Quantifying the Anisotropy of Poroelasticity in Stressed Rock

地球上地壳的岩石通常是多孔的，孔隙和裂缝中充满了水、石油或气体等流体。作用在这些岩石上的力，来自覆盖岩石的重量和板块构造，使颗粒、孔隙和裂缝变形，改变它们的形状和体积。这种变形发生在任何断裂或断层之前，并由称为孔隙弹性的理论描述。该理论指出，孔隙流体所在的裂缝和孔隙的方向对岩石对应力的响应施加主要控制。充满流体的平行裂缝以一定的模式出现在主要的地震倾向断层周围，这些裂缝比随机方向的裂缝或孔隙产生更强的反应。因此，岩石的孔隙弹性性质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用于CO2封存或水力压裂（或“压裂”）。岩石的孔隙弹性性质已经在实验室中测量，但迄今为止测量的所有数据都是在一种非常特殊的应力条件下测量的，这种应力条件可能在地球上不存在。常规三轴应力（CTS）在圆柱形岩石样品上施加垂直应力，然后在侧面周围施加恒定压力。我们知道，地球上的应力在各个方向上都是变化的，这种情况称为真三轴应力（TTS）。然而，我们没有在这种应力状态下测量的多孔弹性数据。一个新委托的设备在伦敦大学学院已专门设计变形流体饱和岩石样品下的真三轴应力，从而提供了一个独特的和及时的机会，以解决核心的科学问题：有没有出版的测量孔隙弹性系数下测量TTS，我们迫切需要更好的数据来约束更好的模型的地震危险。研究人员最近的工作表明，TTS产生显着不同的模式和密度的裂纹相比，类似的加载路径下的CTS：TTS主要产生对齐的平行裂纹，而CTS往往产生径向裂纹。我们必须在地壳内最可能的原地应力条件下（真三轴应力）系统地收集这些数据，我们可以使用这些新数据来建立经过测试的、更可靠的地震危险模型。最近的研究表明，裂缝组构对于流体加压和地震倾向断层的潜在弱化是多么重要。地震活动断层周围的断层平行裂缝阵列可以沿着断层产生等于断层正应力的短期流体压力变化，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。随着对地下应力和应变方向变化的准确预测的需求日益增加（例如，地热能或水力压裂的偏斜钻井），这增加了我们的理由的紧迫性。我们将从我们的研究中产生开源软件，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，并用于帮助预测使用已发表的理论的多孔弹性性能。第二个工具将使用我们新测量的孔隙弹性数据来修改已发表的地震触发模型。在目前的模型中，多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。我们的新代码将联合收割机结合我们以前的工作，在断裂带周围的弹性特性的空间变化与新的实验室测量，使触发地震危险的更强大的预测。

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.