Quantifying the Anisotropy of Permeability in Stressed Rock

岩石中的流体流动对于广泛的自然过程和人类活动至关重要，包括地震的触发，从地下储层中提取石油，天然气和水，以及二氧化碳或放射性废物等废物的储存。地壳中的流体流动通过孔隙、裂缝和裂缝的连通网络发生，并由流体压力的差异驱动。我们用渗透率来衡量岩石传导流体的能力，众所周知，岩石在这一关键的输运性质上表现出很强的方向性变化（或各向异性）。实验室实验和现场钻孔测试表明，渗透率在不同方向上可以变化几个数量级，即100或1000倍。渗透率也被认为高度依赖于固体岩石基质中的应力。同样，精细控制的实验室测试和地下的现场测量结果表明情况确实如此。然而，一个关键问题是，迄今为止进行的实验室测试都是在简化的应力条件下进行的，这与地壳内原地应力的实际各向异性不匹配。这使得很难解释和应用已公布的实验室数据到更一般的地质情况，如地震活跃断层带周围的流体流动或降低裂缝多孔储层中CO2储存的风险，任何程度的信心。我们的建议是使用一个新的仪器在伦敦大学学院，可以施加完全各向异性（真正的三轴）应力流体饱和的砂岩和花岗岩岩石样品。立方体或矩形岩石块将由三对金属冲头压缩，这些金属冲头围绕样品以90度对称布置。这将使我们能够独立地改变3个主应力中的每一个。岩石样本将足够大（例如，5 x 5 x 5 cm立方体），以包含准均匀分布的孔隙和裂缝。我们将修改这个独特的仪器，使渗透率的测量沿着任何三个加载方向压缩岩石。我们的建议建立在阿伯丁最近获奖的研究基础上，在那里，渗透率各向异性已经在来自天然断层带的定向样品中进行了测量，并与岩石内的孔隙结构仔细相关。我们的目标是将渗透率的各向异性与应力的各向异性和空隙空间（=孔隙+裂缝）的各向异性联系起来。我们将根据定量实验室测试和孔隙度表征定义新的经验方程。这些数据和关系将用于断层带的最新计算机模型，以探索流体流动的方向变化（渗透率各向异性）如何影响预期沿着断层带滑动事件的概率和类型。这将使我们更好地了解地壳中易发生地震的断层的风险，更广泛地说，我们将开始了解岩石中流体流动的真正三维性质。

Fluid flow in rocks is vitally important for a wide range of natural processes and human activities, including the triggering of earthquakes, the extraction of oil, gas and water from subsurface reservoirs, and the storage of waste products such as CO2 or radioactive waste. Fluid flow in the Earth's crust takes place through connected networks of pores, cracks and fractures, and is driven by differences in fluid pressure. We measure the ability of rocks to conduct fluid as permeability, and rocks are known to exhibit strong directional variations - or anisotropy - of this key transport property. Laboratory experiments and in situ borehole tests have shown that permeability can vary by several orders of magnitude - i.e. by factors of 100 or 1000 - in different directions. Permeability is also known to be highly dependent on the stress in the solid rock matrix. Again, finely controlled laboratory tests and rather less well constrained in-situ measurements from the subsurface show this to be the case. A key problem though is that the laboratory tests conducted to date have been conducted under simplified stress conditions which do not match the actual anisotropy of in situ stress within the crust. This makes it very difficult to interpret and apply the published laboratory data to more general geological situations, such as fluid flow around seismically active fault zones or reducing risks for CO2 storage in fractured porous reservoirs, with any degree of confidence. Our proposal is to use a new apparatus at UCL which can apply fully anisotropic (truly triaxial) stress to fluid saturated rock samples of sandstone and granite. Cubic or rectangular shaped blocks of rock will be compressed by three pairs of metal rams, symmetrically arranged at 90 degrees to each other around the sample. This will allow us to vary each of the 3 main (principal) stresses independently. Rock samples will be large enough (5 x 5 x 5 cm cubes, for example) to contain quasi-homogeneous distributions of pores and cracks. We will modify this unique apparatus to enable measurement of permeability along any of the three loading directions that compress the rock. Our proposal builds on recent award-winning research at Aberdeen, where permeability anisotropy has been measured in on oriented samples from a natural fault zone, and carefully related to the pore fabric within the rock. We aim to link the anisotropy of permeability with the anisotropy of stress and the anisotropy of the void space (= pores + cracks). We will define new empirical equations from our quantitative laboratory tests and porosity characterisations. These data and relationships will be used in state-of-the-art computer models of fault zones to explore how directional variations in fluid flow (permeability anisotropy) affect the probability and the type of slip events expected along a fault zone. This will provide a much improved understanding of the risks from earthquake-prone faults in the crust, and more generally, we will begin to understand the truly 3D nature of fluid flow in rocks.