Investigation on the fracture mechanics characteristics and crack initiation of deep dense shale

Abstract

The mechanical behavior of shale fractures significantly affects the structure and stability of hydraulic fracturing networks in shale gas reservoirs. Currently, there is insufficient research on the mechanical characteristics of fractures, crack initiation, and the prediction of fracture trajectories in deep dense shale. This study involved conducting direct tension, shear, and three-point bending fracture mechanical experiments on shale samples, complemented by numerical calculations and mechanistic analyses. The results show a significant influence of the bedding plane angle (γ) on the initiation properties and locations of shale fractures. With γ ranging from 0° to 90°, shale fractures transition from alignment with the bedding plane to penetration through it. Fracture initiation locations show a significant degree of randomness when fractures propagate along the bedding plane. The horizontal tensile stress changes with increasing γ, initially increasing and then decreasing as it moves away from the center. The evolution of the horizontal tensile stress field primarily causes alterations in shale fracture paths, reaching its peak value at γ = 45°. Shale initiation fractures can be classified into four types: tensile fractures through the bedding plane, tensile fractures along the bedding plane, shear fractures across the bedding plane, and shear fractures along the bedding plane. Established a criterion called the “maximum stress strength ratio” to determine fracture initiation, which can predict the initiation properties, locations, and paths of shale fractures. When γ is small, fractures mainly propagate along the bedding plane, with a higher likelihood of initiation occurring at the center of the specimen. Conversely, when γ is large, fractures mainly propagate through the bedding plane. The spacing of the bedding planes minimally affects the predicted fracture paths of shale. This study is of significant theoretical importance for reservoir stimulation and stability assessment in deep dense shale gas reservoirs.