Produced waters from shale gas reservoirs are often highly saline and may contain elevated levels of trace toxic metals and naturally occurring radionuclides. Sequential extraction studies of two Michigan Basin shales have demonstrated that a majority of trace hazardous elements including ~90% of total arsenic are present in reduced mineral phases.  The addition of acids and oxidizers to fracturing fluids can thereby contribute to the release of these trace metals and radionuclides. This presentation will discuss reactive transport modelling and experimental results from two core-flooding studies where synthetic hydraulic fracturing fluids were injected through proppant-packed fractured shale cores under reservoir temperature and pressure conditions. The rate of arsenic release increased over time in both experiments and was positively correlated with the extent of carbonate mineral dissolution along the flow path. Non-uniform distribution of calcite led to the generation of preferential flow paths and exposure of more recalcitrant mineral phases, such as pyrite, to the acidic fracturing fluids. The evolution of fracture geometry and surface mineralogy was investigated by a combination of X-ray computed tomography and scanning electron microscopy (SEM). Post-reaction SEM analysis demonstrated the co-occurrence of calcite-depleted regions and exposed pyrite at the fracture face. These observations indicate that erosion of calcite by the acidic fracturing fluids increased the surface area of other less soluble minerals exposed to the oxic influent fluids. This combination of acid-driven and oxidative dissolution processes enhanced the rate of arsenic leaching from the fractured shale. Reactive transport modelling of this system suggests that the development of transport limited zones within calcite-depleted regions adjacent to the primary flow path impact the extent of continued pyrite dissolution and subsequent arsenic release during injection of hydraulic fracturing fluids.<br/> <br/>

Arsenic mobilization in carbonate-rich shales during hydraulic fracturing

253rd National Meeting (2017)

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