Structural and mechanical characterisation of natural hydrofractures in 3D (HyFract3D)

水力裂缝主要是由相对高的孔隙流体压力产生的张性裂缝。水力压裂的自然实例是由孔隙流体或岩浆超压促进的矿脉、节理和岩脉。水力压裂控制着地壳中的流体和热传递，反过来又影响着各种地质因素和过程。这些构造为地壳流体提供了优先通道，并经常引导经济流体和矿床。水力压裂也可以是人工的，并在各种工业操作过程中进行，包括石油或地热储层的刺激。 水力压裂在理想情况下应在3D中呈现硬币形状。这种理想的形状在自然界中是不期望的，因为3D裂纹的传播发生在高度非均匀的介质中，并且应力和结构各向异性都倾向于转移和/或限制裂纹路径。然而，目前对真实的地质介质中水力压裂力学的认识主要是基于二维现场资料和理论研究。天然水力压裂的完整3D暴露确实很少，除非工业界允许进入地下挖掘。到目前为止，还没有尝试量化水力压裂系统的力学在三维空间，使用最先进的有限元建模校准的三维现场曝光和古应力重建的结构映射。在这个项目中，我们建议解决水力压裂系统的三维结构和力学，强调应力和结构各向异性对其复杂的传播路径的影响。为此，我们将研究部分的石英脉暴露在选定的露头的Panasqueira矿山，葡萄牙，作为自然的例子。广泛的地下挖掘网络（即~600 km）允许地质结构的特殊3D暴露。我们将更详细地研究矿脉和预先存在的结构之间的结构关系，以隔离它们的机械相互作用。此外，我们将利用断层滑动和矿脉数据进行古应力分析，并辅以流体包裹体和地质测温研究。这种方法的组合将允许完全量化的应力状态，包括孔隙流体压力，coeval的形成的水力压裂。最后，我们将使用一种创新的数值方法（即X-FEM），以模拟水力压裂在真实的三维地质介质中的传播，并探讨应力和结构各向异性的影响。模型将根据我们的构造观测和古应力重建进行校准。

Hydrofractures are mostly tensile fractures created by relatively high pore fluid pressures. Natural examples of hydrofractures are mineral veins, joints and dykes promoted by pore fluid or magma overpressures. Hydrofractures control fluid and heat transfer in the crust that, in turn, influence a variety of geological factors and processes. These structures provide preferential pathways for crustal fluids and, often, conduct economic fluids and host mineral deposits. Hydrofractures can also be artificial and made in the course of various industrial operations, including stimulation of oil or geothermal reservoirs. Hydrofractures should ideally present a penny shape in 3D. This ideal shape is not expected in nature, because propagation of 3D cracks occurs in highly heterogeneous media and both stress and structural anisotropies tend to divert and/or confine crack paths. However, the current knowledge on hydrofracture mechanics in real geological media is mainly based on 2D field data and theoretical studies. Complete 3D exposure of natural hydrofractures is indeed seldom, unless access to underground excavations is granted by industry. Until now, no attempt has been made to quantify the mechanics of hydrofracture systems in three dimensions, using state of the art finite element modelling calibrated by structural mapping on 3D field exposures and paleostress reconstructions. In this project, we propose to address the 3D structure and mechanics of hydrofracture systems, with emphasis on the influence of stress and structural anisotropies on their complex propagation paths. To this aim we will study parts of the network of quartz veins exposed in selected outcrops of the Panasqueira Mines, Portugal, as natural examples. The extensive network of underground excavations (i.e. ~600 km) allows for exceptional 3D exposure of the geological structures. We will study in much detail the structural relationships between the veins and the pre-existing structures in order to isolate their mechanical interactions. In addition, we will conduct paleostress analyses using both fault slip and vein data, complemented by fluid inclusion and geothermometry studies. This combination of methods will allow for full quantification of the stress state, including pore fluid pressure, coeval to the formation of the hydrofractures. We will finally use an innovative numerical method (i.e. X-FEM) in order to model the propagation of hydrofractures in real 3D geological media and explore the effects of both stress and structural anisotropies. The models will be calibrated according to our structural observations and paleostress reconstructions.