层理对页岩水力裂缝扩展的影响研究

doi:10.13722/j.cnki.jrme.2015.02.002

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Abstract The existence of bedding planes，cracks and other structural planes is the precondition of stimulated reservoir volume of shale formations. In order to analyze the effect of bedding planes on propagation of hydraulic fractures in shale formations，three-point bending tests of notched cylindrical specimens with different bedding orientations were carried out，based on the distribution of stress field around a crack tip of anisotropic materials. The mechanisms of anisotropic failure were revealed through the analysis of the anisotropy of Mode-I fracture toughness. The important role of bedding planes in the formation of fracture network was discussed according to the extension of hydraulic fractures in shale formations under true triaxial stress conditions. The results show that the stress and displacement fields around a crack tip of anisotropic materials depend not only on the stress intensity factor but also on the elastic constants. The notable anisotropy of Mode-I fracture toughness of shale is observed with the fracture toughness to be the largest when the pre-crack is crack-arrester orientated and to be the lowest when the pre-crack was crack-splitter orientated. The cracking of weak bedding planes and the deflection of fracture path were found to be the main mechanisms led to the strong anisotropy of Mode-I fracture toughness of shale. Bedding planes are weak in preventing crack propagation due to smaller fracture toughness induced by weak cementation，while the matrix is strong in preventing crack propagation due to much larger fracture toughness. Branching and re-orientation of hydraulic fractures in bedding planes and then interconnecting with natural fractures or bedding planes are found to be the main mechanisms of the formation of fracture network when the fractures extending perpendicular to the bedding planes.

Key words： rock mechanics shale bedding planes hydraulic fracturing fracture propagation anisotropy fracture toughness

Received: 14 April 2014

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https://rockmech.whrsm.ac.cn/EN/10.13722/j.cnki.jrme.2015.02.002 OR https://rockmech.whrsm.ac.cn/EN/Y2015/V34/I2/228

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