Investigation of Heterogeneous Failure and Fracture Behavior Differences in Shale Based on Continuous Scratch Tests

Abstract

The diverse internal structures of Gulong shale reservoirs significantly impact hydraulic fracturing, yet traditional evaluation methods often fail to characterize the continuous failure features of different shale types. Therefore, this study aims to systematically analyze the mechanical heterogeneity and fracability of four typical shale lithofacies. The research focuses on distinguishing their mechanical responses and failure modes to address the limitations of conventional bulk mechanical parameter assessment. Typical samples of foliaceous shale, siltstone shale, diamictic shale and felsic shale from the Gulong formation were selected for centimeter-scale continuous scratch experiments. Real-time data on cutting force, normal force, and friction response were recorded to extract key mechanical parameters, including hardness and fracture toughness. The study integrates these mechanical metrics with microscopic observations of the scratch track surface morphology. This approach allows for a comparative analysis of the mechanical response differences and failure characteristics during the cutting process, specifically focusing on the structural heterogeneity reflected by parameter fluctuations. Results indicate that continuous scratch profiles effectively reveal the mechanical heterogeneity controlled by shale structures. Foliaceous shale exhibits distinct parameter mutations and discontinuous failure characteristics during the scratch process. Cracks predominantly initiate and propagate along bedding planes, demonstrating a strong structural control effect. In contrast, laminated shale lithofacies displays a relatively smooth mechanical response. Its failure mode is dominated by the synergistic propagation of multi-scale micro-fractures, reflecting a more continuous energy dissipation process. Consequently, the two shale types demonstrate fundamental differences in failure modes, crack morphologies, and propagation paths. Foliaceous shale tends to undergo brittle, plane-dominated failure, while laminated shale exhibits a complex network of micro-damage. This study elucidates the differences in fracability among various shale lithofacies from the perspective of failure mechanisms. It provides a novel experimental basis and mechanical explanation for optimizing target layers in Gulong shale reservoirs and effectively regulating fracture morphology during hydraulic fracturing operations.