A considerable amount of research has been carried out in the past few decades trying to understand the mechanics of hydraulic fracturing. Conventional modeling methods using a fracture mechanics suggest that a tensile crack is generated in hydraulic fracturing, and elongates in the major-principal-stress direction. Whereas, according to the acoustic emission (AE) events recorded during the laboratory and field hydraulic fracturing experiment, most AE classified as the shear type mechanisms. Thus, the hydraulic fracturing mechanism has not been sufficiently clarified.
In this study, the flow-coupled DEM (distinct element method) simulations are performed to better understand the hydraulic fracturing mechanism and the influence of fluid viscosity. The DEM can directly represent grain-scale microstructural features of rock without complicated constitutive laws. This suggests that the DEM model may be more appropriate for the analysis of rock fracturing than other numerical analysis techniques. The simulation results were in good agreement with the actual experimental results. As the results, the followings were found. When the low viscous fluid is used, the fluid is infiltrated into the fracture instantaneously, and the energy emitted from the tensile crack is small compared with that from the shear crack. Such a small AE is easily buried in a noise and hard to be measured in an experiment, the shear type AE with large energy is dominantly observed in AE measurement experiment. On the other hand, when the highly viscous fluid is used, the fluid is infiltrated slowly into the crack after the fracture elongates first. At this time, fluid pressure becomes very high. Highly pressurized fluid causes large fracture opening, and generates tensile cracks which emit large energy. Due to this reason, both tensile and shear types of AE are observed during the injection of high viscous fluid in an AE monitoring.
