NATIONAL AND REGIONAL APPLICATIONS OF THE USGS PRODUCED WATERS GEOCHEMICAL DATABASE

The composition of water co-produced with oil and gas can be used to answer a number of scientific, economic, and environmental questions, including determining the suitability of natural formation water for use, investigating regional hydrogeology, and establishing environmental impacts and disposal plans. This study illustrates a big data approach to understanding produced water geochemistry, including some example applications, using data from the U.S. Geological Survey Produced Waters Geochemical Database.

To provide a classification scheme tailored to produced waters, we created a cluster analysis based on relevant major and minor components. Deep basin brines generally have a restricted range of major element composition compared to surface waters, and except for coal-bed methane (CBM) reservoirs, are mostly Na-Cl dominant. Adding trace elements, which can be diagnostic of certain reservoir types, to the cluster analysis can aid in classification of the brines.

Basin-scale hydrogeologic trends for representative basins can be inferred from salinity vs. depth, Na-Cl-Br, and stable isotope relationships. For example, the Appalachian and Williston Basins show a trend of increasing maximum salinity with depth. The high salinity formations, however, also contain regions of abundant fresh water, likely due to recharge or fluid injection. The Permian and Rocky Mountain basins do not show the same salinity-depth trends, but have regions of elevated salinity where recharge waters interact with subsurface evaporites.

Understanding where heavy metals, naturally occurring radioactive materials (NORM) and organic compounds (e.g. BTEX – benzene, toluene, ethylbenzene, and xylene) are prevalent in produced waters can help inform water use and disposal plans. Seawater normalized enrichment factors of trace metals show that Pb, Hg, and Mn are enriched relative to modern seawater for all well types. Shale gas and tight oil wells show notable enrichments in Ba, Sr, and Zn. BTEX concentrations are limited almost entirely to CBM wells and are highly variable, even within single coal formations. Though there are limited NORM data, shale gas produced waters show the highest concentrations of ²²⁶Ra. These types of analyses help drive future sampling to better understand impacts and uses of produced waters.