Abstract

Water produced from oil, gas, and geothermal reservoirs contains natural radioactivity that ranges from background to levels found in uranium mill tailings. Radioactivity in fluids and in the scale that forms in oil-producing, gas-processing, and water-handling equipment increases concerns for worker and public safety as well as costs of handling and disposing of naturally occurring radioactive materials (NORM). This study explored natural controls on such radioactivity to identify criteria by which high NORM activity might be anticipated on the basis of geologic or geochemical information.

NORM in oil and gas operations is caused mainly by ²²⁶Ra and ²²⁸Ra in produced water, the daughter products of ²³⁸U and ²³²Th, respectively, in reservoir rocks. To explore natural controls on radioactivity, investigations were conducted on geologic and geochemical controls on U, Th, and Ra in sedimentary basins and reservoirs and Ra activity in produced water from well-characterized oil, gas, and geothermal wells.

Results suggest that small amounts of U- and Th-enriched materials within reservoirs, rather than bulk reservoir strata, are the major source of high NORM activities in formation water and that Ra activity greater than ~100 pCi/L occurs only if chlorinity exceeds ~20,000 mg/L (total dissolved solids exceeding ~35,000). These findings suggest that the occurrence of elevated NORM levels in a particular field or reservoir can be predicted if the specific reservoir lithology and formation water chlorinity are known.