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In this study, the hydraulic fracture propagation mechanism in low-permeability layered rocks was investigated using the true triaxial hydraulic fracturing test system and the Continuum-based Discrete Element Method. Firstly, the mechanical properties of natural cores in reservoirs and interlayers were determined using uniaxial and triaxial testing machines. Secondly, the Kaiser stress effect method was employed to calculate the in-situ stress. To characterize the thickness distribution characteristics of reservoirs/interlayers in layered rocks, the concept of reservoir thickness ratio k δ was proposed. The influence law of k δ on the fracture pressure P f of layered rocks was explored. The fracture elongation r was proposed to describe the extension state of fractures in reservoirs and interlayers, and the effect of k δ on fracture elongation r were analyzed. The spatial morphology and geometrical characteristics of fractures were characterized based on the 3D fractal dimension and fracture opening. The 3D damage variable D V was defined to reveal the damage evolution law of layered rocks during fracture propagation. The research results reveal the fracture propagation mechanism in hydraulic fracturing of layered rocks and provide theoretical guidance for the effective implementation of hydraulic fracturing technology. This study is important for the efficient reconstruction of low-permeability reservoirs with layered media and improving oil and gas recovery. • Reservoir thickness ratio k δ charactering reservoir/interlayer thickness proposed. • 3D reconstruction models of fractures spatial morphology established. • Influence of k δ on the fracture propagation behaviors of layered rock explored. • Spatial morphology and geometrical characteristics of fractures quantified. • 3D damage variable and damage evolution law during fracture propagation obtained. [ABSTRACT FROM AUTHOR]

Tight multilayered hydrocarbon reservoirs are typical in unconventional oil and gas extraction, in which widespread bedded interfaces have crucial effects on hydraulic fracturing propagation behavior. However, because of the complex natural properties of bedded interfaces in natural layers, such as geometric structure, distribution, strength, and contact conditions, their effects on fracture propagation and governing mechanisms are not well understood. In particular, traditional theoretical models are not suitable for characterizing the propagation behavior of hydraulic fractures in multilayered reservoirs and conventional numerical methods cannot effectively simulate the interactions between hydraulic fractures and bedded interfaces. In this study, the adaptive finite element-discrete element method was introduced to probe the effects of bedded interfaces on hydraulic fracture propagation. Different numerical models of five deviation angles of representative bedded interfaces were established based on the geometric and physical parameters of natural multilayered reservoir rocks. Some novel techniques (local remeshing near the fracture tips to guarantee accurate fracture propagation path, mesh coarsening to improve the computation efficiency, characterization technology for bedded interfaces and contacts between layers, and hydro-mechanical coupling between fluid flow in fracture network and porous rock matrix in the fracturing process) were comprehensively utilized. Furthermore, the fracturing-induced microseismic damaged and contact slip events were identified based on the computed moment tensors to detect the interactions between the hydraulic fractures and bedded interfaces. The results of the cases with single and multiple perforations in horizontal wells show that the bedded interfaces disturb the stress continuity, reselect propagation direction of hydraulic fractures in multiple perforations, induce hydraulic fracture growth, and promote the occurrences of fracturing-induced damaged and contact slip events. The offset and deflection of hydraulic fractures increase with increasing deviation angles between hydraulic fractures and bedded interfaces and the offset vanishes when the deviation angle is 0°. • Adaptive finite element-discrete element method was proposed. • The effects of bedded interfaces on hydraulic fracture propagation were studied. • Models of bedded interfaces of multilayered reservoir rocks were developed. • Bedded interfaces disturb the stress continuity and provoke fracture growth. • Bedded interfaces promote fracturing-induced microseismic events. [ABSTRACT FROM AUTHOR]