Experimental study on vertical propagation of fractures of multi-sweet of spots shale oil reservoir

Shale oil resources are developed richly in the Yanchang Formation of the Ordos Basin, which is deposited with multiple layers of sandstone and shale, and the results of exploration and development in recent years have shown that its conservatively assessed resources can reach more than billions of tons. The shale oil reservoirs are stacked with developed bedding and natural fractures and obvious anisotropy and heterogeneity, so the fractures height of in the longitudinal extension distance is usually short, making it difficult to accomplish reservoir reconstruction. Based on indoor true triaxial fracturing physical experiments on the full-diameter shale and sandstone cores obtained from the downhole reservoir section of Changqing 7, the full-diameter core is wrapped by concrete to test the initiation and vertical propagation of hydraulic fracturing in a true triaxial environment. Experiment are carried out to reveal the vertical propagation mechanism of fractures of multi-sweet spots with different lithologic reservoirs in 7 shale oil formation of Changqing. It is found that the shale oil reservoir bedding is in the shape of a "thick cake", and the bedding is cemented weakly. Fracturing fluid is easy to percolate along the bedding. When the difference between the vertical stress and the minimum horizontal stress is less than 12 MPa, the shape of the hydraulic fracture generally exhibits a horizontal fracture, and the fracturing fluid is percolated along the bedding. If the displacement is large (30 mL/min), the samples will be caused to produce shear slip damage, resulting in a high-inclination horizontal fracture or a jumping step fracture. When the difference between the vertical stress and the minimum horizontal stress reaches 14 MPa, the obvious vertical fracture will be produced, and it will connect multi-sweet spots. At this time, regardless of whether the flowing rate is high or low, the vertical fractures will communicate with multiple stratification fractures, and the fracturing fluid will be captured by the weakly cemented stratification surface during the expansion of the vertical fractures, then fracturing fluid is percolated and expands along the bedding fractures, resulting in a complex cross shape or road-network complex fractures. During the fracturing construction, it is recommended to perforate a layer with a large difference between the vertical stress and the minimum horizontal principal stress, which is conducive to create vertical hydraulic fractures, then the complex fractures will beformed to enhance the effect of reservoir reconstruction.