层状岩石断裂能各向异性对水力裂缝扩展路径影响研究

The influence of fracturing energy anisotropy on hydraulic fracturing path in layered rocks

作为一种横观各向同性材料,层状岩石在沿层理和垂直层理方向上其断裂能差异明显。基于能量分析方法,提出考虑断裂能各向异性的水力裂缝扩展路径判定准则,该准则在线弹性条件下可转化为断裂韧性、弹性参数和应力强度因子之比组合的三参数模型。通过包含该准则的Cohesive有限元计算,讨论断裂能各向异性对层状岩石中水力裂缝扩展路径的影响。研究发现,断裂能各向异性是限制水力裂缝扩展的控制因素,且不同岩石存在垂直层理与沿层理2个方向断裂能比的极限值。当垂直层理和沿层理2个方向的断裂能比值小于该极限值时,裂缝穿层扩展;当这个比值在该极限值附近时,裂缝穿层伴有部分偏转扩展;当这个比值大于该极限值时,裂缝仅沿岩层界面扩展。研究结果对优选射孔位置和施工排量、提高压裂的增产效果具有参考意义。

For transversely isotropic material,there is a significant difference in fracturing energy along and perpendicular to the bedding plane of layered rocks. In this paper,a fracture path criterion taking the fracture energy anisotropy into consideration was proposed. Under the linear elastic condition,the criterion is transformed into a three-parameter model of fracture toughness,elastic parameters and the ratio of stress intensity factors. The influence of the fracturing energy anisotropy on the hydraulic fracturing path was studied through finite element simulations with the criterion considered in the cohesive elements. The study indicates that the fracturing energy anisotropy is a dominant factor limiting the hydraulic fracture propagation. There is a limit in the ratio of the fracturing energy perpendicular to/along the bedding in different rocks. When the ratio for the fracturing energy perpendicular to/along the bedding is less than the limit value,the fracture will propagate across the layer. As the ratio is around the limit value,the fracture will extend across the layer with deflection. For the ratio is greater than the limit value,the fracture will be deflected along the layer interface.