The flow-coupled DEM simulations are performed to better understand the hydraulic fracturing mechanism and the influence of fluid viscosity. The simulation results were in good agreement with the actual experimental results which containing the AE measurement data. As the results, the followings were found. When the low viscous fluid is used, the fluid is infiltrated into the fracture instantaneously. On the other hand, when the highly viscous fluid is used, the fluid is infiltrated slowly into the crack after the fracture elongates first. Although the tensile cracks are dominantly generated in the simulation, the energy released from a tensile crack becomes small because the tensile strength of rock is obviously small compared with the compressive strength. Such a small AE is easily buried in a noise and hard to be measured in an experiment. Therefore, in AE measurement experiment, shear type AE with large energy is dominantly observed, as many previous researches have indicated.
To better understand the mechanics of hydraulic fracturing, a considerable amount of research has been carried out in the past few decades. Conventional theory suggests that hydraulic fracturing is formed by tensile crack generation .
On the other hand, most of the acoustic emission (AE) events recorded during the laboratory and field hydraulic fracturing experiment classified as the shear type mechanisms . Ishida et al. carried out a laboratory hydraulic fracturing experiment with low viscous water and high viscous oil. The source mechanisms of AE events indicate that shear type mechanisms are dominant when low viscous fluid is injected, and tensile type mechanisms are dominant when high viscous fluid is injected . Thus, the hydraulic fracturing mechanism has not been sufficiently clarified.
Another approach to reveal the hydraulic fracturing mechanisms is numerical simulation. Various numerical analysis techniques have been developed and applied to various problems of rock engineering. Among these techniques, the distinct element method (DEM) can directly represent grain-scale microstructural features of rock, such as pre-existing flaws, pores, microcracks and grain boundaries by considering each grain as a DEM particle [4,5]. These grain-scale discontinuities in the DEM model induce complex macroscopic behaviors without complicated constitutive laws. This suggests that the DEM model may be a strong tool to understand the fracture behavior of the rock.
In this research, the flow-coupled DEM code was developed and simulations of hydraulic fracturing are performed to better understand the hydraulic fracturing mechanism and the influence of the fluid viscosity.
Formulation of mechanics of bonded particlesIn two dimensional DEM, the intact rock is modeled as a dense packing of small rigid circular particles. Neighboring particles are bonded together at their contact points with a set of three kinds of springs as shown in Fig. 1 and interact with each other.
(Equation in full paper)
Fig. 5 illustrates the rock model and loading condition for the hydraulic fracturing. The rock model is expressed by the assembly of particles bonded with each other. The particle radius was chosen to have a uniform distribution between maximum radius and minimum radius.