Abstract: Geologic and Geochemical Controls on Radioactivity in Produced Water and Scale

FISHER, R. STEPHEN

Water produced from oil and gas wells contains natural radioactivity ranging from detection limits to levels found in uranium mill tailings. In some cases, public concern has arisen because of this natural radioactivity in produced water and scale. Controls on radioactivity in oil and gas reservoirs were examined to help predict high levels of naturally occurring radioactive materials (NORM) from geologic and geochemical information.

NORM in oil and gas operations is caused mainly by ²²⁶Ra and ²²⁸Ra in water, decay products of ²³⁸U and ²³²Th, respectively, in reservoir rocks. A literature survey showed that U and Th concentrations in typical reservoir strata are <20 ppm, whereas igneous and metamorphic accessory minerals, black shales, and asphaltite can have 1,000 to 10,000 ppm U and Th. Therefore, reservoirs containing igneous, metamorphic, or volcanic rock fragments, black shales, or asphaltite will produce more Ra than reservoirs lacking U- and Th-enriched materials. Analyses of 158 produced-water samples from Texas, Louisiana, and New Mexico showed no correlation between Ra and basin type or location, gross reservoir lithology, production depth, or major solutes. However, high Ra activities (>200pCi/L) were found only in water having >35,000 mg/L total dissolved solids, demonstrating that Ra requires moderate to high salinity to remain in solution. These results suggest the potential for high NORM accumulation can be predicted if detailed reservoir composition and formation water salinity are known.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky