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Numerical study of forward and reverse flow characteristics of rough-walled tree-like fracture networks
Geomechanics and Geophysics for Geo-Energy and Geo-Resources ( IF 5 ) Pub Date : 2021-06-01 , DOI: 10.1007/s40948-021-00260-7
Decheng Zhang , Hao Gao , Qianqian Dong , Chengwang Xiong

Abstract

The transport properties of complex fracture networks are of great significance to predicting the subsurface flow. Tree-like fracture networks were generated according to the fractal theory, and the roughness of fracture surfaces was created by using the Weierstrass–Mandelbrot function. The forward and reverse flows were numerically investigated via the lattice Boltzmann method by employing positive and negative pressure differences respectively. The results indicate that the bifurcation angle θ, fractal dimension D and flow direction have significant influences on the transport properties of fracture networks. The maximum velocity increases quaveringly from the entrance or the last bifurcation point but suddenly decreases at the next bifurcation point, which is mainly due to the variation of local roughness. Both the energy loss ratio and the local pressure loss coefficient increase with the bifurcation angle. The cavity vortex can form within the concave of rough surface for fractal dimensions larger than 1.2 which result in the reduction of the hydraulic aperture and the alteration of the streamline from being parallel with the surface to a favorable flow path. The ratio of fracture hydraulic width to physical width decreases with the fractal dimension and fracture level. The flow rate of reverse flow is higher than that of forward flow under the same driven pressure because the flow convergence has lower local resistance than the flow diversion at the bifurcations. The theoretical solution of the flow rate through the rough-walled tree-like fracture networks was derived and verified by the modelling results.

Article highlights

  1. 1.

    The vortex forms within the concaves of the rough surface resulting in the reduction of the hydraulic aperture.

  2. 2.

    The ratio of fracture hydraulic width to the physical width decreases with the fractal dimension and fracture level.

  3. 3.

    A theoretical solution of the flow rate through the rough-walled tree-like fracture networks is derived.



中文翻译:

粗糙壁树状裂缝网络正向和反向流动特性的数值研究

摘要

复杂裂缝网络的输运特性对预测地下流动具有重要意义。根据分形理论生成树状断裂网络,并使用 Weierstrass-Mandelbrot 函数创建断裂表面的粗糙度。正向和反向流动通过格子玻尔兹曼方法分别采用正压差和负压差进行数值研究。结果表明,分岔角θ、分形维数D流向对裂缝网络的输运性质有显着影响。最大速度从入口处或最后一个分叉点开始呈颤动增加,但在下一个分叉点处突然减小,这主要是由于局部粗糙度的变化。能量损失比和局部压力损失系数都随着分叉角的增加而增加。当分形维数大于1.2时,粗糙表面的凹面内会形成空腔涡,导致水力孔径减小,流线从平行于表面变为有利的流动路径。裂缝水力宽度与物理宽度的比值随着分形维数和裂缝水平的增加而减小。在相同的驱动压力下,反向流动的流速高于正向流动的流速,因为在分叉处,流动的汇合比流动的分流具有更低的局部阻力。通过建模结果推导出并验证了通过粗糙壁树状裂缝网络的流量的理论解。

文章亮点

  1. 1.

    涡流在粗糙表面的凹部内形成,导致液压孔径减小。

  2. 2.

    裂缝水力宽度与物理宽度之比随着分形维数和裂缝水平的增加而减小。

  3. 3.

    推导出通过粗糙壁树状裂缝网络的流速的理论解。

更新日期:2021-06-01
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