A continuum model for coupled stress and fluid flow in discrete fracture networks
A continuum model for coupled stress and fluid flow in discrete fracture networks
复制标题
DOI：
10.1007/s40948-015-0020-0
复制
发表时间：
2016-01
影响因子：
5
通讯作者：
Q. Gan;D. Elsworth
中科院分区：
文献类型：
--
作者：
Q. Gan;D. Elsworth
通过将连续介质模拟器TF_FLAC3D与变形和流动的逐单元不连续定律相耦合，我们提出了一个由连续介质表示的不连续裂隙体内应力和流体流动的耦合模型。等效介质裂隙刚度和渗透率张量方法被用来描述预先存在的离散裂缝。这种方法的优点是，它允许在储集层内创建裂缝网络，而不依赖于裂缝几何形状或网格。该模型针对嵌入在切割多个网格块的无限多孔介质中的单个应力裂缝周围的热损耗进行了验证。将孔洞的演化与其他模拟器的结果进行比较，证实了合并的本构模型的准确性，该模型考虑了不同应力状态下的应力相关孔洞，包括正常闭合、剪切膨胀和拉伸载荷下脱离接触的断裂壁。冷注入条件下有明显的诱导热卸载效应，比等温注入条件下产生更大的孔径和渗透率。将该模型应用于离散裂缝网络，以跟踪由于应力状态(均值和偏差值)和裂缝方向的影响而导致的裂缝渗透率的演化。裂缝系统的正常闭合是主要的机制，在这种情况下，平均应力以0.65的恒定应力倾角比增大，从而导致渗透率降低。相反，对于不同的应力倾角(0.65-2)，剪切变形是导致渗透率增加的主要机制。准平行于主应力的裂缝是近临界应力，最容易滑动、扩张和增加渗透率。垂直于主应力方向的岩层被压实，渗透率降低。这些机制增加了岩体中渗透率的各向异性。此外，随着网络变得越来越稀疏，失去连通性会导致渗透率降低，加压区被锁定在注入器附近--有可能导致压力上升和诱发地震活动水平上升。
We present a model coupling stress and fluid flow in a discontinuous fractured mass represented as a continuum by coupling the continuum simulator TF_FLAC3Dwith cell-by-cell discontinuum laws for deformation and flow. Both equivalent medium crack stiffness and permeability tensor approaches are employed to characterize pre-existing discrete fractures. The advantage of this approach is that it allows the creation of fracture networks within the reservoir without any dependence on fracture geometry or gridding. The model is validated against thermal depletion around a single stressed fracture embedded within an infinite porous medium that cuts multiple grid blocks. Comparison of the evolution of aperture against the results from other simulators confirms the veracity of the incorporated constitutive model, accommodating stress-dependent aperture under different stress states, including normal closure, shear dilation, and for fracture walls out of contact under tensile loading. An induced thermal unloading effect is apparent under cold injection that yields a larger aperture and permeability than during conditions of isothermal injection. The model is applied to a discrete fracture network to follow the evolution of fracture permeability due to the influence of stress state (mean and deviatoric) and fracture orientation. Normal closure of the fracture system is the dominant mechanism where the mean stress is augmented at constant stress obliquity ratio of 0.65—resulting in a reduction in permeability. Conversely, for varied stress obliquity (0.65–2) shear deformation is the principal mechanism resulting in an increase in permeability. Fractures aligned sub-parallel to the major principal stress are near-critically stressed and have the greatest propensity to slip, dilate and increase permeability. Those normal to direction of the principal stress are compacted and reduce the permeability. These mechanisms increase the anisotropy of permeability in the rock mass. Furthermore, as the network becomes progressively more sparse, the loss of connectivity results in a reduction in permeability with zones of elevated pressure locked close to the injector—with the potential for elevated pressures and elevated levels of induced seismicity.