层理对页岩水力裂缝扩展的影响研究

层理、裂隙等结构面的存在是实现页岩气藏储层体积改造的前提。为分析层理对页岩水力裂缝扩展的影响,在各向异性材料裂纹尖端应力场分布特征的基础上,开展切口与层理呈不同方位的圆柱形试样三点弯曲试验,研究页岩断裂韧性的各向异性特征,并揭示其断裂机制的各向异性,进而根据真三轴条件下页岩水力裂缝的延伸规律,探讨了层理在页岩网状压裂缝形成过程中的重要作用,结果表明:(1)各向异性材料裂纹尖端的应力场和位移场不仅由应力强度因子决定,还与材料的弹性常数有关;(2)切口与层理呈crack-arrester,crack-divider和crack-splitter三种方位时,页岩断裂韧性在crack-arrester时最大,crack-splitter时最小,各向异性显著,而层理面开裂和断裂路径偏移是引起断裂韧性各向异性的主要原因;(3)页岩层理的弱胶结作用使其断裂韧性较小,阻止裂纹失稳扩展的能力较弱,而在垂直层理方向,断裂韧性较大,阻止裂纹扩展的能力较强,当水力裂缝垂直层理扩展时,在弱层理面处会发生分叉、转向,且在继续延伸的过程中会进一步沟通天然裂缝或弱层理面而形成复杂的裂缝网络,达到体积压裂。研究结果可为深入认识页岩气藏储层体积压裂形成条件及机制提供一定参考。

INFLUENCE OF BEDDING PLANES ON HYDRAULIC FRACTURE PROPAGATION IN SHALE FORMATIONS

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.