ABIOTIC AND MICROBIALLY-MEDIATED BARITE DISSOLUTION AT HIGH IONIC STRENGTH

Produced water (PW) from hydraulic fracturing of shales is composed of injected fracturing fluids and natural brines from the formation that returns to the surface during gas production. PWs are often highly saline and contain elevated concentrations of naturally occurring radioactive materials (NORMS) from the formation. We are interested in biogeochemical processes that control Ba and Ra contents in PW. Barite (BaSO₄) could be a possible source and sink for Ba and Ra within the shale gas system. This study examined abiotic and microbially-mediated dissolution of barite in simulated PW. Three bacterial enrichment cultures were established from PW collected from shale gas wells in Tioga County, Pennsylvania. These cultures were dominated by Halanaerobium species, which are anaerobic, extremely halophilic, fermentative bacteria. The enrichments were cultivated at 30ºC in an anaerobic, artificial brine medium designed to simulate the chemistry of PW. Tabular barite crystals were cleaved to expose the (001) surface, added to each culture and allowed to incubate for ~50 days. Rates of Ba release were analyzed by ICP-OES. Barite crystal surfaces were periodically observed by SEM/AFM to evaluate etch pit formation. Enhanced barite dissolution rates were observed in the presence of bacteria, as Ba concentrations increased to over 10 ppm with bacteria, but were only 2 ppm in controls. Distinct etch pits were observed on barite surfaces. In treatments with bacteria, the pits had rectangular shapes, were smaller than 10 μm in length, and most showed diagonal step edges. However, the pits formed in controls were over 50 μm in length and narrow and elongated in [010] direction. The distinct etch pit morphology seen in the biotic treatments was also observed when barite crystals were isolated from bacterial contact using a 50,000 MWCO dialysis membrane. These initial results suggest that chelating materials, organic acids, metabolites or even proteins formed by bacteria may be promoting barite dissolution. We are currently investigating bacterial and chemical pathways responsible for the observed barite etch patterns, and quantifying the rates of etch pit formation. Uncovering mechanisms for barite dissolution has implications for scale treatment and remobilization mechanisms for Ba and Ra in shale gas systems and nature.