Correlations Between Microscale Dimensions and Elastic Anisotropy in Shale

ABSTRACT: Shale exhibits significant anisotropy on both macroscopic and microscopic scales, which can influence the efficacy of the fracturing process. Existing research lacks investigations on the laws governing the variation of mechanical anisotropy at the microscopic scale relative to size This study focus on the variation of mechanical anisotropy in shale within the size range of 50-2000 nm. Through nano-indentation experiments conducted on shale samples aligned parallel and perpendicular to the bedding planes under varied loading conditions, we have discerned that the micro-elastic mechanical properties of shale exhibit distinct variations across different sizes. Our findings are as follows: (1) The anisotropy ratio (bed-parallel to the bed-perpendicular) of the Young's modulus in shale is substantial, with minimal fluctuation across sizes, decreasing from 1.61 to 1.50 as the indentation radius increases from 1.1μm to 3.5μm; The anisotropy ratio of the hardness is lower and demonstrates significant variation with size, decreasing from 1.46 to 1.18; (2) The variation in micromechanical properties of shale with respect to indentation depth requires a holistic analysis that incorporates mineral distribution and the indentation process; (3) The microscopic orientational arrangement of clay minerals plays a pivotal role in the emergence of anisotropic characteristics and their variation with size. 1. INTRODUCTION Shale oil and gas represent a crucial component of the anticipated growth in the production of hydrocarbon resources. These resources are found within shale formations rich in organic material, typically characterized by their low permeability and narrow hydrocarbon-bearing layers (Li et al., 2022; Zhijun et al., 2021). The exploitation of such formations necessitates the application of hydraulic fracturing technologies to enhance extraction outcomes (King, 2012). Shale, a sedimentary rock, emerges from the consolidation of clay minerals, carbonates, quartz, and other minerals, showcasing prominent bedding features. The interlayer bonding is notably weak, and its compressibility is significant, leading to pronounced anisotropy in its mechanical properties, such as elastic modulus and compressive strength. This anisotropy influences the magnitude of in-situ stresses, potentially triggering uncontrollable fractures during hydraulic fracturing processes, thereby compromising reservoir stimulation efficiency (King, 2012).