Naturally occurring radioactive materials (NORM) in water, sludge, and scale from Texas oil, gas, and geothermal wells can increase concern for worker safety as well as handling and disposal costs. We combined published data, new analyses of water samples, and geochemical modeling to explore controls on this radioactivity.
NORM are mainly caused by 226Ra and 228Ra, the alpha-decay daughter products of 238U and 232Th, respectively, in water and barite scale. Geologically short half-lives of the Ra isotopes, slow basinal flow velocities, short transport distances during reservoir production, and the distribution of U and Th in sedimentary reservoirs result in local lithologic variations being a major control on NORM levels. Ra activity correlates with salinity and chlorinity in Gulf Coast Frio formation water but not elsewhere in the state. Formation water composition does, however, determine the type and amount of scale that can form. Scale mineralogy, Ra/Ba and Ba/SO4 solute ratios, and reservoir temperature in turn determine the Ra activity of scale. Although our data base is small relative to the number of geologic, geochemical, and geographic variables, we find several associations of Ra isotope activity with reservoir and formation water characteristics. These associations can be used to anticipate NORM levels at the play, field, and reservoir scale.