Abstract

Shales are fissile sedimentary rocks composed of clay-sized particles. They are crucial in the energy sector as a source of natural gas and for CO₂ sequestration. However, their complex hydro-thermo-mechanical behavior, affected by stress state, pore-fluid chemistry, thermal conditions, and geological history, presents challenges and opportunities for energy applications. This thesis investigates particle-scale mechanisms contributing to the development of anisotropy in shales through laboratory experiments and numerical simulations to understand their effects on engineering applications, such as hydraulic fracturing and drilling operations.

The study first examines shale formation, focusing on the interaction between shale’s distinct mineral phases and organic matter, and their contribution to shale fissility. Results show that the alignment of clay minerals and deformation of organic matter significantly increases anisotropy, highlighting that shale’s fissility may originate from particle-scale features.

Next, the thesis explores the coupled effects of fracture network topology, stress state, and fluid properties on hydraulic fracture propagation. Simulations and experiments reveal that fracture network topology and stress state collude to control fluid invasion and fracture propagation. Furthermore, kinematic dilation occurs beyond the invaded zone and significantly impacts fracture transmissivity in the far-field.

The thesis also introduces new shape-changing particles designed to mitigate drilling fluid loss into large-aperture fractures. Experiments indicate that these particles have a shape and size that promote clogging and withstand the pressure differential between the wellbore and the formation.

Further analysis focuses on proppant interactions within fractures, revealing the emergence of oscillatory forces akin to atomic-scale solvation forces. The wavelength of these oscillations depends on particle size and sample polydispersity. Additionally, the study introduces a new simulation environment that takes advantage GPU parallel computing to reduce computation time.

Finally, the study examines the volumetric changes and reduction in hydraulic conductivity of proppants under high effective stresses. Crushing reduces hydraulic conductivity by orders of magnitude as fines produced during compaction fill pore spaces and clog pore-constrictions.

This research provides a multi-scale perspective on shale behavior, focusing on mechanisms at the particle-scale, and offers insights applicable not only to shales but also to other pre-structured rock formations.