Spatiotemporal Fracture Competition of Hydraulic Fractures under the Coupled Effects of Goaf Stress Disturbance and Bedding Planes

Abstract

The efficient extraction of abundant coalbed methane resources underlying goaf areas has long been hindered by a critical challenge: the complex stress-damage field induced by intense mining-induced stress disturbances, along with bedding plane effects, severely distorts the dynamic propagation behavior of hydraulic fractures, rendering conventional fracturing theories based on continuum mechanics assumptions ineffective. To address this issue, this study, for the first time, to the best of our knowledge, from a discontinuous medium mechanics perspective, we conducted experiments to investigate hydraulic fracture propagation in large-scale bedding coal ( 300    mm × 300    mm × 300    mm ) under three distinct stress disturbance conditions. We analyzed the damage evolution of coal under varying stress disturbances and its control mechanism on fracture initiation and propagation. The experimental findings identify three constitutive types of stress-induced damage: initial stress damage (ISD), bedding plane damage (BPD), and coal matrix damage (CMD). Their sequential dominance defines a “two-stage critical paradigm” for damage evolution. In the first stage, governed by ISD and BPD, damage is characterized by bedding-plane tensile failure, which enhances the bedding effect and leads to the formation of a single transverse fracture, offering limited permeability enhancement. Once the evolution crosses a critical threshold into the second stage, CMD becomes dominant, marked by the widespread coalescence of microfractures within the coal matrix. This triggers a fundamental transformation in fracture morphology, resulting in a complex, orthogonal tree-like network that increases the stimulated reservoir volume by an order of magnitude. Furthermore, a damage-degree-based predictive model confirms that these damage processes significantly reduce both the fracture initiation and propagation pressures, providing a theoretical tool for designing precise “damage-promoted-fracturing” operations.