Abstract

Many salinity sources other than petroleum brines have caused salinization of water resources in oil and gas fields. These include natural saltwaters from dissolution of evaporites, formation waters not associated with petroleum production, salinization of water by evapotranspiration concentration, seawater intrusion, salt mine wastes, dissolved salt used for deicing and conventional water softeners, waste leachates, and chemicals such as hydrochloric acid. Correct differentiation of these saltwater sources is necessary for application of appropriate remediation, protection, and management actions. Methods are presented that more clearly differentiate saltwater sources than previously used procedures.

Conservative mixing relationships for freshwaters and suspected sources can be used to identify salinity sources and estimate chloride concentration contributions from multiple sources. The methods found most useful involve examination of changes in Br/Cl and SO₄/Cl ratios with increasing chloride concentration relative to conservative mixing curves. Bromide and chloride are generally the most conservative constituents dissolved in water and often provide the most conclusive identification. Sulfate and chloride are also relatively conservative in most water resource systems, although the effects of calcium sulfate precipitation and dissolution should be considered in instances such as evapotranspiration concentration. Comparison of observed Na/Cl and (Ca + Mg)/Na ratios to those based on conservative mixing of waters can indicate the relative age of saline water and help identify recent contamination versus natural saltwaters. Cation exchange on clays decreases Na/Cl and increases (Ca + Mg)/Na ratios in saltwater recently flowing through clastic aquifers in comparison with softening often observed during long-term flushing of salinity. Case histories of natural saltwater, evapotranspiration concentration salinization, and water softener salt sources are compared to oil and gas brine contamination, including differentiation between two different brines.