When interpreting hydraulic fracturing in low permeable rock such as granite, the effect of fluid infiltration into rock matrix is commonly neglected. There are laboratory experiments of hydraulic fracturing on granite samples indicating the fluid infiltration may play important role in the breakdown pressure. This paper presents a numerical simulation of hydraulic fracturing in low permeable rock with the consideration of fluid infiltration into rock matrix. A 2D Distinct Element Method code UDEC was employed where the polygonal blocks were generated by a Voronoi tessellation feature to provide incipient fractures to consider fluid diffusion into rock matrix. This Voronoi tessellated model was verified to be hydraulically representative as a low permeable rock. The simulations of hydraulic fracturing with varying pressurization rates detected three regimes in terms of the effect of pressurization rate on the magnitude of breakdown pressure, which is generally consistent with that predicted by the existing analytical models. It was demonstrated that the varying breakdown pressure with the pressurization rate can be attributed to the varied flow diffusion conditions in the surrounding rock matrix and associated change of stress at wellbore wall. Extending the flow diffusion length such as cyclic injection mode tends to reduce the breakdown pressure.
There is growing interest in performing hydraulic fracturing in low permeable formation for the economical production of resources and energies. The hydraulic fracturing in low permeable shale plays a critical role in shale gas production (e.g., Clerk et al., 2012). Another emerging application of hydraulic fracturing is to create fluid circulation pathways for extracting deep geothermal energy which is stored in deep crystalline formation (e.g., Legarth et al., 2005).
There have been numerous studies involving hydraulic fracturing in low permeable rocks (e.g., Chen et al., 2015) and the infiltration of injection fluid into the surrounding rock has rarely been considered. Some laboratory hydraulic fracturing tests in low permeable rock demonstrated that the infiltration of fluid into the rock and its influence might not be negligible. The hydraulic fracturing experiment on Lac du Bonnet granite showed the penetration of fluid into the rock and its influence on the wave velocities (Falls et al., 1992). Ishida et al., (2004) carried out hydraulic fracturing on granite samples by water and oil injection, and found that the influence of fluid viscosity on hydraulic fracture was mainly caused by the varying fluid infiltration conditions. Recent hydraulic fracturing experiments on Pocheon granite demonstrated the water infiltration could significantly impact hydraulic fracturing behavior and the breakdown pressure was increased with the injection rate (Zhuang et al., 2018).