预制横缝条件下水力裂缝的起裂和扩展

针对高角度天然裂缝发育地层中的水平井水力压裂问题，开展了水力裂缝自天然裂缝处起裂扩展的理论和试验研究。尝试将天然裂缝简化为与井筒轴线垂直的横向裂缝，基于线弹性断裂力学理论和最大拉应力准则，给出了水力裂缝起裂压力和扩展过程中应力强度因子的计算方法。利用预制横缝模拟高角度天然裂缝，开展了室内水力压裂试验，对水力裂缝的扩展形态和起裂压力进行了研究。理论计算表明，（1）水力裂缝自预制横缝端部起裂后，扩展距离超过1倍的预制横缝端部半径时可将预制横缝和水力裂缝合并起来，整体视作一条横向裂缝来计算应力强度因子；（2）水力裂缝尖端距井壁处的距离大于4倍的井筒半径时，应力强度因子的计算可忽略井筒的影响，近似采用硬币形裂缝的计算公式。试验研究发现，（1）水力裂缝在预制横缝端部起裂并扩展，形成与井筒轴线垂直的横向裂缝，裂缝的扩展呈现出Ⅰ型断裂的特点，形态近似呈圆形，未发现与井筒轴线平行的纵向裂缝的起裂和扩展；（2）排量对破裂净压力和起裂净压力有重要影响，大排量会导致较高的破裂净压力和起裂净压力，在大、小两种排量下起裂净压力的离散性均较小，计算得到的KⅠ临界断裂值的离散性也较小。研究结果可为改善裂缝发育储层的近井裂缝形态提供指导，也可为煤矿开采中预制横向切槽的水力压裂设计提供参考。

Initiation and propagation of hydraulic fractures under the condition of prefabricated transverse fracture

Aiming at the problem of horizontal well hydraulic fracturing in formation with dip angle natural fractures, theoretical and experimental studies on the initiation and propagation of hydro-fracture from natural fracture are carried out. Simplifying the natural fracture as a transverse fracture perpendicular to the well axis, the method of calculating initiation pressure and stress intensity factor (SIF) are given, based on the theory of linear elastic fracture mechanics and the maximum stress criterion. Meanwhile, using a prefabricated transverse fracture to simulate natural fracture, laboratory hydraulic fracturing test is carried out to investigate the fracture geometry and initiation pressure. Theoretical calculation shows that: (1) after initiating from the prefabricated transverse fracture tip, the prefabricated transverse fracture and the fracture can be regarded as a whole and treated as a transverse fracture to calculate SIF when the fracture propagates longer than the radius of the notch tip. (2) when the distance between the fracture tip and the wellbore wall is longer than four times of the wellbore radius, the wellbore effect can be neglected and SIF equation for penny-shaped crack can approximately be used. Experimental studies show that (1) hydraulic fracture initiates at the tip of the prefabricated transverse fracture and propagates to form a transverse fracture orthogonal to the wellbore; fracture propagation shows the feature of model Ⅰ fracturing; the fracture geometry is near round; no longitudinal fracture is observed. (2) pumping rate has a significant effect on the net breakdown pressure and net initiation pressure with larger pumping rate leading to higher net pressures; the discreteness of net initiation pressure and calculated critical fracturing value KⅠ are small under both large and small pumping rate conditions. Research results can provide guidance for improving near wellbore fracture geometry of naturally fractured reservoir and reference for designing hydraulic fracturing with prefabricated transverse fracture in coal mining.