Permeability evolution of natural fractures during shearing is one of the critical issues in enhanced geothermal system (EGS). In this study, we conducted a series of numerical simulations of shear-flow tests under different normal stresses to investigate the permeability evolution and shear behavior of natural fractures during shearing. All numerical simulations in this study are executed based on a coupled hydro-mechanical pore network model (PNM) for fluid flow within the discrete element method (DEM). The numerical model uses a 33 mm × 25 mm fracture surface profile extracted from X-ray computed tomography scanning data of joints in a rock core retrieved from a 4.2-km-deep well at the Pohang EGS site. The results demonstrate a positive correlation between fracture permeability and shear displacement when the normal stress is relatively high. The central aspect of permeability evolution lies in the variation of local aperture distribution along the pathway: an increase in the variance of fracture aperture distribution leads to an increase in fracture permeability. Permeability evolution of fractures is significantly affected by normal stress, fracture roughness, and relative flow direction. Larger fracture roughness facilitates the formation of high-permeability pathways. Higher local normal stress often corresponds to lower local permeability. Moreover, high normal stress amplifies the effect of fracture roughness on permeability evolution. These findings enhance our comprehensive understanding of hydraulic-mechanical coupling effects during EGS stimulation.

中文翻译：

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花岗岩天然裂缝剪切诱发的渗透率演化：对 EGS 储层增产的影响


剪切过程中天然裂缝的渗透率演化是增强型地热系统（EGS）的关键问题之一。在本研究中，我们对不同法向应力下的剪切流试验进行了一系列数值模拟，以研究剪切过程中天然裂缝的渗透率演化和剪切行为。本研究中的所有数值模拟都是基于离散元法 (DEM) 内流体流动的耦合流体力学孔隙网络模型 (PNM) 执行的。该数值模型使用从浦项 EGS 现场 4.2 公里深井取回的岩心节理的 X 射线计算机断层扫描数据中提取的 33 mm × 25 mm 断裂面轮廓。结果表明，当正应力相对较高时，裂缝渗透率与剪切位移之间呈正相关。渗透率演化的核心在于沿路径局部孔径分布的变化：裂缝孔径分布方差的增加导致裂缝渗透率的增加。裂缝的渗透率演变受到法向应力、裂缝粗糙度和相对流动方向的显着影响。较大的裂缝粗糙度有利于高渗透率通道的形成。较高的局部正应力通常对应于较低的局部渗透性。此外，高正应力放大了裂缝粗糙度对渗透率演化的影响。这些发现增强了我们对 EGS 增产期间水力-机械耦合效应的全面理解。