Large diameter boreholes drilled to a depth of 1000–2000 m are currently investigated in Australia as a potential solution for waste emplacement and long-term containment for long-lived intermediate-level radioactive waste (ILW). Different potentially suitable types of host rocks are considered, including granite, shale and salt. This study is aimed at assessing the risk of borehole instability for a large diameter deep borehole in shale. The investigation is conducted with numerical simulations using the CSIRO-developed modelling tools FRACOD and FRCAOD^3D, and the commercial code Irazu. Previous laboratory experiments on boreholes in deep shale reported in literatures have been used as validation tests of the numerical models. Under hydrostatic stresses, the FRACOD model showed extensive shear-dominated “log-spiral” fractures around a borehole in shale which is similar to the laboratory testings reported in literatures. Subsequently, 2D and 3D numerical models representing a 0.7-m diameter vertical borehole drilled to the depths of 1000 m and 2000 m have been used. “Strong” and “weak” shales are simulated. The strong and weak shales represent the higher and lower ends of the shale strength reported in literature, respectively. Results showed that a “strong” shale exhibited only small borehole breakouts at the depth of 2000 m. The “weak” shale showed large breakouts (>1.5 m) at depths of 1000 m and 2000 m caused by extensive shear fracturing. In the scenario where a pre-existing 3D fracture crosscuts the borehole, the existing fracture may propagate deeper into the rock formation depending on its orientation and on the in situ stress magnitudes. Predictions based on analytical solutions indicate a 0.02-m and 0.04-m thick casing could prevent severe failure of rock mass at 1000 and 2000 m depths, respectively. Insights from model-based investigations of borehole breakouts are critical during the site selection of rocks with suitable geomechanical properties for waste disposal and for optimising the design of large-diameter boreholes for drilling, waste emplacement and long-term safety.

澳大利亚目前正在研究钻探深度为 1000-2000 米的大直径钻孔，作为废物安置和长期遏制长寿命中水平放射性废物 (ILW) 的潜在解决方案。考虑了不同潜在合适类型的主岩，包括花岗岩、页岩和盐。本研究旨在评估页岩中大直径深钻孔的钻孔不稳定风险。该调查是使用 CSIRO 开发的建模工具 FRACOD 和 FRCAOD ^3D以及商业代码 Irazu 进行数值模拟的。文献中报道的先前的深层页岩钻孔实验室实验已被用作数值模型的验证测试。在静水应力下，FRACOD 模型显示页岩钻孔周围存在广泛的剪切主导的“对数螺旋”裂缝，这与文献中报告的实验室测试类似。随后，使用了代表钻探深度为 1000 米和 2000 米的 0.7 米直径垂直钻孔的 2D 和 3D 数值模型。模拟“强”和“弱”页岩。强页岩和弱页岩分别代表文献报道的页岩强度的上限和下限。结果显示，“坚固”的页岩仅在 2000 米深度处出现小钻孔破裂。“弱”页岩在 1000 m 和 2000 m 深度处显示出由大范围剪切压裂引起的大破裂（>1.5 m）。在预先存在的 3D 裂缝横切钻孔的情况下，现有裂缝可能会深入岩层，具体取决于其方向和原位应力大小。基于解析解的预测表明，0.02 米和 0.04 米厚的套管可以分别防止 1000 米和 2000 米深度岩体的严重破坏。在选择具有合适地质力学特性的岩石进行废物处理以及优化大直径钻孔设计以进行钻孔、废物安放和长期安全时，基于模型的钻孔破裂研究的见解至关重要。