高能复合射孔爆生气体作用下预存裂缝起裂扩展研究

Initiation and extension of existing cracks under detonation gas loading in a high energy combined perforation

在高能复合射孔压裂过程中,受爆生气体流动速度、裂缝表面粗糙度、裂缝周围渗透性等因素的影响,裂缝内压力梯度随时间而变化。气体压力分布影响裂缝尖端应力强度,因而不能用传统的破裂压力来判断裂缝起裂扩展。通过设定与时间相关的气体压力分布函数,推导高地应力约束条件下预存裂缝尖端应力强度,得到了井壁岩层起裂扩展条件,该条件可以反映裂缝内爆生气体压力梯度随时间变化的特性。从相应的算例分析,井眼内气体升压越缓,起裂压力和止裂压力越低,且使止裂时间越推后,能得到较长的岩层裂缝长度。

In a high energy combined perforation, the pressure gradient of detonation gas along cracks varies with time due to the influences of flow velocity, crack surface roughness and media permeability. Since the pressure gradient affects stress strength of crack top, it is unsuitable to judge crack initiation by the traditional breakdown pressure. A function describing gas pressure distribution along the crack is proposed to derive the corresponding stress strength of crack top, and then the conditions of crack initiation in borehole rocks are concluded. The conditions reflect that gas pressure gradient varies with time. The example analysis reveals that slower borehole pressurization gives rise to lower crack initiation and arrest pressure, greater extension of crack arrest time, and longer cracks in the formation.