Almost all reservoir rocks contain natural fractures. The presence of such discontinuities affects reservoir stimulation to a certain degree. The interaction between the natural and hydraulic fractures determines the overall fracture trajectory and the stimulated reservoir volume. In contrast to conventional hydraulic fracturing, natural fractures do not propagate in a tensile-dominated regime. Propagation of mechanically closed fractures lies in the mixed-mode propagation regime which differs from that of open fractures. Therefore, the response of natural fractures to pore pressure and stress perturbations should be carefully addressed within the context of reservoir stimulation. This study aims at investigating the response of closed natural fractures to water injection in poroelastic rocks. A poroelastic displacement discontinuity (DD) model is developed and used to shed light on the propagation of closed natural fractures. Mohr–Coulomb contact elements are used in this study to identify the contact status of the elements in the transverse direction. Moreover, fracture propagation is effectively accounted for based on linear elastic fracture mechanics. Hydro-mechanical coupling is achieved by combining an implicit finite difference scheme with the poroelastic DD model. Our simulation results indicate that natural fracture shear slip and propagation are likely to occur when the closed natural fractures are subject to direct water injection. The injection pressure required to maintain the propagation in poroelastic rocks was found to be consistently higher than the pressure in its elastic counterpart. The propagation trajectory of the wing cracks was, on the other hand, found to be similar to that in elastic rocks. It was found in our results that the formation of wing cracks and shear (secondary) cracks is an integral part of the reservoir stimulation in geothermal systems where natural fractures are in direct contact with the injection wells. This point is often overlooked or poorly treated and has led to erroneous interpretations from the reservoir stimulation standpoint.

中文翻译：

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储层岩石中孔隙弹性在天然裂缝扩展方式中的作用

几乎所有的储层岩石都含有天然裂缝。这种不连续性的存在在一定程度上影响了储层增产。天然裂缝和水力裂缝之间的相互作用决定了整体裂缝轨迹和增产储层体积。与传统的水力压裂相比，天然裂缝不会以拉伸为主的方式扩展。机械闭合裂缝的扩展存在于不同于开放裂缝的混合模式扩展机制中。因此，天然裂缝对孔隙压力和应力扰动的响应应在储层增产的背景下仔细解决。本研究旨在研究闭合性天然裂缝对多孔弹性岩石中注水的响应。开发了多孔弹性位移不连续性 (DD) 模型并用于阐明闭合天然裂缝的扩展。本研究中使用 Mohr-Coulomb 接触单元来识别单元在横向上的接触状态。此外，基于线弹性断裂力学有效地解释了断裂扩展。流体力学耦合是通过将隐式有限差分格式与多孔弹性 DD 模型相结合来实现的。我们的模拟结果表明，当闭合的天然裂缝受到直接注水时，很可能发生天然裂缝剪切滑移和扩展。发现在多孔弹性岩石中维持传播所需的注入压力始终高于其弹性岩石中的压力。另一方面，发现翼裂纹的传播轨迹与弹性岩石中的相似。在我们的结果中发现，在天然裂缝与注入井直接接触的地热系统中，翼裂缝和剪切（二次）裂缝的形成是储层增产的一个组成部分。这一点经常被忽视或处理不当，并导致从储层增产的角度做出错误的解释。