A numerical approach for assessing effects of shear on equivalent permeability and nonlinear flow characteristics of 2-D fracture networks
A numerical approach for assessing effects of shear on equivalent permeability and nonlinear flow characteristics of 2-D fracture networks
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DOI：
10.1016/j.advwatres.2017.11.022
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发表时间：
2018
影响因子：
4.7
通讯作者：
Ri-cheng Liu;Bo Li;Yujing Jiang;Liyuan Yu
中科院分区：
文献类型：
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作者：
Ri-cheng Liu;Bo Li;Yujing Jiang;Liyuan Yu
岩石裂隙的水力力学特性是许多地球科学和工程实践的核心问题。以往的实验和数值研究表明，剪切作用可以显著提高单裂隙岩体的渗透性，但剪切作用对裂隙岩体水力特性的影响却很少受到关注。在大多数以前的裂缝网络模型中，通常假定单个裂缝由平行板形成，并且假定流动服从立方定律。然而，相关研究表明，平行板模型不能真实地代表天然岩石裂隙的表面特征，并且在足够大的雷诺数下，流量和压降之间的关系将不再是线性的。在本研究中，建立了一个数值方法来评估剪切对二维离散裂隙网络（DFN）的水力特性的影响，在线性和非线性制度。利用DFNGEN程序生成了考虑断裂面粗糙度和孔径空间变化的DFN。通过求解Navier-Stokes方程进行了数值模拟，以模拟通过这些DFN的流体流动。剪切通过每个模型的裂缝，并通过改变剪切和正常的位移，等效渗透率和DFN的非线性流动特性的剪切的影响进行了估计。结果表明，从线性流态到非线性流态转变的临界条件为：10−4<J< 10−3，其中J为水力梯度。当流体流处于线性状态（即，J< 10−4），剪切引起的等效渗透率的相对偏差δ2与J呈线性相关，变化很小，而对于非线性区域（J> 10−3）的流体流动，δ 2与J呈非线性相关。当J = 1，剪切位移Ds = 7 mm，法向位移Dn = 1 mm时，剪切过程将使垂直于剪切裂缝方向的等效渗透率显著降低53.86%。通过对计算结果的拟合，建立了δ 2的数学表达式，为求解裂隙网络渗流问题时选择合适的控制方程提供了依据。
Hydro-mechanical properties of rock fractures are core issues for many geoscience and geo-engineering practices. Previous experimental and numerical studies have revealed that shear processes could greatly enhance the permeability of single rock fractures, yet the shear effects on hydraulic properties of fractured rock masses have received little attention. In most previous fracture network models, single fractures are typically presumed to be formed by parallel plates and flow is presumed to obey the cubic law. However, related studies have suggested that the parallel plate model cannot realistically represent the surface characters of natural rock fractures, and the relationship between flow rate and pressure drop will no longer be linear at sufficiently large Reynolds numbers. In the present study, a numerical approach was established to assess the effects of shear on the hydraulic properties of 2-D discrete fracture networks (DFNs) in both linear and nonlinear regimes. DFNs considering fracture surface roughness and variation of aperture in space were generated using an originally developed code DFNGEN. Numerical simulations by solving Navier–Stokes equations were performed to simulate the fluid flow through these DFNs. A fracture that cuts through each model was sheared and by varying the shear and normal displacements, effects of shear on equivalent permeability and nonlinear flow characteristics of DFNs were estimated. The results show that the critical condition of quantifying the transition from a linear flow regime to a nonlinear flow regime is: 10−4<J< 10−3, whereJis the hydraulic gradient. When the fluid flow is in a linear regime (i.e.,J< 10−4), the relative deviation of equivalent permeability induced by shear,δ2, is linearly correlated withJwith small variations, while for fluid flow in the nonlinear regime (J> 10−3),δ2is nonlinearly correlated withJ. A shear process would reduce the equivalent permeability significantly in the orientation perpendicular to the sheared fracture as much as 53.86% whenJ= 1, shear displacementDs= 7 mm, and normal displacementDn= 1 mm. By fitting the calculated results, the mathematical expression forδ2is established to help choose proper governing equations when solving fluid flow problems in fracture networks.