Physical Property Evolution and Intrinsic Mechanisms of shale hydration: Insights from Multi-lithofacies and Multi-Scale Experiments

ABSTRACT: The effects of shut-in on shale production remain a topic of debate, underscoring the importance of investigating hydration mechanisms across various lithologies. This study offers a comprehensive analysis of the hydration mechanisms and their impact on the structural and mechanical properties of three distinct shale types from the Subei Basin. Using a combination of advanced techniques, including XRD, NMR, CT, SEM, and triaxial compression tests, this research adopts a novel, multi-scale approach to examine the differential hydration responses of these shale lithofacies. The findings show that laminated calcareous shale experiences substantial structural alterations, such as carbonate dissolution, pore enlargement, and the formation of microfractures, resulting in significant weakening of its mechanical strength. In contrast, grayish-black mud shale, which is rich in clay content, initially exhibits pore expansion but later undergoes microfracture closure due to varying stages of clay hydration and fluctuations in water adsorption rates. Laminated dolomitic shale, however, remains largely resistant to hydration, with only minor changes observed, attributed to its absence of natural fractures, low porosity, and stable mineral composition. This study's integrated multi-experimental approach across multiple lithofacies highlights the contrasting hydration behaviors of different shale types and links these behaviors to specific mineralogical and microstructural changes. The insights gained from this research have significant implications for optimizing hydraulic fracturing in shale reservoirs, particularly in terms of selecting appropriate fracturing fluids, determining shut-in durations, and designing treatments tailored to the unique properties of each shale reservoir type.