Time-Dependent Microscale Evolution of Mineral-Filled Fractures in Deep Shale During Water Immersion: Insights from Fixed-Field SEM–EDS Imaging

Abstract

The morphological evolution of mineral-filled fractures in low-clay shale during water exposure remains poorly understood, particularly regarding how infilling mineralogy, fracture geometry, and fluid accessibility control the response. In this study, fixed-field scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS) was used to track selected fracture regions in Longmaxi Formation shale recovered from a depth of approximately 4072 m in the southern Sichuan Basin, China. The same regions were examined before immersion and after 3 and 6 days of static immersion in deionized water (18.2 MΩ·cm; essentially zero initial ionic strength). Changes in fracture trace length, apparent aperture, area fraction, and box-counting fractal dimension were quantified using a consistent image-analysis procedure and interpreted together with elemental distributions. Three fracture-evolution patterns were identified: sustained enlargement, initial enlargement followed by partial reduction, and moderate aperture increase accompanied by pronounced trace-length extension. Calcium-sulfate-rich fractures exhibited the first two patterns. Their initial enlargement was associated with dissolution of the mineral infilling, whereas the contrasting later-stage responses appeared to be influenced by fracture geometry and fluid accessibility. Relatively open fractures continued to enlarge, while more confined fractures showed partial loss of the initial enlargement and local accumulation of crystalline material. However, the composition and origin of this material (including possible drying-induced crystallization) remain uncertain. Clay-rich fractures showed modest aperture enlargement and continued trace-length extension, consistent with clay–water interaction, although the underlying mechanism could not be resolved conclusively. These results demonstrate that fractures with similar mineral infillings can follow different temporal evolution paths during water exposure. The fixed-field SEM–EDS workflow provides a reproducible method for quantitatively tracking microscale fracture evolution and improves understanding of mineral- and geometry-dependent fracture responses in low-clay shale, though the results are derived from a single specimen and should be interpreted as illustrative observations rather than population-level statistics.