Use of radium isotopes to determine the age and origin of radioactive barite at oil-field production sites

Radium-bearing barite (radiobarite) is a common constituent of scale and sl udge deposits that form in oil-field production equipment. The barite forms as a precipitate from radium-bearing, saline formation water that is pumpe d to the surface along with oil. Radioactivity levels in some oil-field equ ipment and in soils contaminated by scale and sludge can be sufficiently hi gh to pose a potential health threat. Accurate determinations of radium iso topes (Ra-226 + Ra-228) in soils are required to establish the level of soi l contamination and the volume of soil that may exceed regulatory limits fo r total radium content. In this study the radium isotopic data are used to provide estimates of the age of formation of the radiobarite contaminant. A ge estimates require that highly insoluble radiobarite approximates a chemi cally closed system from the time of its formation. Age estimates are based on the decay of short-lived Ra-228 (half-life = 5.76 years) compared to Ra -226 (half-life = 1600 years). Present activity ratios of Ra-228/Ra-226 in radio-barite-rich scale or highly contaminated soil are compared to initial ratios at the time of radiobarite precipitation. Initial ratios are estima ted by measurements of saline water or recent barite precipitates at the si te or by considering a range of probable initial ratios based on reported v alues in modern oil-field brines. At sites that contain two distinct radiob arite sources of different age, the soils containing mixtures of sources ca n be identified, and mixing proportions quantified using radium concentrati on and isotopic data. These uses of radium isotope data provide more descri ption of contamination history and can possibly address liability issues. ( C) Published by Elsevier Science Ltd.