裂缝性地层水力裂缝张性起裂压力分析

HYDRAULIC FRACTURE TENSILE INITIATION PRESSURE ANALYSIS FOR FRACTURED FORMATIONS

室内实验和矿场压裂实践表明,裂缝性地层水力裂缝在近井区域可能扩展为复杂的径向缝网,这与均质地层水力压裂产生的平面对称双翼裂缝具有巨大差异。由于水力裂缝延伸形态受起裂和延伸2个过程的控制,为此,研究裂缝性地层水力裂缝起裂机制是认识径向缝网扩展的前提。基于弹性力学和岩石力学理论,考虑天然裂缝与射孔孔眼相交,结合张性起裂准则,建立裂缝性地层水力裂缝沿天然裂缝张性起裂的起裂压力计算模型。计算结果表明:天然裂缝与孔眼相交点越靠近孔眼指端,张性起裂压力越小;天然裂缝与孔眼相交点越靠近孔眼顶部,张性起裂压力越小;受天然裂缝和水平地应力方位的影响,井眼周向上不同方位孔眼的张性起裂压力差可能急剧减小,这种作用效应将导致水力裂缝从井眼周向上不同方位的孔眼同时起裂延伸,产生径向缝网。实例计算表明,文中建立的起裂压力模型计算精度高,计算结果可靠,可用于裂缝性地层水力裂缝起裂压力计算和径向缝网扩展分析。

Laboratory experiments and field fracturing practices have shown that complex radial fracture-network extended system is easily formed in the area around borehole due to the effect of natural fractures during fracturing of fractured reservoirs, which greatly differs from the single, bi-wing, symmetric planar fracture generated in the homogeneous reservoirs. As the forming of radial fracture-network is mainly controlled by the initiation and propagation of hydraulic fracture at the beginning stage of hydraulic fracturing, studying the initiation mechanism of hydraulic fracture for fractured formations is the prerequisite for understanding the forming of radial fracture-network. Based on the theories of elastic mechanics and rock mechanics, taking account of the spatial relation of natural fractures and perforations under intersection, and integrating with the criterion of tensile initiation, the calculating model of initiation pressure in which the hydraulic fracture is tensile initiation along natural fractures is established. The calculation results show that the deeper position of natural fractures intersecting with perforation is, the lower the tensile initiation pressure is. Tensile initiation pressure in the position where natural fractures intersect the top of perforation is the minimum value. The difference of tensile initiation pressure may sharply decreases from perforations in different azimuths around borehole due to the effect of natural fractures and geostress azimuth, which leads to the extending of hydraulic fracture simultaneously from perforations in different azimuths and forms radial fracture-network. The example calculation results indicate that this calculating model can obtain high calculating accuracy and reliable calculating results, and can be used to calculate initiation pressure of fractured formations and analyze the probability of radial fracture-network propagation during fracture-network fracturing.