Characterization and Modeling of Secondary Fe(OH)₃ Phases in Stimulated Shale

Hydraulic fracturing involves the injection of large volumes of water-based fluids into shale formations to create complex fracture networks, leading to opportunities for chemical interactions between shale and injectates. This study examines the mineralogical alterations resulting from the interaction between acidic stimulation fluids and a shale core using experimental and modeling approaches, focusing on secondary precipitation of ferric (hydr)oxides, Fe(OH)3. Two experimental conditions were used: a brine-only case, where the shale was reacted with formation brine throughout, and a B + S case, where stimulation fluid was introduced midway to mix with the reacting brine. Focused ion beam-equipped scanning electron microscopy (FIB-SEM) and SEM provided the morphology and spatial distribution of minor secondary Fe(OH)3 phases in shale. Two phases of secondary Fe(OH)3 were revealed: (1) one phase replaced pyrite while preserving its framboidal structure (spherical clusters of microcrystalline pyrite), and (2) the other formed loosely clustered aggregates in secondary pores generated by ankerite dissolution. Both secondary phases of Fe(OH)3 precipitated within nanoscale spaces. Following solid-phase characterization, a reactive transport model was developed based on the experimental setup to explore the key factors controlling secondary Fe(OH)3 distribution within the shale matrix. Calibration against experimental observations suggested that (i) both secondary Fe(OH)3 phases exhibited similar solubilities; (ii) pyrite-replacing Fe(OH)3 had a lower reaction rate; and (iii) the distribution of secondary Fe(OH)3 was influenced by the experimental design. Findings from this study provide a framework for interpreting experimental results within the context of the experimental design. Additionally, they contribute to a better understanding of secondary Fe(OH)3 formation in shale and its potential impact on transport processes within shale matrices.