Temperature variations often trigger coupled thermal, hydrological, mechanical, and chemical (THMC) processes that can significantly alter the permeability/impedance of fracture-dominated deep geological reservoirs. It is thus necessary to quantitatively explore the associated phenomena during fracture opening and closure as a result of temperature change. In this work, we report near-field experimental results of the effect of temperature on the hydraulic properties of natural fractures under stressed conditions (effective normal stresses of 5–25 MPa). Two specimens of naturally fractured granodiorite cores from the Grimsel Test Site in Switzerland were subjected to flow-through experiments with a temperature variation of 25–140 °C to characterize the evolution of fracture aperture/permeability. The fracture surfaces of the studied specimens were morphologically characterized using photogrammetry scanning. Periodic measurements of the efflux of dissolved minerals yield the net removal mass, which is correlated to the inferred rates of fracture closure. Changes measured in hydraulic aperture are significant, exhibiting reductions of 20–75% over the heating/cooling cycles. Under higher confining stresses, the effects in fracture permeability are irreversible and notably time-dependent. Thermally driven fracture aperture variation was more pronounced in the specimen with the largest mean aperture width and spatial correlation length. Gradual fracture compaction is likely controlled by thermal dilation, mechanical grinding, and pressure dissolution due to increased thermal stresses exerted over the contacting asperities, as confirmed by the analyses of hydraulic properties and efflux mass.

中文翻译：

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天然裂缝花岗岩的热驱动裂缝孔径变化

温度变化通常会触发热，水文，机械和化学（THMC）耦合过程，这些过程会显着改变以裂缝为主的深部地质储层的渗透率/阻抗。因此，有必要定量研究由于温度变化而在裂缝打开和闭合过程中的相关现象。在这项工作中，我们报告了在应力条件下（有效法向应力为5-25 MPa）温度对天然裂缝的水力特性影响的近场实验结果。来自瑞士格里姆瑟尔试验场的两个天然断裂的花岗闪长岩岩心标本经受了25-140°C温度变化的流通实验，以表征断裂孔/渗透率的演变。使用摄影测量扫描对所研究样品的断裂表面进行形态学表征。定期测量溶解矿物质的流出量会得出净去除质量，该净去除质量与推断的裂缝闭合速率相关。液压孔径的变化非常显着，在整个加热/冷却循环中减少了20-75％。在较高的围限应力下，裂缝渗透率的影响是不可逆的，尤其是时间依赖性。在最大平均孔径宽度和空间相关长度最大的样品中，热驱动裂缝的孔径变化更为明显。逐渐的裂缝压实很可能由热膨胀，机械研磨和压力溶解所控制，这是由于施加在接触粗糙表面上的热应力增加所致，