Fluid flow in the Earth: the influence of dehydration reactions and stress

岩石可能含有孔洞（孔隙），就像湿海绵中的孔洞一样，可能充满了水或其他液体。这些孔隙中的流体压力可能与岩石所支撑的固体压力不同。流体压力的变化可能引起地震，因此了解是什么控制着它是很重要的。我们将讨论对流体压力的三个相互关联的影响。1. 许多矿物在固体结构中含有水，在加热或在周围压力下降等其他情况下，水会以流体的形式释放出来：这被称为脱水反应。释放出来的流体可能会改变流体压力。流体压力影响并受反应速率的影响——这是反馈。但是固体压强也会影响反应过程吗？这一点是反应行为中最不为人所知的方面，但却与地球上相当普遍的情况有关。如果岩石被压实或变形（如挤压海绵），流体压力就会改变。岩石比海绵坚固得多，所以我们需要了解它改变形状的速度（改变孔隙的体积，从而改变孔隙内的压力）。3. 流体可以流过连通的孔隙，从而消散异常压力。我们需要测量和了解渗透率，即流动的容易程度。一般情况下，反应伴随着变形和流体流动。我们的研究将通过对天然材料的实验和理论建模来揭示这三个过程的关联效应。我们将在实验中使用矿物石膏（在相当低的温度下发生脱水反应）和蛇纹石：作为完整的块体（实际岩石，流体流动困难）和作为高多孔性粉末（高渗透性，流体流动容易）。一组实验将在反应温度以下进行，因此我们可以在没有其他影响的情况下检查变形行为。其他组则在反应温度以上，考察固体压力、流体压力和时间对反应过程的影响，以及由此产生的流体压力反馈。需要数学模型来推断大型岩石（太大而无法直接进行实验）和地质时间尺度（几年到数百万年）的结果；我们将在进行实验的同时进行这项工作。我们的结果将有助于理解流体和岩石压力不同的各种情况：下面是两个例子。地震可能是由流体压力的变化引起的，但地震本身也会改变流体压力。这反过来可能导致脱水反应（或相反，再水合反应），再次改变压力场。在地热能源场，流体穿过多孔岩石，但通常是化学反应。我们将应用我们的结果来理解地热田的变质变化是如何发生的，以及这些变化是如何改变孔隙度和流体流动的。我们的目标是NERC主题“地球系统科学3.3b：地球内部动力学及其在地表的表现”的核心，我们引用其中的内容：“许多自然灾害过程依赖于材料特性和动态过程，而这些特性和动态过程的特征或理解都很差。这些问题可以通过实验室测量和天然或模拟材料的实验来解决。由于许多危险过程涉及复杂的多相混合物（气体、固体、液体），其性质要么不清楚，要么不了解，因此会出现重大挑战。”变形介质中的反应流体流动就是这样一个动态过程。

Rocks may contain holes (pores) which, like those in a wet sponge, may be filled with water or other fluids. The fluid pressure in these pores may not be the same as the solid pressure the rock is supporting. Changes in fluid pressure may cause earthquakes, so it is important to understand what controls it. There are three linked influences on fluid pressure which we will address. 1. Many minerals contain H2O within the solid structure and this is given off as fluid during heating, or under other circumstances including a drop in surrounding pressure: this is called dehydration reaction. The fluid given off may then change the fluid pressure. The fluid pressure influences and is influenced by the reaction rate - this is feedback. But does the solid pressure influence reaction progress as well? This point is the least understood aspect of reaction behaviour, yet relates to a quite general situation in the Earth. 2. Fluid pressure is changed if the rock is compacted or otherwise deformed (like squeezing the sponge). Rock is much stronger than sponge, so we need to understand just how fast it will change shape (changing the volumes of pores and hence the pressure within them). 3. Fluid can flow through connected pores, thus dissipating anomalous pressures. We need to measure and understand the permeability, that is, the ease of flow. In general the reaction is accompanied by deformation and by fluid flow. Our research will unravel the linked effects of these three processes, using experiments on natural materials as well as theoretical modelling. We will use the minerals gypsum (which undergoes dehydration reactions at quite low temperatures) and serpentine in experiments: as intact masses (actual rock, with fluid flow difficult), and as highly porous powders (high permeability, fluid flow easy). One group of experiments will be run below the reaction temperature, so we can examine the deformation behaviour in the absence of other effects. Other groups will be above the reaction temperature so as to examine the effects of solid pressure, fluid pressure and time on reaction progress, and the consequent feedbacks on fluid pressure. Mathematical modelling is required to extrapolate results to large bodies of rock (too large to run experiments on directly) and geological timescales (years to millions of years); we will conduct this in parallel with experiments.Our results will inform understanding a great variety of situations in which fluid and rock pressures are different: here are two examples. Earthquakes may be triggered by fluid pressure changes but will themselves change fluid pressure. This in turn may lead to dehydration reactions (or the reverse, rehydration reactions) which again modify the pressure field. In geothermal energy fields, fluids move through porous rock but are often chemically reactive. We will apply our results to understanding how metamorphic changes in geothermal fields occur, and how these modify porosity and fluid flow. Our objectives are core to NERC Theme "Earth System Science 3.3b: Dynamics of the Earth's Interior and their Manifestation at the Surface", from which we quote:"Many natural hazards processes depend on material properties and dynamic processes that are poorly characterised or understood. These can be best addressed through laboratory measurements and experiments on natural or analogue materials. Major challenges arise because many hazardous processes involve complex multiphase mixtures (gas, solid, liquid) whose properties are either poorly characterised or understood".Reactive fluid flow in deforming media is one such dynamic process.