Experimental and numerical investigations on density-driven dissolution of CO2 and related carbonate dissolution in karst water

到目前为止，密度驱动的二氧化碳溶解在岩溶系统中的作用还不是很清楚。众所周知，溶解在水中的二氧化碳推动了岩溶过程，同样，人们普遍认为，其主要来源是生物来源，即由土壤中的微生物或根呼吸产生的。碳酸盐的溶解主要发生在地表附近，在那里渗入的大气水富含二氧化碳，导致所谓的剥蚀，本质上是对陆地表面积的磨损。但为什么洞穴也会在岩石深处生长呢？目前通过两种不同的流行机制来解释这一点：(1)混合腐蚀指的是两股水流混合时产生的溶解电位，因为混合的水总是方解石侵蚀性的。(2)非线性溶解动力学假定水可以保留一定量的溶解潜势，直到进入岩石深处。我们的新主张是，第三种重要的、迄今为止被低估的补充水体中二氧化碳的机制--我们将其称为密度驱动的溶解--也可能在深处的洞穴发育中发挥作用。我们在最近的一篇文章中证明，密度驱动的二氧化碳在水中的溶解可以用二氧化碳补充停滞的水体，从而在几周到几个月的时间尺度上实现溶解潜力。到目前为止，由于其特别高的复杂性，尚未研究的是密度驱动的二氧化碳溶解与石灰岩的相互作用，即反应运输系统。人们可以假设，密度驱动的二氧化碳溶解发生在特定孔径的裂缝内。密度引起的渗流取决于实际的裂缝开度，而实际裂缝开度可能会因碳酸盐溶解而增加，从而触发自增强过程。该项目的总体目标是帮助更好地理解密度驱动的二氧化碳溶解在已知机制方面的作用，如混合腐蚀和非线性溶解动力学。要了解地质时间尺度上不同岩溶作用机制之间的相互作用，唯一合适的工具是建模，并通过复杂和受控良好的实验室和野外实验进行验证。数值模型求解密度依赖于溶解组分浓度的Navier-Stokes方程。验证的目的是包括钙碳系统中密度驱动的溶解和石灰岩表面引起的形态变化所引起的反应流的耦合。综上所述，我们建议通过使用来自良好控制的实验的数据系统地验证我们的数值模拟器DuMux来提高数值模拟能力。-量化密度驱动的石灰岩表面溶解和反应导致的二氧化碳进入岩溶水的速率。-量化相应的碳酸盐溶解速率和石灰岩表面的改性。

The role of density-driven CO2 dissolution in karst systems is not well understood to date. It is known that CO2 dissolved in water drives the karstification process, and it is likewise generally understood that its major source is biogenic, i.e. produced by microorganisms in the soil or by root respiration. Dissolution of carbonates takes place primarily near the ground surface, where percolating meteoric water is enriched with CO2 and leads to so-called denudation, which is, in essence, a wearing-down of the terrestrial surface area. But why do cavities also grow deep inside the rock? This is currently explained through two different prevailing mechanisms: (1) Mixing corrosion refers to dissolutional potential created when two water streams mix, since the mixed water is always calcite-aggressive. (2) Non-linear dissolution kinetics are assumed on the basis that water may retain a certain residual amount of its dissolutional potential until deep into the rock. Our new claim is that a third important and, so far, underestimated mechanism for replenishing CO2 in water bodies - we refer to it as density-driven dissolution - may also play a role in cavity development at depth. We demonstrated in a recent article that density-driven dissolution of CO2 in water can replenish stagnant water bodies with CO2 and, thus, the dissolution potential on a time-scale of weeks to months. What has not been investigated to date, due to its particularly high complexity, is the interaction of density-driven CO2 dissolution with limestone, i.e. the reactive-transport system. One can assume that density-driven dissolution of CO2 takes place within a fracture of a certain aperture. Density-induced flow then depends on the actual fracture opening, which may increase as a result of carbonate dissolution, thus, triggering a self-enhancing process.This project's overall aim is to contribute to a better understanding of the role of density-driven CO2 dissolution in relation to already known mechanisms, such as mixing corrosion and non-linear dissolution kinetics. To understand the interaction between different karstification mechanisms on geologic time scales, the only appropriate tool is modelling, validated by sophisticated and well-controlled laboratory and field experiments. The numerical model solves Navier-Stokes equations with density dependent on the concentrations of dissolved components. The validation is aimed at including a coupling of reactive flow due to density-driven dissolution in calco-carbonic systems and induced morphological changes on the surface of a limestone.In summary, we propose to- improve numerical modeling capabilities by systematic validation of our numerical simulator DuMux with data from well-controlled experiments.- quantify CO2 entry rates into karst waters resulting from density-driven dissolution and reaction at limestone surfaces.- quantify corresponding dissolution rates of carbonates and modification of the limestone surface.