NORM dans le secteur du pétrole et du gaz

Les eaux de formation des réservoirs de pétrole et de gaz contiennent sous forme dissoute une certaine concentration en radium (Ra-226 et Ra-228) et ses produits de désintégration. Ces radionucléides sont entraînés à la surface lors du pompage du pétrole et les modifications de pression et température qui en résultent peuvent conduire à la formation de scales – des précipités solides de sulfates ou de carbonates. La fraction de radium qui n’est pas précipitée sous forme de scales se retrouve dans les boues d’extraction. Par-ailleurs, le gaz radon (Rn-222) présent dans la formation sera entraîné à la surface avec la fraction gazeuse. Bien qu’il n’y ait pas en Belgique d’activités d’extraction de gaz et de pétrole, les secteurs du transport du gaz naturel et du raffinage du pétrole peuvent également être affectés.

Processus à investiguer

- Transport du gaz naturel : Les produits de désintégration du radon, Pb-210 et Po-210, sont susceptibles de former des dépôts sur les faces internes des équipements de transport : canalisations, valves de contrôles, compresseurs, pompes, etc. Les concentrations en Pb-210 peuvent atteindre ~ 200 Bq/g.

- Secteur du raffinage : Des concentrations significatives de substances radioactives naturelles dans les boues de fond de cuve se rencontrent parfois: ces concentrations sont très variables mais des valeurs de 7 Bq/g en Ra-226 et de 19 Bq/g en Pb-210 ont déjà été rapportées. Les catalyseurs usés peuvent également présenter une concentration en radionucléides significative. Points d’attention – radioprotection des travailleurs Les opérations de maintenance sur les éléments « à risque » : - démontage et remplacement des valves et autres équipements du transport de gaz naturel;

- nettoyage des fonds de cuve dans les raffineries. L’identification d’éventuelles contaminations doit se faire préalablement aux opérations de maintenance.

Points d’attention – gestion des résidus - Mesure de la radioactivité des fonds de cuve, catalyseurs usés, ou autres déchets « à risque » avant leur valorisation ou traitement.

- Si réalisable, décontamination des équipements contaminés avant leur réutilisation ou recyclage.

 ORIGIN AND RADIOLOGICAL CHARACTERISTICS OF NORM

The radionuclides identified in oil and gas streams belong to the decay chains of the naturally occurring primordial radionuclides 238U and 232Th. These parent radionuclides have very long half-lives and are ubiquitous in the earth’s crust with activity concentrations that depend on the type of rock. Radioactive decay of 238U and 232Th produces several series of daughter radioisotopes of different elements and of different physical characteristics with respect to their half-lives, modes of decay, and types and energies of emitted radiation (Figs 22 and 23, and Table I) [50]. Analyses of NORM from many different oil and gas fields show that the solids found in the downhole and surface structures of oil and gas production facilities do not include 238U and 232Th [49]. These elements are not mobilized from the reservoir rock that contains the oil, gas and formation water (Figs 22 and 23). The formation water contains Group II (Periodic Table) cations of calcium, strontium, barium and radium dissolved from the reservoir rock. As a consequence, formation water contains the radium isotopes 226Ra from the 238U series (Fig. 22) and 228Ra and 224Ra from the 232Th series (Fig. 23). All three  radium isotopes, but not their parents, thus appear in the water co-produced with the oil or gas. They are referred to as ‘unsupported’ because their long lived parents 238U and 232Th and also 228Th remain in the reservoir. The 228Th radionuclide sometimes detected in aged sludge and scale is likely to be present as a product of the decay of the mobilized 228Ra. When the ions of the Group II elements, including radium, are present in the produced water, drops in pressure and temperature can lead to the solubility products of their mixed sulphates and carbonates being exceeded. Referring to Fig.24, this causes their precipitation as sulphate and carbonate scales on the inner walls of production tubulars (T), wellheads (W), valves (V), pumps (P), separators (S), water treatment vessels (H), gas treatment (G) and oil storage tanks (O). Deposition occurs where turbulent flow, centripetal forces and nucleation sites provide the opportunities. Particles of clay or sand co-produced from the reservoir may also act as surfaces initiating scale deposition or may adsorb the cations. If seawater, used to enhance oil recovery, mixes with the formation water, it will increase the sulphate concentration of the produced water and enhance scale deposition. Mixing may occur in the formation if ‘breakthrough’ occurs, which will result in scale deposits in the well completion, or the waters may be combined from different producing wells and mixed in topside plant and equipment. The mixed stream of oil, gas and water also carries the noble gas 222Rn that is generated in the reservoir rock through decay of 226Ra. This radioactive gas from the production zone travels with the gas–water stream and then follows, preferentially, the dry export gases (Fig. 22). Consequently, equipment from gas treatment and transport facilities may accumulate a very thin film of 210Pb formed by the decay of short lived progeny of 222Rn adhering to the inner surfaces of gas lines. These 210Pb deposits are also encountered in liquefied natural gas processing plants [33–37]. A quite different mechanism results in the mobilization, from the reservoir rock, of stable lead that contains relatively high concentrations of the radionuclide 210Pb. This mechanism, although not well understood [49], has been observed in a number of gas production fields and results in the deposition of thin, active lead films on the internal surfaces of production equipment and the appearance of stable lead and 210Pb in sludge. Condensates, extracted as liquids from natural gas, may contain relatively high levels of 222Rn and unsupported 210Pb. In addition, 210Po is observed at levels in excess of its grandparent 210Pb, indicating direct emanation from the reservoir