Shale

a kind of sedimentary rocks

is characterized by significant bedding.The investigation of the impact of bedding planes of shale on hydraulic fracturing is of great significance for the exploitation of shale gas.The characteristics of sedimentary structures at various scales (ranging from meter to ten micron scales) of shale in the Yanchang Formation in southern Ordos Basin were analyzed

and the hydraulic fracturing tests for multi-sized shale specimens under different dip angles of bedding plane were carried out

both of which function to explain the characteristics of the bedding planes of shale and their controlling effects on hydraulic fracturing process and outcome.The results show that the average thickness of laminae is of fractal features with a fractal dimension of 1.06

and is 2.26 m

2.09 dm

1.70 cm

1.48 mm and 11.7 μm

corresponding to meter

decimeter

centimeter

millimeter and 10-micron scales respectively.The behaviors of hydraulic fracturing are significantly influenced by the bedding planes of shale

clearly shown by the contrast of fracture geometry before and after fracturing and the fracturing pressure.When the dip angles of the bedding planes are less than 30°

the fractured shale specimen is remarkably different from its original one in fracture geometry

featuring much more artificial fractures

and higher fracturing pressure

which tends to slump as the dip angle of bedding plane increases; while when the dip angles of the shale specimens are above 45°

the fracture geometry of the shale specimens after fracturing is almost the same with that of the original specimens

featuring lower fracturing pressure which tends to fluctuate a little with the increase of the dip angle of the bedding planes.Generally

the fracturing pressures of shale under different dip angles of bedding plane fluctuates in the shape of oblique "S".Besides

the controlling effects of structural dip angles of the bedding planes on the fracture propagation

hydraulic pressure curve and fracturing pressure are also reflected in the tests.