Characteristics of pore structure are crucial to oil exploration and development in shale reservoirs, but practical evaluation of shale pore structure remains a longstanding challenge. In this study, based on nuclear magnetic resonance transverse relaxation time (T₂) spectra under water-saturated and irreducible conditions, total pore space and moveable fluid space were skillfully determined. Meanwhile, multifractal spectra were used to characterize T₂ distributions of total pore space and movable fluid space. The results show characteristic parameters of high probability measure areas of multifractal spectra (multifractal parameters) have strong negative correlations with T₂ geometric mean value, and average radii of pore and throat increase with a decrease in the multifractal parameters, indicating the shale reservoirs with low values of the multifractal parameters may have good pore structure. Moreover, clay mineral content has strong positive effects on the multifractal parameters. In contrast, (quartz+feldspar) mineral content negatively influences on the multifractal parameters. Finally, new accurate models for permeability estimation in core scale and well-logging scale of shales are both proposed based on the multifractal parameters, which are also applicable for the other reservoirs. Overall, this study is important and timely for evaluating pore structure and predicting permeability in shale reservoirs.

Caprock sealing capacity is paramount for the safety and efficacy of geological carbon storage. This study investigates the evolution of mudstone caprock sealing capacity under CO₂–mechanical coupling, integrating experimental rock mechanics with fluid–solid coupling numerical simulations. Laboratory experiments reveal that caprock permeability exhibits strong stress sensitivity, decreasing exponentially with increasing effective stress. The stress sensitivity coefficient is highly dependent on initial pore pressure and porosity, being greatest under low-pore-pressure and high-porosity conditions. Furthermore, permeability loss during loading is partially irreversible due to plastic deformation. Numerical simulations, conducted using an integrated Petrel + Visage + Eclipse workflow, quantify the influence of caprock physical and mechanical properties on sealing capacity during CO₂ injection. The results demonstrate that vertical total stress increases with increasing porosity, Young’s modulus, and permeability, with permeability exerting the most significant control. Conversely, vertical effective stress decreases with increases in these parameters, with porosity causing the largest variation. We conclude that lower caprock permeability and porosity are most critical for enhancing sealing integrity, while a higher Young’s modulus improves mechanical stability. These findings provide a theoretical basis and practical methodology for evaluating caprock sealing capacity and ensuring the secure storage of CO₂.

In natural micro-nanoscale porous media, the movable fluid porosity can effectively represent storage and permeable properties, but various mineral compositions have complicated effects on it. Taking saline lacustrine shale as an example, this study researched the effects of mineral composition on movable fluid porosity, based on nuclear magnetic resonance (NMR), focused ion beam (FIB), and x-ray diffraction (XRD) experiments. The results show that movable fluid porosity exhibits a stronger dependence on porosity than movable fluid saturation does. Micropores (1000 nm) are mostly developed in silicate and gypsum minerals, and have a highly heterogeneous distribution. In contrast, carbonate intercrystalline pores are dominated by mesopores (100−1000 nm), and behave strongly heterogeneously. Many mesopores play a positive role in generating highly movable fluid porosity, but the development of micropores and macropores is not conducive to an increase in movable fluid porosity. Overall, a significant negative effect is observed between silicate mineral content and movable fluid porosity, and carbonate mineral content has a strong positive effect on movable fluid porosity, whereas movable fluid porosity exhibits a relatively small reduction with an increase in the gypsum.