Geomechanical characterization of gondwana shale across nano-micro-meso scales

Abstract

We study multiphase and multiscale mechanical properties of the Gondwana shale samples obtained from the Raniganj Formation of central and eastern India. The mechanical properties of black and gray shale samples cored perpendicular to the bedding plane direction are analyzed across nano-to-micro scales using the nanoindentation method. We characterize the multiphase structure of the two shales in terms of three mechanical phases–quartz/feldspar/pyrite, organic matter, and kaolinite–and use that to explain the heterogeneous elastoplastic behavior observed during nanoindentation. We quantify the fracture toughness of black and gray shales at microscale using our modified energy method, which relates the energy released during a pop-in event to a radial fracture model. We extend the deconvolution approach over multiple properties to estimate the uncertainty in multiphase structure and phase volume fractions. We used the viscosity-toughness scaling analysis to study the difference in hydraulic fracturing behavior of the black and gray shales. We show the influence of nano- and micro-scale multiphase mineralogical properties on characteristic length, time and pressure scales of fracture propagation at mesoscale.