Radiochemistry supports management of TENORMs from oil and gas industries by helping to determine radionuclide inventories in scales, sludge and other oil and gas wastes containing radionuclides. In particular, as during processing the equilibrium between radionuclides can be disturbed, it is very important to characterize specific radionuclides for which radiochemists can provide effective solutions. Mobility of radionuclides in such wastes will govern their future fate in the environment and radiochemists can determine this to aid decision making on how to efficiently manage disposal of such wastes.