Study of brittle/ductile layering effect on fracture geometry and mechanical behavior by tri-axial testing
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Hydraulic fracturing has been a widely used technology to produce hydrocarbon from shale plays. A better understanding of the fracturing process is needed to improve oil and gas production. Understanding fracture height growth is one of the main concerns and fracture systems are usually influenced by the presence of layers with contrasting mechanical properties. This study uses a tri-axial test to investigate the fracture geometry and mechanical behavior of brittle/ductile layered samples. Synthetic hydrostone is used as brittle rock, and uncemented sand is used to mimic ductile rock. A series of experiments evaluate the effect of loading speed, confining stress, and layer thickness on the mechanical behavior and fracture geometry of the layered samples. A discrete element method is also used to calculate the mechanical behavior of layered samples and investigate the layering effect. The tri-axial test results show that the ductile/brittle multilayer becomes more brittle by increasing the number of layers. According to the results, the loading rate has less effect on thicker layer samples, and the samples are more ductile under higher confining stress. A sensitivity analysis using the discrete element method includes interface properties, number of layers, layer thickness, boundary conditions and edge effects. The results show that the mechanical behavior of brittle/ductile layered samples is highly dependent on the interface properties as well as on the number of layers. The layered samples become stronger and more brittle by increasing interface roughness and friction as well as the number of layers. This work will help better understand brittle ductile behavior of rocks and provide guidelines for the investigation of the brittle ductile layering effect on fracture height containment.