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，包括触发地震、从地下水库中开采石油、天然气和水，以及储存二氧化碳或放射性废物等废物。地壳中的流体流动通过连通的孔隙、裂缝和裂缝网络进行，并由流体压力的差异驱动。我们测量岩石作为渗透率传导流体的能力，众所周知，岩石表现出强烈的方向性变化--或各向异性--这一关键传输特性。实验室实验和现场钻孔测试表明，渗透率可以在不同方向上变化几个数量级--即100或1000倍。渗透率也被认为高度依赖于坚硬岩石基质中的应力。同样，严格控制的实验室测试和来自地下的相对较少约束的现场测量表明，情况就是如此。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，与地壳内地应力的实际各向异性不匹配。这使得将已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，例如地震活动断裂带周围的流体流动或降低裂隙多孔储层中二氧化碳储存的风险，以任何程度的信心。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。立方体或长方形的岩石块将被三对金属闸板压缩，在样品周围对称排列，彼此成90度角。这将允许我们独立地改变三个主要(主要)重音。岩石样品将足够大(例如，5x5x5厘米立方体)，以包含准均匀分布的气孔和裂缝。我们将对这种独特的仪器进行改装，使之能够沿压缩岩石的三个加载方向中的任何一个测量渗透率。我们的建议建立在阿伯丁最近获奖研究的基础上，在该研究中，渗透率各向异性是在天然断裂带的定向样品上测量的，并与岩石中的孔隙组构密切相关。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的各向异性(=孔洞+裂缝)联系起来。我们将从我们的定量实验室测试和孔隙度特征中定义新的经验公式。这些数据和关系将用于最先进的断裂带计算机模型，以探索流体流动的方向变化(渗透率各向异性)如何影响沿断裂带预期的滑动事件的概率和类型。这将大大提高对地壳易地震断层的风险的了解，更广泛地说，我们将开始了解岩石中流体流动的真正3D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.