Recent laboratory experiments of hydraulic fracturing on granite samples carried out by Korea Institute of Civil Engineering and Building Technology (KICT) indicated that fluid infiltration into rock matrix plays an important role in the breakdown even though the granite has very low permeability. This paper presents numerical simulations of hydraulic fracturing with the consideration of fluid infiltration into rock matrix by a two dimensional distinct element method code UDEC. Polygonal blocks are generated by Voronoi tessellation feature to provide incipient fractures for fluid infiltration. Fully coupled hydro-mechanical (HM) modellings are performed to generate hydraulic fractures and the stress change surrounding injection borehole is investigated to study the injection rate dependent breakdown pressure. It is found the variation of stress change of different injection rates results in the observed phenomenon that the breakdown pressure increases with injection rate.
Laboratory experiments of hydraulic fracturing on granite samples were carried out by Korea Institute of Civil Engineering and Building Technology (KICT) with various pumping rates (Zhuang et al., 2017). Test results indicate the fluid infiltration has significant impacts on hydraulic fracturing behaviors, even though the granite samples have extremely low permeability. A sufficient high injection rate was required to cause breakdown and the observed breakdown pressure magnitude increased with applied injection rate (Fig. 1), which can not be understood if the fluid infiltration is excluded. In this paper, we performed fully coupled hydro-mechanical (HM) simulation of hydraulic fracturing using a 2D distinct element method code UDEC. The water infiltration into rock matrix was included by adopting random polygonal joints generated by Voronoi tessellation feature.