Visual Methods for Geochemical Screening of Possible Impacts to Groundwater by Oilfield Brines

The primary components of oilfield brines (sodium and chloride) occur in groundwater naturally and from other contaminant sources. Therefore, sodium and chloride content alone are not reliable indicators of possible low-level oilfield brine impacts. Graphical methods used to evaluated chemical characteristics facilitate comparison of groups of samples (e.g., groundwater and brine) based on relative proportions of multiple ions. A recent investigation in Texas used histograms, Piper plots, and Stiff diagrams, to visually compare major ion content of well water with suspected brine impacts to produced water from a nearby oilfield, and to regional groundwater chemistry from a published database. Graphical comparison showed allegedly impacted well water to be geochemically distinct from oilfield brine, and similar to regional groundwater quality. To demonstrate a complimentary approach, the data from the Texas study were reanalyzed using cluster analysis, which combines multivariate statistics with simple graphics to aid interpretation of the similarities among multiple variables. Samples are clustered with similarity expressed as a simple percentage displayed on a single dendrogram plot, facilitating visual discernment of similarities or differences among multiple samples without the need to compare numerous individual plots. Reanalysis of the Texas data provide an additional line of evidence to support the conclusion that well water chemistry is not the result of anthropogenic brine impacts.