单轴加载条件下页岩层理角度对水力压裂缝扩展规律影响研究

EFFECT OF BEDDING ORIENTATION ON HYDRAULIC FRACTURE PROPAGATION IN SHALE UNDER UNIAXIAL COMPRESSION

水力压裂作为一种改造储层渗透性、压裂增产的技术,对页岩气开采具有重要意义。为研究射孔附近水力压裂过程中页岩各向异性特征对破裂压力及裂缝扩展的影响规律,开展了单轴试验条件下不同层理角度的页岩水力压裂试验。研究表明:页岩的破裂压力存在明显的各向异性,破裂压力随层理角度的分布曲线呈U型分布,其中0°和90°破裂压力最大,30°最小;页岩的破裂形态主要有两种,一种为沿着最大主应力方向即竖直方向起裂并延伸,另一种模式为裂缝先沿着最大主应力方向起裂并延伸,延伸过程中直接穿过层理面,随后渐渐转向为沿层理面方向扩展;破裂机制则包括拉张破坏和拉张剪切混合破坏。研究结果对于深入了解页岩裂缝起裂和延伸机理、水力压裂施工设计等具有重要的意义。

Hydraulic fracturing is a well-stimulation technique and has been used as one of the key methods of extracting unconventional oil and unconventional gas resources. Uniaxial hydraulic fracturing experiments are carried out in order to study the effect of shale bedding plane orientation with respect to loading direction on breakdown pressure and hydraulic fracture propagation. Fracturing is completed on cylindrical samples containing a single blind axial borehole under simple uniaxial conditions with axial stress of lOMPa. The results show that the effect of bedding plane orientation with respect to principal stress on the breakdown pressure of shale is obvious. Under the same applied stress conditions, 90°sample returns the highest breakdown pressure, followed by 0°s a m p l e , w h i l e 30°sample returns the lowest. As the result, the breakdown pressure curve shows a U-like shape. In terms of fracture morphology, there are two types of fracturing patterns founded in samples. One type is the cracks extending along the loading direction and the other is the crack extending along the loading direction at first and then turning to the bedding plane direction. Fracture propagation by hydraulic fracturing is induced by both shear and tensile stresses, which means the failure modes are tensile splitting, sliding failure or both. The experiment result has a significant impact on understanding mechanism of fracture extension and provides references for fracture optimization design in exploiting shale gas.