The paper analyzes microcracks nucleation under predominantly compressive stress states. It is proposed that microcracks are nucleated from pores, and the nucleation is driven by a local tensile stress. It is shown that local tension can be readily induced by remote compression, as long as the remote stress state is sufficiently different from pure hydrostatic pressure. The mechanism is analyzed in a setting involving pressurization and depressurization of a macroscopic tunnel crack. It is shown that the mechanism is microcrack nucleation can be enhanced upon unloading, as long as the pressurization induces plastic deformation.
Gas shales are fine-grained rocks that appear homogeneous in hand samples but are heterogeneous at the micron scale. Gas shale constituent phases exhibit a wide range of constitutive behavior. In particular, the mechanical behavior of quartz or calcite dominated regions are essentially elastic and brittle, clay/organic dominated regions can undergo significant plastic strains, and bituminous lumps exhibit viscous behavior over relatively short time scales. It is clear that the heterogeneity may result in rather complex stress and strain fields, especially under loading conditions relevant to gas production. Furthermore, the heterogeneity may result in microstructural changes that may accelerate (or decelerate) the production. In addition, those changes may lead to various undesirable environmental and geotechnical activities that must be avoided. Thus there is a clear need for a fundamental understanding of processes at the microscopic scale. Prior to hydraulic fracture stimulation, a typical permeability of gas shales is about 0.05 µd [1]. This low permeability is difficult to reconcile with observed gas production rates. In this paper, we suppose that the effective permeability can be significantly increased as a result of microcracking induced by hydraulic fracturing. Accordingly, we focus on conditions for microcrack nucleation.
