Summary In multistage fracturing operations of shale, water-rock interaction caused by fracturing fluid invasion into reservoirs will promote the initiation and propagation of microcracks and will also lead to a decrease in matrix porosity. However, a single measurement method or analysis at a specific scale cannot fully reflect the actual changes in the complex pore system of shale. For this study, we used multiple methods, such as nuclear magnetic resonance (NMR), computed tomography (CT) scanning, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS), to study systematically the influence of water-rock interaction on the microstructure of deep shale. The behaviors of water-rock interaction under liquids with different ionic compositions and mineral compositions deionized water, 3% potassium chloride (KCl) solution were tested, and the influences of water-rock interaction on shale pore-crack structure and physical properties were studied. The results indicated that during the core saturation process, clay swelling leads to the closure of microcracks. However, the dehydration process not only mitigates the microcrack closure effect induced by clay swelling but also significantly promotes the initiation and propagation of microcracks, and the effect of promoting the initiation and propagation of microcracks is significantly strengthened with the increase in dehydration rate. Under vacuum dehydration conditions, the porosity increases by 43.17–71.19% and the permeability increases by 126.28–165.69%. Under oven-drying dehydration conditions, the porosity increases by 96.4–202.54% and the permeability increases by 472.99–482.36%. The latter is more conducive to the expansion and generation of pores and cracks. Pores with a diameter of 0.2–2 μm exhibit the greatest degree of pore size change in the experiment and sustain the most significant damage from water-rock interaction. Based on the evolution of water invasion dynamics, the pressure saturation process can be divided into three stages: the capillary force-dominated stage, the transition stage, and the pressure difference-dominated stage. During these stages, the driving effect of capillary force weakens gradually, while the driving effect of pressure difference becomes increasingly prominent. During the water saturation process of the sample, the NMR porosity gradually increases to a maximum value and then gradually decreases under the influence of water-rock interaction. Clay hydration and swelling can cause a decrease in the strength of clay and its interfaces, leading to the detachment and migration of clay minerals. However, the addition of potassium ion (K+) can significantly inhibit the decrease in clay and its interface strength, as well as the detachment and migration of clay minerals. In shale gas fracturing, a 3% KCl solution can be added to prevent mineral detachment and maintain fracture conductivity.
Peng et al. (2026) studied this question.