Two-phase flows in open geological fractures: a combined experimental and numerical investigation

许多岩土工程和环境工程问题，如地热能源的生产或深层地层流体的储存，都涉及地下裂缝介质中的流动过程。当两种不混相流体共存时，流动涉及它们之间界面的位移。这种所谓的两相流可以导致非常复杂和复杂的流体空间分布，这取决于平均流速、流体的粘度和密度、流体界面的机械性能以及可渗透介质的几何形状。对于多孔介质，近20年来人们对两相流的丰富现象进行了深入的研究。相反，在地质裂缝中，对这种丰富现象学的表征还处于起步阶段。该项目的第一个主要目标是系统地研究它与流体性质、流动条件、裂缝几何形状和闭合的关系。这一目标将通过以下两方面的结合来实现：(i)在透明装置上进行实验室实验，再现真实的裂缝几何形状，但允许测量两种流体及其速度的空间分布，以及（ii）基于流体力学第一原理的裂缝空间中的两相流数值模拟。实验室的实验将在雷恩学院建立和进行，而数值研究将在汉诺威大学发展和进行，从而利用两个小组的互补性专门知识。这两个团队还将共同开发一种名为深度平均二维数值模拟的创新技术，该技术可以直接计算裂缝孔径上的平均流体速度。它有望在预测精度和计算效率之间提供一个很好的折衷。从实验和数值数据中系统表征粗裂缝中的两相流将由两个小组共同完成。在实际应用中，涉及到几十米到几千米的长度尺度，流体-流体界面的长度尺度无法解决。为了预测现场案例研究中的流动，因此使用了所谓的连续尺度数值模型，这种模型不是基于流体力学的第一原理。众所周知，这些模型通常不能很好地再现空间流体分布。例如，当注入的流体取代了之前存在的流体后，留在地下的天然流体的数量，或者流体在给定距离内的移动时间，通常很难预测。该项目的第二个主要目标是通过使用新获得的两相流现象学知识来改进这种大规模预测，以找到合适的大规模模型参数，这些参数将优化对大规模观测结果的预测，例如地质地层中剩余的驱替流体量。

Many geotechnical and environmental engineering problems, such as geothermal energy production or storage of fluids in deep formations, involve flow processes in subsurface fractured media. When two immiscible fluids coexist, the flow involves the displacement of the interfaces between them. This so-called two-phase flow can result in very complex and intricate spatial distributions of the fluids, depending on the mean flow velocity, the viscosities and densities of the fluids, the mechanical properties of their interface, and the geometry of the permeable medium. For porous media, the rich phenomenology of two-phase flows has been investigated at length in the last 20 years. In geological fractures, on the contrary, the characterization of this rich phenomenology is still in its infancy. The first main objective of this project is to systematically investigate it, as a function of the fluids’ properties, flow conditions, as well as fracture geometry and closure.This objective will be tackled through a combination of (i) laboratory experiments on transparent setups reproducing a realistic fracture geometry but allowing for measurements of the spatial distributions of the two fluids and of their velocities, and (ii) numerical simulation of the two-phase flow in the fracture space, based on the first principles of fluid mechanics. The laboratory experiments will be set up and run at Géosciences Rennes, while the numerical study will be developed and run at University Hannover, thus building on the complementary expertise of the two groups. An innovative so called depth-averaged two-dimensional numerical simulation, which directly computes fluid velocities averaged over the fracture aperture, will also be developed jointly by the two groups. It is expected to provide an excellent compromise between prediction accuracy and computing efficiency. Systematic characterization of two-phase flow in rough fractures from the experimental and numerical data will be done jointly by the two groups.For practical applications, which involve length scales of tens to thousands of meters, the length scale of fluid-fluid interfaces cannot be resolved. To predict flow in field case studies, so-called continuum scale numerical models, which are not based on the first principles of fluid mechanics, are therefore used. These models are known to not often reproduce the spatial fluid distributions well. For example, the amounts of naturally-occurring fluid that remains in the subsurface after an injected fluid has displaced that previously-resident fluid, or the travel times of fluids over a given distance, are often poorly predicted. The second main objective of this project is to improve such large scale predictions by using the newly-acquired knowledge of two-phase flow phenomenology to find appropriate large scale model parameters that will optimize the prediction of large scale observables such as the amount of displaced fluid remaining in the geological formation.