Abstract: Due to the lithology difference between layers and pronounced vertical heterogeneity, the main fractures are difficult to extend vertically in thin interbedding tight sandstone reservoirs. Increasing the fracture extension rate can effectively promote the fracture extension crossing the layer. A three-point bending fracture experiment was carried out using prefabricated cement-sandstone specimens to study the effect of extension rate on fracture path. The digital image correlation method monitored the fracture process zone (FPZ) development characteristics when the fracture extends to the bedding. A prediction model of fracture extension path considering rate effect is proposed based on fracture dynamics theory. The results indicate that at low fracture extension rates, the fracture geometry exhibits a tortuous pattern, accompanied by a short and wide FPZ. Conversely, at high extension rates, the fracture geometry becomes smooth, with a long and narrow FPZ. FPZ is discrete and has the characteristics of mutual attraction. The fracture expands from low-strength rock to high-strength rock. At low propagation speeds, a high-strain zone (FPZ) forms in advance at the bedding plane, facilitating the extension of fractures along the bedding, causing the cracks to propagate along the layers upon reaching these interfaces. In contrast, this phenomenon is not observed during high-speed crack propagation. A positive correlation exists between the tensile strength of the rock and the average tensile strength of fractures traversing the unit. Fractures with low extension rates preferentially propagate along micro-defects, leading to a reduction in the tensile strength of the rock. Conversely, fractures with high extension rates preferentially propagate along self-similar directions, causing an increase in the tensile strength of the rock due to the tearing of numerous high-strength components. As the angle between the fracture and bedding increases, the fracture's ability to traverse the layer improves. The effect of the angle is maximized when it reaches 30° between the fracture and bedding, gradually diminishing beyond this threshold. These research findings hold significant implications for optimizing hydraulic fracturing parameters, enhancing fracture height, and boosting oil and gas production in thin interbedded tight sandstone reservoirs.

Key words: thin interbedding tight sandstone, interlayer mechanical properties, rate dependent fracture, fracture process zone, energy release rate