Modeling interactions between hydraulic fracture and pre-existing microcracks in crystalline rocks using hydro-grain-texture model

The heterogeneity of the mineral grains and pre-existing microcracks significantly impacts the cracking behavior of crystalline rocks. However, these characteristics have not been adequately addressed in the existing hydraulic fracturing studies. To bridge this gap, this study introduces a fluid-solid coupling method enhanced by a microcrack simulation component, called Hydro-Grain-Texture Model (HGTM), to investigate the influence of mineral structures and pre-existing microcracks on hydraulic fracturing processes in granite. Compared with existing methods, the HGTM incorporates a "grain growth" algorithm that can more accurately represent the characteristics of mineral grains. This study investigates the base case of intact rock, and four additional cases featuring microcracks oriented in various directions, i.e., horizontal, diagonal, vertical and complex microcracks, under both hydrostatic and non-hydrostatic in-situ stress conditions. The HGTM effectively captures a range of microscopic behaviors during the hydraulic fracturing of granite, including the formation of rock fragments, fracture branches, and dry fractures, among others. Furthermore, by comparing numerical results for breakdown pressure with analytical results, the accuracy of the HGTM in predicting breakdown pressure is substantiated. The findings indicate that mineral structures (grain boundaries) and microcracks contribute to a more intricate pattern of hydraulic fractures propagation. The pre-existing microcracks significantly influence the propagation path of primary hydraulic fractures. Under fluid-driven pressures, these pre-existing microcracks experience shear failure distinct from tensile failure observed in the surrounding rock matrix.