An insight into mechanical properties of alkaline minerals (eitelite and shortite) deposited in Fengcheng Formation, Mahu Sag

Mechanical properties of minerals in shale plays would impact the design of hydraulic fracturing, a major tool to increase production from unconventional shale reservoirs. Unlike other lacustrine shale formations in China which are mainly formed in freshwater lake environment, Fengcheng Formation in the Mahu sag was deposited in a volcanic-alkali lake evaporation system with abundant alkaline minerals. However, up to date, the mechanical properties of these alkaline minerals are still unknown, which limits a proper stimulation operation design and implementation. In this study, several samples from the Fengcheng Formation were collected, characterized for their mineralogy, geochemical attributes, pore morphology via electron microscopy and mechanical properties (Young's modulus, hardness, fracture toughness) via nanoindentation testing. The results show that mechanical parameters (Young's modulus, hardness and fractures toughness) of eitelite are generally smaller than that of the shortite. Furthermore, mechanical parameters of eitelite and shortite were found close to that of the feldspar, making us consider them as brittle minerals and propose a modified brittleness index (BI) formula. Ultimately, mechanical properties of these alkaline minerals did not vary in each sample with points to the heterogeneous nature of such properties. Overall, this study is the first investigation of mechanical properties of alkaline minerals in Fengcheng Formation which can assist the hydraulic fracturing design in such area to ensure economic gains and avoid financial loss during field operations.