Quantifying the Anisotropy of Poroelasticity in Stressed Rocks

地球上地壳中的岩石通常是多孔的，孔隙和裂缝中充满了水、石油或天然气等流体。由于上覆岩石的重量和板块构造作用，作用在这些岩石上的力使颗粒、孔隙和裂缝变形，改变了它们的形状和体积。这种变形发生在任何压裂或断裂之前，用一种叫做孔隙弹性的理论来描述。该理论认为，孔隙流体所在的裂缝和孔隙的方向对岩石对应力的响应起着重要的控制作用。充满流体的平行裂缝以主要地震易发断层周围的模式出现，这些裂缝产生的响应比随机方向的裂缝或孔隙强得多。因此，岩石的孔隙弹性特性对于我们预测大断层上的地震和人类活动引起的地震活动的能力非常重要，例如在钻孔中注入流体以封存二氧化碳或水力压裂（或“水力压裂”）。岩石的孔隙弹性特性已经在实验室中测量过了，但迄今为止所有测量的数据都是在非常特殊的应力条件下测量的，这种应力条件在地球上可能不存在。传统的三轴应力（CTS）在圆柱形岩石样品上施加垂直应力，然后在侧面施加恒定压力。我们知道地球上的应力在各个方向上都是不同的，这种情况被称为真三轴应力（TTS）。但是我们没有在这种应力状态下的测量数据。伦敦大学学院新近委托的一台仪器专门设计用于在真三轴应力下变形流体饱和岩石样品，从而提供了一个独特而及时的机会来解决核心科学问题：在TTS下测量的孔隙弹性系数没有公布的测量结果，我们迫切需要更好的数据来约束更好的地震危险模型。研究人员最近的工作表明，与相似加载路径下的CTS相比，TTS产生的裂缝模式和密度明显不同：TTS主要产生平行裂缝，而CTS倾向于产生径向裂缝。我们必须在地壳内最可能的原位应力条件下系统地收集这些数据——真正的三轴应力——我们可以使用这些新数据来制作经过测试的、更可靠的地震危险模型。最近的研究表明，裂缝结构对于地震易发断层的流体加压和潜在减弱是多么重要。地震活跃断层周围的断层平行裂缝阵列可以沿断层产生与断层正应力相等的短期流体压力变化，从而使断层在地震中滑动。这对评估主要断层的地震风险具有潜在的巨大影响。再加上对准确预测地下应力和应变方向变化的需求不断增加（例如，地热能源或水力压裂的斜井钻井），这增加了我们的基本原理的紧迫性。我们将从我们的研究中开发开源软件，免费提供给其他科学家、工程师和更广泛的公众。目前正在测试的第一个工具将量化孔隙和裂缝的三维（3D）模式，包括它们的方向、大小和形状。这些特征的统计分布将被量化，并使用已发表的理论来帮助预测孔隙弹性特性。第二个工具将使用我们新测量的孔隙弹性数据来修正已发表的地震触发模型。当前模型中包含的孔弹性变形与摩擦行为混合在一起，但这是非常不同的物理现象。我们的新代码将把我们以前关于断层带周围弹性特性的空间变化的工作与新的实验室测量相结合，以对触发的地震危险做出更可靠的预测。

Rocks in the upper crust of the Earth are often porous, with the pores and cracks filled with fluids like water, oil or gas. Forces acting on these rocks, arising from the weight of the overlying rocks and from plate tectonics, deform the grains and pores and cracks, changing their shape and volume. This deformation occurs before any fracturing or faulting, and is described by a theory called poroelasticity. This theory states that the orientations of the cracks and pores, where the pore fluid resides, exerts a major control on the response of the rock to stress. Fluid-filled parallel cracks occur in patterns around major earthquake prone faults, and these produce a much stronger response than random orientations of cracks or pores. Therefore, the poroelastic properties of rocks are important for our ability to forecast earthquakes on big faults and induced seismicity from human activities such as fluid injection in boreholes for CO2 sequestration or hydraulic fracturing (or 'fracking'). The poroelastic properties of rocks have been measured in the laboratory but all the data measured to date has been under a very special stress condition that probably does not exist in the Earth. Conventional triaxial stress (CTS) applies a vertical stress on a cylindrical rock sample, and then a constant pressure around the sides. We know that the stresses in the Earth vary in all directions, a condition known as true triaxial stress (TTS). And yet we have no poroelastic data from measurements under this stress state. A newly commissioned apparatus at UCL has been specifically designed to deform fluid saturated rock samples under true triaxial stresses and thus provide a unique and timely opportunity to address the core scientific issues: there are no published measurements of poroelastic coefficients measured under TTS and we urgently need better data to constrain better models of seismic hazard. Recent work by the investigators has shown that TTS produces significantly different patterns and densities of cracks in comparison to similar loading paths under CTS: TTS produces predominantly aligned parallel cracks, whereas CTS tends to produce radial cracks. We must systematically collect these data under the most likely in situ stress conditions within the crust - true triaxial stress - and we can use these new data to make tested, more robust, models of seismic hazard. Recent work has shown how important crack fabrics are for the fluid pressurisation, and potential weakening, of earthquake-prone faults. Arrays of fault parallel cracks around seismically active faults could produce a short-term fluid pressure change along the fault equal to the fault normal stress, allowing the fault to slip in an earthquake. This has potentially massive consequences assessing earthquake risk on major faults. Married with the increasing demand for accurate predictions of directional variations in stress and strain in the subsurface (e.g. deviated drilling for geothermal energy or hydraulic fracturing), this adds urgency to our rationale. We will produce open source software from our research, freely available to other scientists, engineers and the wider public. The first tool, currently being tested, will quantify the three-dimensional (3D) patterns of pores and cracks, including their orientations, sizes and shapes. The statistical distributions of these features will be quantified and used to help predict the poroelastic properties using the published theory. The second tool will use our newly measured poroelastic data to revise published models of earthquake triggering. The inclusion of poroelastic deformation in the current models is mixed with the frictional behaviour, but these are very different physical phenomena. Our new code will combine our previous work on the spatial variations of elastic properties around fault zones with the new laboratory measurements to make more robust forecasts of triggered earthquake hazard.