Study on the Micromacro Cross-Scale Propagation Law of Hydraulic Fractures in Conglomerate Reservoirs

The propagation of hydraulic fractures in conglomerate reservoirs is not only controlled by geological factors (geostress) and engineering factors (viscosity and volume of fracturing fluid) but also influenced by the strong heterogeneity of gravels. Based on the heterogeneity features of gravels in terms of "structure" and "material", the sandy conglomerates of the Cretaceous Yagremu Formation in the Kuche Preclassic Kelasu Structure Belt were taken as the object of study, and a three-phase (gravel-sand-mud) composite rock skeleton model combining image analysis and modeling experiments was established to investigate the gravel-encountering behavior and mechanical response characteristics of hydraulic fractures under plane stress loading conditions and reveal the micromacro cross-scale propagation law of hydraulic fractures in conglomerate reservoirs. In addition, the heterogeneity indexes of "structure" and "material" were introduced to evaluate the main controlling factors affecting the propagation of hydraulic fractures in conglomerate reservoirs. The results show that (1) the difference between the gravels and matrix is the fundamental reason for the structural and material inhomogeneity of conglomerates; (2) higher gravel content and larger grain size significantly enhance the shielding effect of gravel, leading to a higher probability of the gravel-winding phenomenon of hydraulic fractures with increasing tortuosity; conversely, lower gravel content and smaller grain size effectively weaken the shielding effect of gravel, causing the gravel-penetrating phenomenon of hydraulic fractures with simple and flat shapes. The study helps predict the gravel-encountering extension behavior of hydraulic fractures during the fracturing of conglomerate reservoirs, thus providing theoretical guidance for the optimization of on-site fracturing design.