Investigation of Mechanical Properties of Quartz and Illite in Shale Using Molecular Dynamics Simulation

Shale oil production has been significantly boosted since the large-scale hydraulic fracturing revolution, determining that the mechanical properties of shale are essential for hydraulic fracturing. The macro-mechanical properties of shale have been investigated broadly. However, shale is comprised of multiple minerals and it is characterized by heterogeneity, anisotropy, and multi-scale properties of micro-components that impact its macro-mechanical properties. Firstly, this paper investigated the micro-mechanical properties of the main minerals (quartz and illite) in shale based on previous research results. Then, by considering the influence of anisotropy, we utilized a molecular dynamics (MD) technique to analyze the deformation mechanism of tensile failure as well as calculate and verify the mineral elastic parameters. Next, the surface energy density and fracture toughness of quartz and illite were determined based on the Griffith theory of brittle fracture. Finally, we compared the differences in mechanical properties between inorganic and organic matter and discussed the microscopic mechanical properties of various components in shale and their impacts on hydraulic fractures. The results indicated the following: (1) The tensile failure process of illite and quartz follows a sequence of "elastic response -> bond breaking/hole failure -> crack nucleation -> crack propagation." (2) The bulk modulus, shear modulus, Young's modulus, and Poisson's ratio of quartz were 39.38 GPa, 53.42 GPa, 110.36 GPa, and 0.03, respectively, and those of illite were 79.1 GPa, 44.3 GPa, 117.99 GPa, and 0.264, respextively. (3) Brittle failure mainly occurred in quartz; in contrast, illite had apparent plastic deformation in the Y direction and brittle failure in the XZ direction; in addition, the G(c) (critical energy release rate) and K-IC (critical stress intensity factor) of quartz were larger than those of illite in all directions. (4) The mechanical properties of different components in shale were as follows: quartz easily developed hydraulic fractures with branches due to high elasticity and high brittleness; illite and organic matter were ductile, especially organic matter, resulting in rock fractures characterized by short length, blunt, and poorly connected. The micro-mechanical properties of the primary components in shale impact its macro-mechanical properties. This finding provides insights into macro-simulations.