Naturally Occurring Radioactive Materials (NORM) in Produced Water and Scale from Texas Oil, Gas, and Geothermal Wells

Water produced from oil, gas, and geothermal reservoirs contains natural radioactivity that ranges from background levels to levels found in uranium mill tailings. Radioactivity in fluids and in the scale that forms in oil-producing and gas-processing equipment increases concerns for worker and public safety as well as costs of handling and disposing of naturally occurring radioactive materials (NORM), which include water, sludge, scale, and affected equipment. This study explored natural controls on such radioactivity to identify screening criteria by which high NORM activity can be anticipated on the basis of geologic or geochemical information. Such criteria can help State and Federal agencies target disposal regulations for situations likely to incur high radioactivity levels and can also prevent operators from making costly measurements if the likelihood of encountering elevated NORM activity is low. NORM in oil and gas operations is caused mainly by radium-226 (226Ra) and radium-228 (228Ra), daughter products of uranium-238 (238U) and thorium-232 (232Th) respectively, in barite scale. To explore natural controls on radioactivity, we considered the (1) geographic distribution of NORM in oil-producing and gas processing equipment, (2) geologic controls on uranium, thorium, and radium in sedimentary basins and reservoirs, (3) mineralogy of NORM scale, (4) potential of Texas formation waters to precipitate barite scale, (5) radium activity in Texas formation waters, and (6) geochemical controls on radium isotope activity in barite scale. Our approach combined compilations of published data, collection and analyses of new water and scale samples, and geochemical modeling of scale precipitation and radium incorporation in barite. We found that (1) elevated NORM levels (90th percentile in both major oil- and gas-producing regions of Texas) occur where volcanic rock fragments are abundant in sandstone reservoirs, (2) lithologic or mineralogic heterogeneities within reservoirs are a major control on NORM activities in produced water, (3) barite scale is the most likely NORM host in sludge and scale, (4) the potential for barite to precipitate from produced water increases with reservoir temperature, (5) high radium activity (> 200 pCi/L) occurs only if water salinity exceeds about 35,000 mg/L total dissolved solids, and (6) the radium activity of barite scale is predicted to vary significantly between major Texas basins. These results suggest that multivariate analysis of geologic and geochemical parameters may be useful in predicting NORM activity in produced water and scale in individual basins, plays, or reservoirs.