Microscale Mechanical Anisotropy of Shale
Authors: Wilson Espinoza W. E. , Fereshteh Rahmani F. R. 
Published: August 2024

ABSTRACT:
The hydrocarbon production in the United States, which was dominated by vertical drilling methods, underwent a shift towards combining horizontal and hydraulically fractured wells in 2012. By the end of 2018, horizontal wells accounted for 96% of the natural gas production. Despite such transition, the anisotropic behavior of shale formations remains a challenge affecting borehole integrity, fracture morphology, production rates and hydrocarbon leakage. This study focuses on measuring the microscale mechanical properties (e.g., elastic modulus, hardness, and creep deformation) of shales at directions parallel and perpendicular to their bedding plane. Shale samples were tested through instrumented nanoindentation using grid arrangements with maximum loads of 4,000 μN and 10,000 μN. The results suggest that the mechanical properties of the tested shales showed an inverse relationship with respect to the bedding plane. The elastic modulus and hardness parallel to the bedding plane were, respectively, 19.02% and 51.38% greater than at their orthogonal counterparts. Additionally, constant load analyses suggested that creep deformation was 35.95% greater in the direction perpendicular to the bedding plane than parallel to it. The characterization of multidirectional behavior of shales presented in this study can contribute to the calibration of numerical models predicting microscale strength properties and time effects of shales in directions parallel and perpendicular to their bedding plane.

1. INTRODUCTION
Shale, a complex rock with nanoscale porosity, clay particles, and non-clay inclusions, exhibits diverse mechanical properties influenced by its composition. Parameters such as elastic modulus, hardness, and brittleness play an important role in designing hydraulic fracturing processes (Lin et al., 2017). The emergence of the "Shale Gas Revolution" in the 21st century has made shale reservoirs a prominent global natural gas source. In 2016, over 60% of dry natural gas production originated from shale resources in the U.S. (EIA, 2016, 2017, 2018). In recent years, the exploration of unconventional shale resources has gained attention due to its potential for hydrocarbon extraction, which involves assessing factors including geological characteristics, rock properties, well testing, and hydrocarbon yield (Zou et al., 2012). Due to advances in extraction technologies, especially relating to horizontal drilling, the growth rate of hydrocarbon production in the U.S. reached 38% between 2002 (5.01 trillion m3) and 2008(6.94 trillion cubic m3) (Dudley, 2019). Horizontal drilling and hydraulic fracturing technologies respectively enable access to large shale formations and increase accessibility to hydrocarbon reservoirs by creating more pathways for their extraction (Li et al., 2015). Additionally, the profitability of hydrocarbon production is directly tied to the ability of creating stable fractures in shale formations (Liu et al., 2014). However, shales behave more complexly than other sediments due to their anisotropy, which provides directional dependence on their mechanical behavior. Their laminated structure with distinct bedding planes introduces mechanical variability. Therefore, understanding their orientation-dependent behavior could not only contribute to further optimizing hydrocarbon extraction but also to enhance safety and sustainability. Enabling more efficient and environmentally friendly hydrocarbon extraction methods could reduce environmental impacts, minimize waste, ensure wellbore stability, and enhance the overall sustainability of energy production. Previous research has focused on studying shales' hydraulic fracturing design based on mechanical parameters including elastic modulus, hardness, and brittleness. Smith and Montgomery (2015) characterized elastic modulus and hydraulic fracture behavior to establish connections among rock brittleness, fracture propagation, and fluid flow dynamics in hydrocarbon reservoirs. Gulrajani et al. (2000) investigated links between fracture propagation, rock brittleness, and fluid dynamics. Other studies considering the flow conditions of fracture-stimulated rocks indicated that shale formations with high hardness tend to enhance proppant embedment (Alramahi & Sundberg, 2012). That is because high hardness leads to high brittleness, which in turn could lead to enhanced fracture connectivity in shale formations (Wen et al., 2007). Additionally, hardness can also control proppant embedment and fracture conductivity in shale formations. Soft proppants embedded into fracture networks of hard shale materials could induce fracture closure (Mueller & Amro, 2015). Therefore, compatible hardness between proppants and their embedding rock matrices ensure efficient fluid-flow in hydrocarbon reservoirs, especially under conditions of high fracture-closure stress (Zhang et al., 2017). In addition to studying shale behavior in the context of fracturing and flow processes of hydrocarbon reservoirs, other researchers have studied shale anisotropy through various techniques. Wang and Han (2020) used wave detection, multiload tests, and fracture analyses to find that P-wave and S-wave velocities exhibited anisotropic characteristics influenced by bedding plane orientations. In other studies, mechanical parameters such as elastic modulus and hardness measured in multiple directions highlighted the anisotropic behavior of shale. The hardness of shales tested parallel to their bedding plane have shown to be different from that tested perpendicular to it (Liu et al., 2017; Shi et al., 2018; Shukla et al., 2013). The anisotropic behavior on shales has also shown contrasting mechanisms among their mechanical characteristics. Shale formations from China have exhibited high anisotropy in regards to fracture toughness but negligible anisotropy with respect to Young's modulus. Xiang et al. (2017) studied the fracture toughness in the direction parallel to the bedding plane and found it 20% lower than that perpendicular to it. However, no significant change was observed in regards to anisotropic elastic moduli. Cala et al. (2017) found that the elastic modulus of shales was highly anisotropic while their hardness exhibited low anisotropy with increasing loads. That is, as the testing load increased, hardness became almost isotropic. However, Boulenouar et al. (2017) found no difference between hardness measured parallel and normal to the bedding plane of shales. In their studies, Boulenouar et al. (2017) surmised that anisotropy could be obscured by shale heterogeneity. Additionally, Liu et al. (2021) found multiscale differences in shale behavior spanning from predominant anisotropy at the macroscale and strong heterogeneity at the microscale. Therefore, consensus on the driving factors affecting shale anisotropy is still under development. This study aims to shed light into addressing limitations of shale characterization by investigating the directional dependence of micromechanical properties of shales. This study includes analyses on the elastic modulus, hardness, and creep behavior of shales tested horizontally and perpendicularly to their bedding plane using instrumented nanoindentation. Optimizing hydrocarbon extraction technologies from the perspective of material characterization could enhance hydrocarbon production rates and potentially minimize the amount of new hydrocarbon wells needed in the future. Moreover, a comprehensive understanding of shale mechanics could enhance the safety of exploration, drilling, and hydrocarbon extraction.