(398d) Determination of the Failure Envelope of Kerogen Matrix By Molecular Dynamics Simulations

Shale gas is one of the most important unconventional resources. Hydraulic fracturing is the common method to create fractures in shale media for shale gas production. Generally, kerogen is the predominant component of organic matter in most shale oil/gas formations. Because kerogen has the potential to be the mechanically weak component in shale where the fracture initiates, it is critical to investigate its mechanical properties and failure mechanism. Failure envelope describes the locus of all shear and normal stresses at failure for a given rock material, which is often used as the criterion of rock failure for macroscopic simulations. Because kerogen is often dispersed in shale media in submillimeter range, it is impractical to accurately and systematically perform the mechanical tests on very small kerogen pieces to determine the failure envelope.

In this study, we provide a systematic study on the determination of the failure envelope of kerogen by molecular dynamics simulations. We construct various molecular structures of kerogen matrix, including type I, type II, and type III as well as the kerogen macromolecules with different maturities. The process of tension and compression at various states of stress are simulated. The stress-strain curves are obtained from the simulations, and Young’s modulus, tensile strength, compressive strength, and fracture toughness are analyzed. The effect of pre-existing fracture and large pores are also investigated.

Our molecular simulations reveal that kerogen matrix experiences an elastic deformation first, and then a plastic deformation in both tension and compression. The shear failure approximately follows the Mohr–Coulomb criterion. Different kerogen types present significantly different failure modes. Type I kerogen shows ductile behavior, while type II and type III have a moderate brittle failure. However, all have less brittleness, weaker tensile strength, and weaker compressive strength than other minerals in shale. The fracture toughness is lower than the brittle minerals as well. The pre-existing fractures have a more significant effect on the mechanical properties than the large pores in the tensioning process. These effects will lead to different behaviors at the mesoscopic and macroscopic scale, which may result in different fracturing pressure.

In our work, we propose a practical method to determine the failure envelope of kerogen by molecular simulations for the first time. These results provide the essential parameters for mesoscopic and macroscopic simulations. The investigation at molecular scale is also critical to gain insight into the mechanisms of hydraulic and CO₂ fracturing for shale in the future work.