Carbonate dissolution enhances particulate iron sulfide mobilization during water-shale interaction

Abstract

The interaction between shale and water in the environment can lead to the mobilization of micrometer-sized iron sulfide (pyrite) grains. Because these grains often contain high concentrations of toxic elements, including arsenic, this process can pose a significant hazard. While previous studies have suggested that calcite dissolution plays a major role in iron sulfide mobilization, the relationship between rock composition and the release of particulate matter is unclear. Here, we performed laboratory experiments that simulated water–rock interaction in the subsurface on 5 different shale formations: Eagle Ford, Marcellus, Mancos, and Barnett from the USA, and Ein Zeitim from Israel. We used high resolution imaging to evaluate the impact of a reactive fluid on the shale surface, and used image analysis software to determine the rate of iron sulfide grain mobilization. Comparison of the shale surfaces before and after the experiments showed that the dissolution of calcite cement had a major impact on sulfide mobilization: grains that were primarily surrounded by calcite cement were up to 85 times more likely to be mobilized than grains embedded in the shale matrix, which compromises a complex mixture of submicrometer-scale phases, including phyllosilicates, organics, and carbonates. By contrast, iron sulfide embedded in organic matter typically remained cemented in place. This suggests that while calcite dissolution is a crucial phase in facilitating iron sulfide detachment in shales, organic matter might act as an adhesive that suppresses particulate mobilization. However, the adhesive effect of organic matter is only likely to be of secondary importance. Overall, our results indicate that during hydraulic fracturing operations, which involve the injection of fluid into shales in the subsurface, formations with high levels of carbonate cement are more likely to release particulate iron sulfide, thereby reducing the quality of flowback water and potentially increasing treatment costs.