Multi-Scale Assessment of Plasticity of Organic-Rich Shale: from Core Scale to Mineral Scale

Recently, evaluation plasticity in organic-rich shale has attracted significant attention for unconventional oil and gas extraction. A multi-scale assessment was conducted including triaxial testing, nanoindentation and SEM-EDS to characterize plastic deformation and microstructural changes of individual minerals. Nine Longmaxi shale samples were loaded over a wide range of confining pressure from 0 to 65 MPa and the correlation of plasticity from core scale to mineral scale is discussed. Results demonstrate that macro-plasticity at a core scale becomes severe with increasing confining pressure. However, the local deformation behavior reflects a strong heterogeneity at the scale of the minerals that are present. The nanoindentation and SEM-EDS images provide strong evidence that both cracking and plastic deformation always occur around indentation imprints even at high in-situ stress condition. The heterogeneity of plasticity in shale is controlled by the confining pressure and mineralogy. The response of mineral plasticity to the confining pressure is variable. Some minerals experience plastic hardening while other minerals demonstrate a plastic softening behavior. The uneven distribution of the ductile and brittle minerals provides controls hydraulic fracture complexity even in deep and high in-situ stress reservoirs.