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微流动效应的粗糙单裂隙水力开度影响

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    微流动效应的粗糙单裂隙水力开度影响
    Advances in Geo-Energy Research, 2019, 3(1): 104-114

          深入认识粗糙裂隙中的复杂流动行为对许多工程实践起到重要的作用。随着裂隙尺度的减小,其逐渐接近于流体分子的平均自由程,微流动效应(有时又称为相对稀薄效应、相对离散效应、非平衡效应)将随之变得显著,并且往往会引起相对流量的增加。然而在这一过程中,裂隙的粗糙效应也被加强并且使得相对流量出现相反的变化。为了探讨两者的影响,本文开展了数值模拟与理论分析研究。首先,通过理论分析,对文献中的含粗糙水力开度经验模型进行了考虑微流动效应的修正,初步获得了关于两种效应的认识。进而采用格子Boltzmann方法对根据高斯分布随机构建的二维粗糙单裂隙中的微流动进行了直接模拟研究,揭示了两种效应的竞争关系并利用模拟结果检验了修正模型。发现不论是较粗糙的裂隙还是相对平滑的裂隙,基于Rasouli和Hosseinian(2011)的修正模型均给出了与数值模拟一致的结果。通过进一步研究,发现了在不同粗糙条件下,随着力学开度的减小,水力开度与力学开度之比表现出不同的变化趋势。同时,两种效应能够在一定粗糙条件下获得平衡,即不论力学开度如何变化,水力开度始终与力学开度保持相等。该研究结果揭示了二维粗糙单裂缝中微流动效应的影响机理,并且有望进一步为微裂缝中的天然气运移等工程相关问题提供参考。


    Microflow effects on the hydraulic aperture of single rough fractures

    Ge Zhang, Yudong Zhang, Aiguo Xu, Yingjun Li
    (Published: 2019-01-20)
    CorrespondingAuthor and Email: Aiguo Xu, xu_aiguo@iapcm.ac.cn; ORCID: https://orcid.org/0000-0002-6179-5973

    Citation: Zhang, G., Zhang, Y., Xu, A., Li, Y. Microflow effects on the hydraulic aperture of single rough fractures. Advances in Geo-Energy Research, 2019, 3(1): 104-114, doi: 10.26804/ager.2019.01.09.

    ArticleType: Original article

    Abstract:
          The understanding of flow behavior in rough fractures is essential for many engineering activities. When the aperture of a rough fracture approaches the mean free path of fluid molecules, the microflow effect, sometimes also referred to relative rarefaction effect, relative discrete effect or non-equilibrium effect, becomes pronounced. It was found often to enhance the flow rate. However, the surface roughness shows completely contrary influence. In order to clarify the influences of the two factors, a computer simulation work accompanied with theoretical analyses is conducted. Previous empirical models for hydraulic aperture which already containing roughness effect are modified with consideration of the microflow effect. Direct simulation using the lattice Boltzmann method is conducted on artificially created 2D fractures with random roughness following Gaussian distribution to reveal the competitive relationship of two effects. The simulation results also verify modified models. Among them, the one based on Rasouli and Hosseinian's model agrees with the simulation on the relationship between hydraulic aperture and mechanical aperture for both cases with very rough fractures and relatively smooth fractures. Further investigation confirms that, under various roughness, the ratio of hydraulic aperture over mechanical aperture shows quantitatively different trends as mechanical aperture decreases. This phenomenon exists on a relatively wide scale. An equilibrium point of two effects is also found through analysis of the relationship. The results reveal the mechanism of microflow in 2D rough fractures and also provide a reference for engineering problems like the transport of natural gas through microfractures.

    Keywords: Microflow, single fracture, roughness, hydraulic aperture, lattice Boltzmann method.
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