• Large-scale true triaxial fracturing testing system is used to investigate the impact of cluster spacing on uneven fracture propagation in multi-cluster perforation fracturing affected by stress shadow effect. • A critical spacing of 50 mm is found achieving multi-fracture propagation equilibrium. • Fracture equilibrium index and fluid intake discrepancy coefficient are introduced to analyze the multiple fractures unbalanced propagation and the injection pressure evolution. • A theoretical stress model of multi fracture induced stress is established to clarify the dual role of stress shadow. Multi-cluster perforation fracturing (MCPF) technology has developed into a pivotal technique for enhancing reservoir performance by creating complex fracture networks and expanding the extent of the modified reservoir. Horizontal well multi-cluster synchronous fracturing is critically constrained by stress shadow effects, which cause uneven fracture initiation and imbalanced propagation, ultimately compromising reservoir stimulation. In this paper, a large-scale true triaxial fracturing testing system was employed to investigate the impact of cluster spacing on uneven fracture propagation in MCPF under the influence of stress shadow effects. Multiple-fracture unbalanced propagation, injection pressure evolution and injection flow rate were analysed by introducing the fracture equilibrium index and fluid-intake discrepancy coefficient. The fracture propagation patterns reveal that perforation-cluster spacing exerts substantial regulatory control over synchronous multi-fracture initiation and propagation. A threshold spacing of 50 mm was identified as achieving equilibrium multi-fracture propagation. At narrow spacing (10–40 mm), stress interference intensifies, suppressing central fractures and generating short-but-wide fractures with low fluid distribution (7%–16% flow share). At 50 mm spacing, stress shadow effects diminish significantly, and multiple fractures propagate synchronously with an equilibrium index of 0.97. Fluid distribution becomes balanced, with each perforation cluster accounting for 33% of the flow. However, at wide spacings (60–80 mm), although the fracturing effect improves markedly, fractures on both sides tend to deflect towards the centre, once again leading to uneven propagation. Optimal balanced fracturing is achieved at 50 mm spacing, where fractures exhibit uniform geometry and fluid distribution. A theoretical stress model of multi-fracture-induced stress is established to clarify the dual role of the stress shadow effect: excessive spacing leads to fracture reorientation, whereas insufficient spacing intensifies competition between clusters. This provides theoretical support for the optimisation of multi-cluster fracturing design. The research findings offer important guidance for reservoir stimulation in deep coal-measure coalbed methane reservoirs.
Jian et al. (Mon,) studied this question.