页岩水力压裂水力裂缝与层理面扩展规律研究

在对含天然层理弱面页岩进行水力压裂过程中,水力主裂缝的起裂、扩展及层理面的扩展对缝网的形成有重要影响。为研究水力主裂缝的起裂、扩展规律和层理面对水力裂缝扩展的影响,开展真三轴试验条件下的水力压裂试验,采用声发射系统监测水力压裂过程,并在试验后对试样进行剖切与CT扫描;同时进行定量的理论分析,并通过试验结果验证。研究表明:(1)起裂方向由初始角度转至最大水平主应力方向;垂向应力与水平最大主应力相差极小时,各个方向起裂压力相差极小,裂缝很快转向最大水平主应力方向。(2)水力主裂缝整个扩展过程中所需水压区间与裂缝长度、断裂韧性值相关。(3)形成由层理面与主裂缝构成的网状的裂缝系,层理面在主裂缝的靠近过程中张开区的长度极小,主要在主裂缝接触到层理面后产生较大的张开区与剪切区,层理面的剪切区域长度远大于张开区域长度,剪切区域提供主要的导流通道;剪切区的长度对层理面黏聚力c和水力裂缝与层理面交角?参数敏感性很高。研究结果可以为压裂模型的建立提供几何参数,并对施工参数的设计有指导意义。

PROPAGATION OF HYDRAULIC FISSURES AND BEDDING PLANES IN HYDRAULIC FRACTURING OF SHALE

In order to investigate the interaction of the hydraulic fractures and the bedding planes of shale in hydraulic fracturing，the hydraulic fracturing experiments under triaxial conditions were carried out with the acoustic emission system monitoring the process of hydraulic fracturing and the CT scanning system scanning the sections of rock samples before and after hydraulic fracturing for capturing the distribution of fractures. The initiation and propagation of the hydraulic fracture was captured experimentally. At the situation of small difference of stresses，the critical pressure for initiation was found to be differed little in different angels，the fracture grew along the direction of after initiation. The critical pressure for the propagation of major hydraulic fractures was related to the length of fracture and . The small sections of the bedding planes in shale were opened up when the major hydraulic fracture approached. The passage was created at the intersection of the hydraulic fracture and the bedding plane due to shearing tension. The shear zone was larger than opened one，with a ratio of 13 at maximum，functioning as the major passage for fluid flow. The length of the shear zone is sensitive to the cohesion of bedding planes, and the angle of intersection of hydraulic fracture and bedding plane.