页岩纹层结构对水力裂缝扩展规律的影响

Effects of laminated structure on hydraulic fracture propagation in shale

根据鄂尔多斯盆地延科1井陆相页岩岩心特征,制备了岩性相似的混凝土样品。利用正交实验法四因素三水平设计原理,并采用真三轴水力压裂实验系统进行实验,以模拟水平主应力差、射孔套管与纹层的夹角、纹层厚度和间距对水力裂缝扩展规律的控制作用。结果表明:当水平主应力差较小时,试样的主破裂面为平行于纹层走向的面,一级裂缝走向的模式为:穿过纹层—在纹层层面处发生较大偏转—沿着纹层层面扩展—发生较大偏转—穿过纹层;当水平主应力差较大时,试样的主破裂面为垂直于纹层走向的面,一级裂缝走向的模式为:穿过纹层—发生较大偏转—穿过纹层。水平主应力差越小,裂缝偏转的角度越大,压裂效果越好;射孔套管与纹层的夹角越小,致裂性越好;页岩纹层的厚度适中时,分支裂缝多,压裂效果较好;凝灰岩纹层的厚度较小时,压裂效果好。水力压裂效果对水平主应力差最敏感,射孔套管与纹层的夹角次之。

Concrete samples with properties similar to the continental shale core of Well Yanke 1 in the Ordos Basin in the Shaanxi province, China, were made. The control effects of horizontal principal stress difference, the angle between perforated casing and lamina, the lamina thickness and space on hydraulic fracture propagation were modeled on a real tri-axial hydraulic fracturing experiment system, based on the design principle of four factors-three levels of orthogonal experiment. When the horizontal principal stress difference is small, the main fracture surface is parallel to the laminae strike, and the primary fractures will propagate as follows： propagate through the laminae, then deflect significantly at the surface of the laminae, continue along the laminae surface and finally change direction and propagate again through the laminae. When the horizontal principal stress difference is big, the main fracture surface is perpendicular to the laminae strike and the primary fractures will propagate as follows： run through the laminae, change their propagation direction dramatically on the laminar surface, and then run through the laminae. The smaller the difference of the horizontal principal stress, the bigger the angle of fracture deflection, and the better the fracturing result will be; the smaller the angle between perforated casing and laminae, the better the fracturing result will be; when the thickness of shale laminae is moderate, there will be more branching fractures and the fracturing result is better; tuff layers with smaller lamina thickness have better hydraulic fracturing effect. The hydraulic fracturing effect is most sensitive to horizontal principal stress difference, followed by the angle between perforated well casing and laminae.