Effects of lamina on petrophysical properties and brittleness of shales: Insight from lamina-scale experiment and fracture propagation simulation

Accurate shale characterization is hampered by the lack of high-resolution analytical techniques to probe the in situ petrophysical and mechanical properties at the lamina scale. Here we report for the first time a comprehensive lamina-scale experimental characterization and numerical simulation, including AMICS, digital rock, fracture propagation simulation, pore network modeling, FIB-SEM, and N₂ and CO₂ adsorption experiments to investigate the effects of lamina type and thickness on the petrophysical properties and brittleness of shales. We characterized a variety of shales with laminae dominated by felsic minerals, carbonate minerals, clay minerals, and mixed minerals from the Second Member of the Kongdian Formation in the Cangdong Sag, Bohai Bay Basin. Among these laminae investigated, the felsic mineral lamina exhibits the largest porosity, average macropore radius, pore throat radius, and coordination number, and the best pore connectivity and permeability. The carbonate mineral lamina comprises the largest proportion of micropores and the smallest proportion of macropores. The mixed mineral lamina consists mainly of mesopores and micropores. Shales in the study area mainly comprise felsic-mineral-clay-mineral, carbonate-mineral-clay-mineral, and mixed-mineral-clay-mineral coupled laminasets. The felsic-mineral-clay-mineral laminaset appears to have the best permeability and brittleness. The simulated fractures in the felsic-mineral-clay-mineral laminaset show the farthest propagation and the largest aperture. An increase in the thickness of individual felsic mineral lamina would improve the shale brittleness, increase fracture propagation, and enhance fracture connectivity. Fractures tend to be initiated at the interface between the clay mineral lamina and brittle mineral lamina, especially at the boundaries between the ductile and brittle minerals. Therefore, shales with high proportions of felsic mineral lamina would thus improve the porosity, permeability, and frackability of shales. This study proposes a novel technique for characterizing the in situ petrophysical properties and brittleness of shales at the lamina scale.