Constrained study of nanoindentation-based upscaling of mechanical properties of shales

Shale gas reservoirs are both heterogeneous and multi-mineralic at microscale, potentially including as many as 10 mineral phases. Mechanical interactions among these different minerals significantly impact gas flow characteristics in matrix and fractures and additionally require transformation to macroscale. We use complementary nanoindentation and triaxial deformation experiments to determine the viscoplastic behavior of the Longmaxi shale at both microscopic and macroscopic scales—and link the two. The distribution of the diverse and heterogeneous mineral microstructure was facilitated by a Tescan integrated mineral analyzer (TIMA). A multi-distance clustering method was used to autonomously differentiate the extensive measured data into mineral groups, with proportions consistent with the XRD results. Pyrite exhibits the highest deformation and creep moduli, followed by dolomite, calcite, quartz then clay. A modified Mori–Tanaka method is applied to upscale the micromechanical properties to macroscale, with results revealing only a minor deviation from the triaxial test. Moreover, a positive correlation was observed between the deformation and creep moduli. The shale reservoir exhibits elastic anisotropy at both micro- and macroscales. Notably, the macroscopic anisotropy ratio is greater than its microscopic counterpart, indicating an increase in anisotropy with an increase in length scale. These findings offer new insights into the mechanical characterization of shale, providing a more comprehensive understanding of its behavior across different scales.