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The efficient development of shale oil and gas plays an important role in ensuring energy supply. The key to high-efficiency mining depends on the generation of complex fracture networks: the complex fractures formed by the interaction of weak planes such as bedding and hydraulic fractures provide the production channels for shale oil and gas, just as the multi-layered complex vascular system in the human body transfers nutrients to the body. section. Mechanically, the critical condition for the hydraulic fracture to expand is a key to unlock the mystery of the mesh formation. The provision of this condition allows the engineer to optimize the design of the fracturing project under the guidance of the theory and form a more reasonable crack passage.
Researchers from the Institute of Mechanics, Chinese Academy of Sciences, based on the principle of minimizing the energy release rate, gave an explicit expression of the rate of release of the strain energy in the hydraulic crack in any direction considering the factors such as the ground stress and friction, and deduced the hydraulic force. Theoretic solution to crack propagation condition. These results solve the long-standing fracturing geomechanical paradox: the hydraulic fracturing is intuitively an expanding crack failure mode and experimental observations have shown that the failure mode is mainly shear-type. The theoretical work reveals the following important engineering laws: (1) With the increase of water pressure, shale reservoirs will undergo friction lock stage, type II (shear) fracture stage and mixed fracture stage in turn, and along the weak surface type II ( Shearing) fracture is the main fracture mode in shale hydraulic fracturing; (2) The smaller the friction coefficient of the fracture surface or the greater the geostress difference, the smaller the fracturing pressure required for the hydraulic fracture to expand along the shale weak surface or matrix. The shale reservoir is more prone to fracturing; (3) In type II (shear) fracture stage, the difference between the fracture surface friction force and the ground stress does not affect the crack propagation direction, and the incident angle between the crack and the weak plane is the crack Turning to determinants of behavior.
This work was supported by the National Natural Science Foundation of China (Wei Yujie, No. 11425211) and published in the Journal of the Mechanics and Physics of Solids.
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