Synchronous vertical propagation mechanism of multiple hydraulic fractures in shale oil formations interlayered with thin sandstone

To investigate the synchronous vertical propagation mechanism of multiple hydraulic fractures in shale oil formations interlayered with thin sandstone (SIS), we conduct a series of accurate, triaxial, hydraulic fracturing experiments using artificial SIS samples and analyze the effects of the formation dip angle and vertical stress difference on the penetration behavior of a single hydraulic fracture. Further, we develop a numerical model for the synchronous propagation of multiple fractures in SIS formations on a field scale using a three-dimensional lattice algorithm and investigate the controlling effects of the critical fracturing operation parameters on the penetration behavior of multiple hydraulic fractures. Increasing the formation dip angle inhibits the ability of the hydraulic fractures to penetrate the interlayer in the longitudinal direction significantly, while increasing the vertical, in-situ stress difference can improve this penetration ability. When the length of the segment to be fractured remains fixed, too many or few fractures are unfavorable to the longitudinal extension of the hydraulic fractures. When the segment to be fractured includes five perforation clusters, multiple hydraulic fractures on the outside can better penetrate the sandstone interlayer and enter the adjacent shale layers. Appropriately increasing the injection rate and viscosity of the fracturing fluid can enhance the penetration and extension ability of the outer hydraulic fractures; however, massive injections enhance the communication between the intermediate fractures and weaken the penetration and expansion ability of the outer fractures. An injection rate of the fracturing fluid equal to 12 m³/min can produce a better layer-penetration hydraulic-fracturing effect. When the method of intermittent pumping at a decreased injection rate of the fracturing fluid is used for fracturing under cyclic loading of first high and then low loads, the ability of multiple fractures to penetrate the interlayer in the longitudinal direction can improve further. The obtained results can provide a deeper understanding of the synchronous longitudinal propagation mechanism of multiple fractures in SIS formations, thereby providing more accurate guidance on optimizing layer-penetration fracturing parameters.