Experimental and Numerical Investigation of the Effect of Bedding Layer Orientation on Fracture Toughness of Shale Rocks

Fracture toughness is a critical parameter responsible for fracture initiation and propagation. Fracture toughness in shale gas reservoir, however, is highly variable because of its anisotropy and spatial variation of clay content. In this paper, a series of laboratory and numerical experiments are carried out to estimate shale fracture toughness at different bedding plane orientations with respect to loading direction (angle of bedding layer with respect to Cracked Chevron notched Brazilian disc (CCNBD) test samples) and in different brine solutions. The CCNBD test was conducted on 12 cylindrical samples for fracture toughness. Shale samples were prepared at four different angles (0 degrees, 30 degrees, 45 degrees and 90 degrees) relative to the bedding plane. The prepared specimens were saturated in potassium chloride (KCL) solutions of different concentrations. The laboratory results of toughness have shown to be highly variable with respect to both bedding plane and brine concentration and that the sample at the angle of 90 degrees in 4% KCL concentration exhibited the highest fracture toughness. Numerical simulations based on extended finite element method (XFEM) were also carried out to simulate fracture evolution and propagation in CCNBD samples with different bedding planes. The results have shown that the bedding layers caused the fracture path to deflect. The deviation from straight crack path is caused by mixed mode fracture initiation and propagation instead of tension mode. The mixed mode fracture propagation behavior was verified by analyzing fracture propagation path using both laboratory experiments and numerical simulation.