SORPTION OF AS, SE, SR, AND BA IN PETROLEUM PRODUCED WATER ON THE SURFACE OF DOLOMITE

Petroleum produced water (PPW) from unconventional oil and gas resources contains high concentrations of toxic elements (including heavy metals, metalloids and nonmetals). PPW is frequently disposed via injection into deep saline dolomitic formations. The well barrier or integrity failure of wells could be as high as 75% in some areas. In some cases the disposed PPW may migrate from disposal sites and reach conductive fault zones and/or existing wells with casing and/or cementing failures, which could pose an environmental risk to potable groundwater. In this study, we investigated the effect of brine salinity and competing cations (Ca+2 and Mg+2) on the sorption and transport of Arsenic (As), Selenium (Se), Strontium (Sr), and Barium (Ba) in PPW in dolomite rocks. Batch experiments were conducted to investigate the effect of salinity and competing ions on sorption of these toxic elements under room/subsurface temperatures. Powdered dolomitic rocks (500-600 µm particle size) were used for batch experiments. The starting pH of these experiments was ~6. Batch sorption experiments revealed that the sorption of most of the tested elements on dolomite greatly decreases with increasing brine salinity (0 – 90,000 mg-NaCl/L) and with increasing concentrations of Ca (5,000 mg/L) and Mg (1,000 mg/L). This is not only because of increasing competition of ions over the available surface sites for sorption of tested elements, but also is due to the chloro-complexation reactions between elements and Cl ions in the aqueous phase. Increases in the pH of the solution due to the dissolution of dolomite served to increase sorption of the cationic elements by introducing more negative sites on the dolomite mineral surfaces. The experiments are currently underway to examine As and Se behavior under similar conditions. The partition coefficients and reactive transport models (using surface complexation) were applied to interpret the obtained experimental results. The presented results are important in understanding the fate of toxic elements present in PPW disposed into deep saline aquifers.