AAPG Geoscience Technology Workshop

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Detachment of Particulate Iron Sulfide During Carbonate Rich Rocks-Water Interaction


Many exploration and production activities involve water-rock interaction, drilling, perforation for testing and production, well stimulation and injection wells, many of these are done on carbonate rich rocks, in some of these operations large volumes of water are injected into the subsurface. Although the injected fluid typically contains various reagents and chemicals, it can become further contaminated by interaction with minerals present in the rocks. Iron sulfide like Pyrite, which is common in organic-rich shales, is a potential source of toxic elements, including arsenic and lead, and it is generally thought that for these elements to become mobilized, pyrite must first dissolve. In our work we used atomic force microscopy and environmental scanning electron microscopy to show that during fluid-rock interaction, the dissolution of carbonate minerals in carbonate rich Eagle Ford shale leads to the physical detachment, and mobilization, of embedded pyrite grains. In experiments carried out over a range of pH, salinity, and temperature we found that in all cases pyrite particles became detached from the rock surfaces. On average, the amount of pyrite was over an order of magnitude greater than the rate of pyrite oxidation expected under similar conditions. This result suggests that mechanical detachment of pyrite or similar grains could be an important pathway for the mobilization of pollutants in water involving operations and in groundwater systems containing carbonate rich rocks and has to be addressed in further treatment of these fluids in drilling or production lifecycle.