Investigating the Multiscale Micromechanical Properties of Sembar Shale in the Southern Indus Basin, Pakistan through Nanoindentation

Abstract

The Sembar Shale in Pakistan is considered to possess large volumes of unconventional energy resources. To assess the reservoir micromechanical properties of Sembar shale is essential to ascertain the hardness and elastic modulus while injecting CO₂ and to perform hydraulic fracturing operations; however, no study has yet been conducted to enumerate such properties. Such parameters provide guidelines for proppant selection and may affect the designing and optimization strategies of drilling complex boreholes and fracturing treatments. Thus, to achieve this, the experiments were performed with the hierarchy of mineral identification via X-ray diffraction (XRD), microstructural description via scanning electron spectroscopy (SEM) studies, and elemental compositional analysis via energy dispersive spectroscopy (EDS) and the nanoindentation, atomic force microscopy (AFM),. The mineral composition of shale was assessed via XRD and the shale composition was analysed concerning clay minerals, organic matter, and with other minor mineral contents. The analysis of Young's modulus variation is conducted by examining recorded indentation depth curves and modulus distributions.  It is obvious from our results that there is a significant difference in the modulus of elasticity and hardness of selected samples. The hardness (indentation moduli, GPa) of shale sample B was significantly lower than shale sample A. This shows that the shale sample A relatively displayed elastic deformation and recovery after the loading and unloading sequence. It was further noticed that the micro-cracks were generated in shale sample A corresponding to indent penetration; conversely, the micro-cracks in shale sample B were not propagated for applied indention loading. Such variations occurred in both samples due to the changes in grain orientations and compaction at the time of deposition. The smaller hardness and lower elastic modulus values observed from the shale sample B show an underestimation of rock strength, which in turn may affect the proppant selection and settlement as well as embedment during fracturing jobs. Thus, the study provides an understanding of decision-making and designing drilling operations and fracking treatments for shale gas oil reservoirs.