Ion Chromatographic Determination of Sulfate Ion in Deep Subsurface Water and Subsurface Brine

Ion species contained in water samples accompanying produced oil and natural gas (deep subsurface water, subsurface brine, etc.) are generally measured by ion chromatography. However, abnormalities in the determination of low-concentration sulfate ion have been confirmed, and problems still remain unresolved. The high-salinity of those solutions is mentioned as being a cause. According to the literature, it was reported that abnormal quantitative values have occurred in samples containing high salinity. Therefore, we aimed to accurately measure the sulfate ion concentration of deep subsurface water and subsurface brine samples containing high salinity in this study. We made sure that in the sample-dilution experiments, the shift of the retention time and the decrease of the measured values had taken place due to the effects of the high-salinity of the solutions. We then perfoimed cation-removal tests with H+-type cation exchange resin, and also with OnGuard II cartridge-Ag/H. It was confirmed that there were no problems in the recovery of anions from standard solutions and actual samples. We carried out cation-removal experiments without dilution, because the actual samples used in this study were low in sulfate-ion concentration. As a result, we were able to obtain sulfate ion concentrations higher by 140-491 % than those without any pretreatment. Based on the pretreatment we proposed appropriately, we think, that it is possible to accurately determine the sulfate ion concentrations of deep subsurface water and subsurface brine samples.