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Discrete element modelling of stress-induced instability of directional drilling boreholes in anisotropic rock

机译：各向异性岩石中定向钻孔应力不稳定性的离散元建模

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摘要

Directional drilling is a viable and cost-effective trenchless technology for the installation of pipelines and conduits in shallow formations and the extraction of unconventional energy resources in deep reservoirs. The performance of drilled boreholes is associated not only with drilling practice (e.g., borehole size and drilling direction) but also with geological environment (e.g., in-situ stress and geomaterial property). We conduct discrete element modelling on hollow anisotropic rock samples to elucidate the stress-induced instability along a directional drilling borehole with vertical, inclined, and horizontal sections. We systematically evaluate the influences of confining pressure, borehole diameter, and bedding orientation on the mechanical response of the hollow samples. When the confining pressure increases, the peak stress varies from a linearly increasing fashion for relatively small borehole diameters to an unstable variation trend for relatively large borehole diameters. Both peak stress and elastic modulus decrease with larger borehole diameter. The effects of bedding planes are negligible when the borehole exists perpendicular to bedding planes, while borehole instability likely occur when the borehole is oriented along bedding planes. The presence of bedding planes leads to the concentration of tangential stress along the normal direction of bedding planes, which ultimately dominates the failure mechanisms at the particle scale. With the increase of anisotropy angle, the dominant failure modes transform from the tensile failure of rock matrix to the shear failure along bedding planes.
机译：定向钻探是一种可行且经济高效的非开挖技术，用于在浅层地层中安装管道和导管以及在深层储层中提取非常规能源。钻孔的性能不仅与钻孔实践（例如，钻孔尺寸和钻孔方向）有关，而且与地质环境（例如，原地应力和土工材料特性）有关。我们对空心各向异性岩石样本进行离散元素建模，以阐明沿垂直，倾斜和水平截面的定向钻孔产生的应力不稳定性。我们系统地评估了围压，井眼直径和层理取向对空心样品力学响应的影响。当围压增加时，峰值应力从相对较小的井眼直径线性增加的方式变化到相对较大的井眼直径不稳定的变化趋势。峰值应力和弹性模量都随钻孔直径的增大而减小。当井眼垂直于井底平面存在时，井底平面的影响可以忽略不计，而当井眼沿着井底平面定向时，井眼不稳定可能会发生。顺层平面的存在导致切线应力沿着顺层平面的法线方向集中，最终在粒子尺度上主导了破坏机制。随着各向异性角的增加，主要破坏模式从岩体的拉伸破坏转变为沿层理面的剪切破坏。

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