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，从地下储层开采石油、天然气和水，以及二氧化碳或放射性废物等废物的储存。地壳中的流体流动是通过连通的孔隙、裂缝和裂缝网络进行的，并受到流体压力差异的驱动。我们以渗透率来测量岩石传导流体的能力，已知岩石在这一关键传输特性上表现出强烈的方向性变化或各向异性。实验室试验和现场钻孔试验表明，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。再一次，精确控制的实验室测试和较少约束的地下原位测量表明情况确实如此。然而，一个关键问题是，迄今为止进行的实验室测试是在简化的应力条件下进行的，与地壳内实际的地应力各向异性不匹配。这使得将公布的实验室数据解释和应用于更一般的地质情况变得非常困难，比如地震活跃断裂带周围的流体流动，或者降低裂缝性多孔储层中二氧化碳储存的风险。我们的建议是在伦敦大学学院使用一种新的仪器，它可以对砂岩和花岗岩的流体饱和岩石样品施加完全各向异性（真正的三轴）应力。立方或矩形的岩石块将被三对金属锤压缩，它们以90度对称地排列在样品周围。这将允许我们独立地改变3个主要的（主要的）压力。岩石样品将足够大（例如5 × 5 × 5厘米的立方体），以包含孔隙和裂缝的准均匀分布。我们将改进这种独特的仪器，使其能够沿着压缩岩石的三个加载方向中的任何一个方向测量渗透率。我们的建议建立在最近在阿伯丁获奖的研究基础上，在那里，渗透率的各向异性已经在一个自然断层带的定向样品中进行了测量，并与岩石中的孔隙结构密切相关。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。我们将从定量实验室测试和孔隙度特征中定义新的经验方程。这些数据和关系将用于最先进的断层带计算机模型中，以探索流体流动的方向变化（渗透率各向异性）如何影响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地震易发断层的风险，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.