Grain-scale analysis of fracture paths from high-cycle hydraulic fatigue experiments in granites and sandstone

Previous studies have revealed that hydraulic fracturing behavior in crystalline rock (e.g., granite) is highly dependent on mineral content, grain size, and heterogeneity and is therefore quite different from the behavior observed in sedimentary rock (e.g., sandstone). We investigated hydraulic fracture paths generated via cyclic and monotonic injection in Pocheon granite, Gonghe granite, and gray sandstone. The gray sandstone has a porosity of 9.0% and its anisotropy was not considered in this study. The effect of cyclic injection on the reduction of hydraulic fracturing breakdown pressure was more significant in the two granites compared with that in the sandstone. Computed tomography images were obtained on fractured specimens for a quantitative evaluation of hydraulic fractures that included fracture length, fracture aperture, and tortuosity. Minerals along hydraulic fractures in the two granites were identified via an analysis of thin section collages from microscopic observations. We analyzed the role of different minerals (i.e., quartz and biotite) and pre-existing defects – such as microcracks and natural fractures – in controlling hydraulic fracture propagation in monotonic and cyclic injections.