Hydraulic stimulation of enhanced deep geothermal reservoirs commonly targets pre-existing joint networks with the goal of increasing reservoir permeability. Here, we study the permeability and strength of joint-free and jointed Buntsandstein sandstones from the EPS-1 exploratory borehole at the Soultz-sous-Forêts geothermal site (France). The studied jointed samples contain naturally formed fractures that are variably filled with secondary mineralisation. We find that the permeability of these rocks is more sensitive to the presence and orientation of bedding than to the presence of joints at the scale of the samples: permeability is lowest in samples where bedding is oriented perpendicular to the direction of fluid flow. While well-sealed joints can act as barriers to fluid flow, partially filled joints neither inhibit nor promote fluid flow with respect to their joint-free counterparts. These samples were then deformed under triaxial conditions to assess (1) whether deformation reactivates pre-existing joints, and (2) how permeability changes as a result of deformation. We find that the mechanical response of the rocks depends on the extent to which joints are sealed. Well-sealed joints locally increase rock strength and experimentally induced fractures do not exploit pre-existing joint surfaces; partially sealed joints, by contrast, act as planes of weakness that localise strain. Although the permeability of all samples increased during deformation, permeability increase was largest in samples with poorly filled joints. We conclude that hydraulic stimulation operations must carefully consider the extent to which targeted joint networks are filled. Partially sealed joints are ideal targets for stimulation: these features act as planes of weakness within the rock mass and their reactivation can result in significant increases in permeability. By contrast, well-sealed joints may increase rock strength locally and may never reactivate during stimulation, making them poor targets for permeability enhancement.

中文翻译：

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地热储层岩石中可变密封节理的重新激活和渗透性增强

强化深层地热储层的水力增产通常针对预先存在的节理网络，以增加储层渗透性。在这里，我们研究了来自法国 Soultz-sous-Forêts 地热站 EPS-1 勘探钻孔的无节理和节理 Buntsandstein 砂岩的渗透性和强度。研究的节理样本包含自然形成的裂缝，这些裂缝不同程度地充满了次生矿化。我们发现，这些岩石的渗透率对层理的存在和方向比对样本范围内节理的存在更敏感：层理垂直于流体流动方向的样本中渗透率最低。虽然密封良好的接头可以充当流体流动的屏障，但相对于无接头的接头，部分填充的接头既不会抑制也不会促进流体流动。然后这些样品在三轴条件下变形，以评估（1）变形是否重新激活预先存在的接头，以及（2）变形导致渗透性如何变化。我们发现岩石的机械响应取决于接缝的密封程度。密封良好的节理局部增加了岩石强度，并且实验诱发的裂缝不会利用预先存在的节理表面；相比之下，部分密封的接头充当局部应变的薄弱平面。尽管所有样品的渗透率在变形过程中均有所增加，但在接缝填充不良的样品中渗透率增加幅度最大。我们的结论是，水力增产作业必须仔细考虑目标节理网络的填充程度。部分密封的节理是增产的理想目标：这些特征充当岩体内的薄弱面，它们的重新激活可导致渗透性显着增加。相比之下，密封良好的节理可能会局部增加岩石强度，并且在增产过程中可能永远不会重新激活，从而使其成为渗透率增强的不良目标。