Abstract
Exploring secondary microcracks (SMC) growth during hydraulic fracturing in rocks with native microdefects is a challenging task in unconventional oil and gas exploitation. The effect of native microdefects on the SMC growth currently remains unclear. In this study, conceptual models by discrete element method (DEM) are established for rocks with the native microdefects assuming to be micropores (MP) and microfractures (MF), respectively. Based on the benchmark simulation, the influence of oriental relationship between microdefects and tensile stress on the cracking process is summarized. Furthermore, we investigate the effects of MP and MF on the growth of SMC during the Brazilian splitting loading. The results show that the cluster trend of SMC in the spindle-shaped zone becomes more significant with increasing microdefect densities. Torsion SMC appear outside the zone in the highly dense MP model, which does not occur in the MF model. The central part of MF model induces more SMC than the MP model under the same horizontal tensile stress. The SMC growth is controlled by both microdefect densities and spring stiffness ratios, but the latter could be ignored for the microdefect density over 0.041 (MP) and 0.324 (MF), respectively. The numerical experiments reveal the mechanism of SMC growth in rocks with native microdefects under the tensile stress.
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