Summary

Hydraulic fracturing has already been a powerful tool to increase the total production for unconventional reservoirs such as shale formations. The deformation and fracture properties largely depend on the rock mechanical properties. Due to the mineralogical variability, mechanically and chemically unstable properties of the shale formations, it is very expensive and sometimes even impossible to obtain suitable size samples for standard mechanical testing. Nanoindentation, which only requires a small sample volume, is a promising technique to detect the in situ mechanical properties. In this paper, we picked samples from Bakken Formation, a typical unconventional reservoir in North America. By using FESEM (Field Emission Scanning Electron Microscope), we characterized the pore structures in nanoscale. Then we analyzed the mineral compositions of the samples by element mapping using EDX. After that, we applied grid nanoindentation to calculate Young's modulus and hardness value of the samples and compared. The results showed that the mechanical properties such as Young's modulus and hardness values of the sample which is parallel to the bedding line are different from the value of the sample which is perpendicular to the bedding line, identifying the anisotropy properties. Samples from Middle Bakken Formation have larger Young's modulus and hardness values than the values of Upper Bakken Formation.

Introduction

Due to the depletion of the conventional resources and the increasing demand of the energy by the society, unconventional resources such as shale reservoirs has been widely carried out by many countries. The development in the fracturing technology and the horizontal drilling increases the total production of the oil and gas (Li, et al., 2015). Mechanical properties such as Young's modulus can influence the stability of the fractures, which will affect the performance of the hydraulic fracturing. The deep understanding of the mechanical properties can assist the hydraulic fracturing operations and finally increase profits. Shales are multi-phase, multi-scale heterogeneous sedimentary rocks, which comprise a variable mineralogy, nanoscale pores and clay particles (Bobko and Ulm, 2008; Veytskin et al., 2017). Due to the mineralogical variability, mechanically and chemically unstable properties of the shale formations, sometimes it is difficult to apply the standard mechanical testing methods to derive the mechanical properties of the shale rocks.

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