Natural Radioactive Scale: The Development of Safe Systems of Work
Authors: Waldram I.M. 
Published: December 2007

Since about 1981, operators in the northern U.K. Continental Shelf (UKCS) have become increasingly aware that scale deposited in production tubing and equipment can be mildly radioactive. Although not a production tubing and equipment can be mildly radioactive. Although not a major hazard, such scale could present a long-term risk if handled indiscriminately, and safe systems of work are necessary both offshore and onshore. Such systems must both conform with relevant legislation and be practical in typical offshore working conditions. The training of staff at practical in typical offshore working conditions. The training of staff at all levels is an important feature in the successful control of the hazards presented by naturally radioactive scale. presented by naturally radioactive scale.

Introduction

The phenomenon of scaling in both downhole and topside production equipment is well known in the oil industry where water-drive production equipment is well known in the oil industry where water-drive reservoirs are in production. Sulfate scales, consisting mainly of barium, strontium, and calcium, occur mostly downhole, whereas carbonate scales, mainly calcium and magnesium, occur on topside equipment. Table 1 shows analyses of typical samples from the Thistle field in the northern North Sea; the variation from sulfate to carbonate between downhole and topside can be seen clearly. Traces of radium and thorium may also be present in such scales. This has been reported previously (e.g., see Ref. 1) but is not always common knowledge. These substances can be detected most easily by measurement of the amount of radioactivity per gram of scale (specific activity), information that is also included in Table 1. It will be seen that the specific activity varies roughly in proportion to the barium plus strontium content: the principal effect is proportion to the barium plus strontium content: the principal effect is that radium is coprecipitated with the barium and strontium.

Uranium and thorium minerals occur widely in nature, particularly in shales and clays. This is the basis of downhole gamma particularly in shales and clays. This is the basis of downhole gamma logs, which are used to distinguish between shales and sandstones and thus to identify pay zones. The complex decay series for uranium 238 is shown in Table 2, radium 226 being one of the long-lived daughter isotopes. There are similar decay series for uranium 235 and thorium 232, the other main naturally occurring heavy radio-active elements. The complex interaction between the geology and the water chemistry of typical UKCS oil fields is outlined in Ref. 2. In summary, radium is the most soluble of the long-lived, naturally occurring heavy nuclides, more so in chloride-rich than in sulfate-rich water, and is therefore present in the interstitial water in oil-bearing strata. As this formation water is coproduced with oil, reductions in temperature and pressure as it rises to the surface tend to cause precipitation in the form of scaling. Such scaling is more pronounced in mature fields where treated seawater is injected to maintain reservoir pressure, although it has been observed even where injection is not practiced.

In the oil industry, many workers are familiar with the use of sealed radiation sources, including the concepts of classified persons, personal dose meters, barriers, and signs. High-activity persons, personal dose meters, barriers, and signs. High-activity unsealed sources are also used, for instance, to check cementing programs or reservoir flows, but direct involvement with these is programs or reservoir flows, but direct involvement with these is limited to a few specialists. Work with low-activity unsealed radioactive substances involves additional concepts and less familiar means of measurement, and there is no single number akin to 0.75 mrem/h [7.5 mu Sv/h] to mark the boundary between "normal" and "specialist" work. The phenomenon of natural radioactive scale was first identified in the northern North Sea in 1981–82. Significant effort was then necessary to develop safe systems of work and to ensure that these systems were understood by all those involved with such scale, not by just a few specialists who were already familiar with work with unsealed radioactive sources.

Hazards of Naturally Radioactive Scale

The different effects of alpha, beta, and gamma radiation on tissue must be understood clearly. In each case, the hazard is that the energy of the radiation will either kill the cell or cause it to replicate differently, perhaps to form a cancerous growth. Because alpha radiation is stopped very easily, all its energy is lost over extremely short distances. If alpha radiation is external, only the outer (dead) layers of skin can be affected; therefore, no serious consequences develop. In contrast, if alpha radiation is internal, sensitive organs can be affected quite easily. Gamma radiation is more penetrating, thus affecting more cells as it is absorbed, but each to a lesser extent. The effects of gamma radiation are therefore similar, whether the source is internal or external. Beta radiation can penetrate a few millimeters of tissue. Because the majority of sensitive organs in the body are deeper than this, beta radiation is only a minor external hazard but a significant internal hazard.

Table 2 gives details of the stages in the radium decay chain at which alpha radiation (helium nuclei) and beta radiation (electrons) are emitted. Gamma radiation is also emitted at each decay stage. The decay chain for thorium 232 similarly involves alpha, beta, and gamma radiation. Radium and thorium form only a very small fraction of the scale, which therefore falls into the category of low-specific-activity radioactive material. Thus, for such scale, there is a low-level external hazard, but a much more serious internal hazard, if the substance enters the body by inhalation, ingestion, or skin absorption. One common misconception should be noted. Low-level radioactivity cannot be induced in one substance by another. Once the scale is removed from the contaminated equipment or surfaces, there is no further hazard. The principal difficulty is to do this without creating dust that would present a significant inhalation hazard.

Maximum annual levels of intake (ALI) for various radionuclides are specified by the Intl. Commission for Radiological Protection (ICRP). Levels for the public are one-tenth of those for a radiation worker. The figures for radium 226 are detailed in Table 3, covering both inhalation and ingestion. It can be seen that ingestion is less hazardous than inhalation, basically because the lung is more sensitive to alpha radiation than the stomach. Because accidental ingestion of scale is also much less likely than accidental inhalation, safe working procedures are based on the need to avoid inhalation, and ingestion can be neglected by comparison.

Inhalation ALI's for radium 226, expressed in radiation units, can be converted to weights of dust by dividing by the specific activity (amount of radioactivity per unit weight). This is done in Table 3, with typical specific activities for scale and sand, the two common forms of contamination. Similar figures can be derived for scale or sand containing traces of thorium 232, and numbers can be factored for material of higher or lower specific activity. Note, however, that these ALI's cannot be used simply as a basis for safe working, because the overriding principle is that all exposure to ionizing radiation must be kept as low as reasonably achievable. It is also obvious that reasonably accurate analysis of the scale is an important feature of a safe system of work.