Nanoindentation application in geomechanics analysis of shale under high-temperature treatment/thermal fracturing conditions

Heat treatment can drive the initiation and propagation of fractures and lead to porosity–permeability enhancement of shale reservoirs for efficient gas production. An understanding of shale mechanical behaviour under heat treatment conditions has been limited by the scarcity of macro-sized samples for geomechanical experiments. Nanoindentation was thus utilized for geomechanical evaluation of USA Eagle Ford shales exposed to high-temperature conditions of 450, 550, and 750 ºC. Shale samples were heat treated at set temperatures and SEM–EDX-Mapping was conducted for mineralogical characterization and elemental distribution analysis. The elemental distribution suggested the dominance of calcite and quartz minerals in the Eagle Ford shale. Heat treatment of samples from ambient to 750 ºC led to a progressive decrease in average Young’s modulus (Eavg.) from 43.15 – 28.88 GPa, while the decreases in average Hardness (Havg.) and average Fracture toughness (KICavg.) were non-progressive. Havg. showed slight increases before a drastic decrease of 56.48%, and KICavg. encountered a drastic decrease of 35.08% at 750 ºC. Eavg., Havg., and KICavg. were found to be strongly correlated to sample heat treatment temperature given their associated R-squares from 0.89068 – 0.99827. Fracture toughness was of stronger correlations to Young’s modulus and Hardness with R-squares at 0.57901 and 0.69152 respectively, while the correlation between Hardness and Young’s modulus was considered weak (R-square: 0.36431). The developed correlation-driven models of high R-squares can be of significant relevance in the treatment design and optimization of thermal fracturing as a standalone option or complement to hydraulic fracturing in shale gas production.