Experimental and Numerical Investigation of Lower Huron Shale Mechanical Properties

Abstract:

Understanding mechanical properties of organic rich shale has attracted great attention due to the increasing development of unconventional reservoirs. Shale is typically comprised of a mixture of clay particles, pyrite, kerogen, quartz, and other minerals. During hydraulic fracturing process, the rock strength tends to be weakened by the effect of shale hydration because of the water-adsorption characteristics of clay minerals. In this work, shale elastic mechanical properties were first characterized by Atomic force microscopy (AFM). Next, uniaxial compression and triaxial compression tests were conducted on core-scale Lower Huron Shale samples. Numerical models are constructed to extract mechanical properties from both uniaxial compression and triaxial compression experiments. With AFM measured and numerical model extracted mechanical properties, composite material models were constructed to investigate the role of shale hydration and composition heterogeneity on shale mechanical properties. The effects of various materials content combinations and confining pressure on the effective Young’s modulus, cohesion, and friction angle are also studied and the stress-strain curve and maximum compressive strength under different confining pressures were compared. The simulation results demonstrate that elastic property, Young’s modulus, can be determined using weighted average of each component in the rock when chemical or structural effects are excluded, whereas the plastic properties, cohesive strength and friction angle, cannot be simply determined by the arithmetic average of each components’ value. Furthermore, the cohesive strength and friction angle may play different roles when the sample is subjected to different confining pressures. The mineral property distribution and shale hydration effects have a substantial influence on the triaxial compression strength whereas limited influence on the uniaxial compression strength. Simulation studies also indicate that numerical models account for the heterogeneity of shale can improve the accuracy of mechanical property characterization. The outcome of this research will benefit the understanding of the Lower Huron shale mechanical properties and support hydraulic fracturing design.