不同粒径特征的砾石层井壁围岩破坏机制

Failure mechanisms of borehole wall rocks in gravel beds with different grain sizes

塔里木油田大北区块新近系库车组是一套长段砾石地层,在钻井过程中多发生井壁坍塌、掉块等钻井复杂问题。而砾石地层具有的弱胶结和不连续的特点,使得基于连续介质力学的分析方法不能很好解释井周砾石剥落和掉块的机理。为此,首先对该区砾石层露头进行粒度分析、微观电镜扫描、室内三轴实验,获得了砾石层物理力学特性;进而采用非连续特性的离散元数值模拟方法,深入地分析了该套地层中砾石以及粒径分布对围岩破坏和井周裂缝扩展特征。研究结果表明:①砾岩破坏方式为劈裂,破裂面沿着砾石颗粒的胶结面扩展,形成一条呈锯齿状的破坏面;②井周的大颗粒砾石降低了井壁围岩的抗压强度,在井眼液压作用下,砾石层井壁产生掉块,形成以最小水平主应力方向为长轴的不规整崩落椭圆;③井壁在非均匀构造应力作用下,发育小粒径砾石地层裂缝沿着最小主应力方向扩展,砾石的分布对裂缝扩展的影响较小,而发育大粒径砾石地层中,裂缝在砾石大颗粒周围产生,优先向附近砾石集中区域扩展,改变初始裂缝传播路径,扩展速度更快,延伸距离更远,逐渐形成更大范围的破坏区域,导致井壁失稳。

In Dabei Block of Tarim Oilfield, the Neogene Kuche Formation is a set of long gravel beds, in which borehole wall col- [apse and failings occurred frequently during well drilling. Gravel beds are characterized by weak consolidation and discontinuity, so the mechanisms of gravel scaling and falling around the wells could not be well explained by the analysis methods based on continuum mechanics. In this paper, the physical mechanics characteristics of gravel beds were identified by performing grain size analysis, elec tron microscope scanning and laboratory triaxial tests on the gravel outcrops in this block. And then, analysis was conducted of the effect of gravels and size distribution on surrounding rock failure and circumferential fracture extension by using discontinuity dis crete-element numerical simulation. It is shown that gravel failures are in the mode of splitting, the fracturing planes extend along the consolidation planes of gravel grains and a sawed breaking plane forms. Compressive strength of wall rocks is reduced by the large sized gravels around the well. Under the force of borehole hydraulic pressure, wall fallings occur in gravel beds, and an irregu- lar breaking ellipse is formed with the long axis along the direction of minimum horizontal principal stress. Due to the heterogeneous tectonic stress on borehole walls, fractures in small-sized gravel beds extend along minimum principal stress and fracture extension is less affected by gravel distribution. In large size gravel beds, however, fractures are formed around large-size gravels and extend preferentially to gravel concentration zones, so the original fracture propagation paths are changed with high speed and long distance. And consequently, the failure zone is enlarged and the borehole wall instability occurs.