Experimental study on vertical propagation of hydraulic fracturing in multilayer permeability heterogeneous reservoirs: Based on true triaxial experiment and acoustic emission monitoring

To investigate the influence mechanisms of formation permeability on hydraulic fracture vertical propagation behavior in the southern Songliao Basin of China, this study constructed a physical experimental model simulating multilayer permeability heterogeneous reservoirs. Through true triaxial hydraulic fracturing experiments combined with quantitative fracture area measurement, 3D fracture reconstruction, and real-time AE monitoring, systematic analysis was performed on key geological and engineering factors affecting fracture vertical extension. The results indicate that three typical distribution patterns exist in heterogeneous permeability formations: ascending pattern, sandwiched pattern, and leaping pattern. Interlayer permeability contrast significantly controls fracture propagation, with hydraulic fractures preferentially extending into high-permeability layers while low-permeability layers experience substantial suppression due to fluid competition. As the permeability ratio between adjacent layers and target layers increases, the fracture height-to-length ratio exhibits an upward trend. To quantitatively characterize cross-layer fracture propagation behavior, a critical threshold K_ϕ was innovatively defined. When K_ϕ ≤ 0.5, fractures are permeability gradient dominated; when K_ϕ > 0.5, the regime transitions to middle layer weighting dominance. Increasing injection rate reduces fracturing fluid leak-off at weak interfaces, concentrating hydraulic energy at fracture tips to enhance penetration capacity into low-permeability layers. Low-viscosity fracturing fluids optimize fracture area distribution while maintaining cross-layer propagation capability. Vertical stress contrast positively correlates with fracture extension capacity in high-permeability layers but negatively correlates in low-permeability layers due to hydraulic isolation effects. These findings provide deeper insights into fracture geometry and propagation mechanisms in multilayer heterogeneous permeability formations, offering theoretical foundations for optimizing hydraulic fracturing design parameters.