Water jet drilling (WJD) is an effective technique for drilling micro-holes in the subsurface for reservoir stimulation. This study aims to investigate numerically the failure mechanism of rock during WJD and to assess the WJD performance before drilling. A 3D fluid-solid coupling model is developed for simulating WJD by coupling a mechanical solver based on the combined finite-discrete element method (FDEM) with a fluid solver using an immersed-body method. The new numerical model is capable of simulating crack initiation and propagation and fragment removal under the impact load of a high-speed water jet. The poroelastic effect is implemented via Biot's theory of poroelasticity. The numerical results show that: (1) rock failure is only observed in the Gildehaus sandstone with the lowest strengths among the three types of rock tested, (2) most of the cracks are tensile failures and pure shear cracks are rare, mixed mode cracks account for 15%∼40% of the total crack number depending on the mechanical boundary conditions, (3) increased water back pressure significantly suppresses jet erosion. The poroelastic effect on the rock failure is insignificant in the mesoscale simulations and will be further investigated using a microscale model in future.

水射流钻井 (WJD) 是一种在地下钻探微孔以进行储层增产的有效技术。本研究旨在以数值方式研究 WJD 期间岩石的破坏机制，并在钻井前评估 WJD 性能。通过将基于组合有限离散元法 (FDEM) 的机械求解器与使用浸入体法的流体求解器耦合，开发了用于模拟 WJD 的 3D 流固耦合模型。新的数值模型能够模拟高速水射流冲击载荷下的裂纹萌生和扩展以及碎片去除。孔隙弹性效应是通过 Biot 的孔隙弹性理论实现的。数值结果表明：（1）岩石破坏仅见于 Gildehaus砂岩3种岩石中强度最低，（2）裂缝大多为拉伸破坏，纯剪切裂缝很少，混合模式裂缝占总裂缝数量的15%~40%，取决于力学边界条件, (3) 增加水背压显着抑制射流侵蚀。孔隙弹性对岩石破坏的影响在中尺度模拟中并不显着，未来将使用微尺度模型进一步研究。