7 MN1007-C
radioactive equilibrium, all nuclides will be present at the same level of radioactivity. The natural decay series are
complex and, in theory, up to 48 separate radionuclides may exist in scales which have been derived from a
mixture of the Uranium and Thorium decay products.
In reality the isotopes of uranium-235, uranium-238 and thorium are rarely detected in scales themselves. This is
thought to reflect the low solubility of such materials in the water within the reservoir. It is more usual to detect the
more soluble decay products of each series in the form of radium isotopes and their subsequent daughter products.
The result of these processes is that NORM contains many different radionuclides which emit a complex mix of
alpha, beta and gamma radiations. The relative amounts of radionuclides from the uranium and thorium decay
chains can also vary depending on the history of the reservoir and chemical composition; hence scales taken from
different locations often exhibit variations in radioactive composition.
Many of the isotopes in the various decay chains can be seen to have relatively short half-lives (minutes or hours)
and, if present in isolation, would very quickly decay and cease to generate any further radiological hazards. These
isotopes would technically be described as "unsupported" but in many cases they continue to remain present with
an unchanging level of radioactivity. This occurs because they are continuously being produced by other
radioisotopes in the decay chain such that there is a constant balance between production and decay leading to a
constant level of radioactivity; irrespective of the actual half-life. In this situation the decay scheme would be
considered to be in equilibrium and the shorter lived nuclide fully “supported” by its parent nuclide.
2.2 Radioactivity in Oil and Gas Installations
Radioactive materials from natural sources may arise in a number of forms within the oil and gas industry. A
common form of the material is known as Low Specific Activity or Naturally Occurring Radioactive Material
(LSA/NORM) scale. It frequently arises within oil and gas installations due to a process of selective chemical
deposition. Radioactive isotopes, together with a wide variety of other chemicals, may be leached from reservoir
rock and become dissolved within the reservoir formation water.
In oil and gas installations, where water injection of seawater is used to promote the recovery of oil, the presence of
sulphate-rich injection waters can give rise to the deposition of barium and strontium sulphate scales. These scales
may incorporate radioactive isotopes derived from the presence of uranium and thorium in the formation rock and
can produce significant scaling of pipework and vessels; thereby reducing the flow of liquids and affecting the
efficiency of control systems.
The laydown of radioactive scale can lead to significant external radiation levels on the outside of production
equipment. This raises the possibility of exposure to personnel from contamination by ingestion of radioactive
material when equipment is handled or cleaned during scale removal operations. Similarly, scale and silt may be
deposited in the equipment of onshore terminals where crude oil is delivered from offshore installations via marine
pipelines.
An additional problem may arise in gas production and associated handling systems from the presence of radon
gas generated by radium isotopes; both in formation rock or water and in LSA/NORM scales. This radioactive gas
can contaminate the production gas to significant levels and will decay to particulate-forming radioisotopes such as
lead-210 and polonium-210 which can also be deposited in equipment of the gas production system.
These deposits can be radiologically significant and pose potential hazards to health when equipment is opened
up. It is not uncommon for significant quantities of unsupported lead-210 or polonium-210 to be transported from a
reservoir in the gaseous phase. In such circumstances the availability of these radionuclides reflects their chemical
and physical properties and the historical levels of the radioactivity which over time have built up in the reservoir.
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