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，如地热能的生产或深层流体的储存，都涉及地下裂隙介质中的流动过程。当两种不相容的流体共存时，流动涉及它们之间的界面的位移。这种所谓的两相流可能会导致非常复杂和错综复杂的流体空间分布，这取决于平均流动速度、流体的粘度和密度、其界面的机械性质以及可渗透介质的几何形状。对于多孔介质，在过去的20年里，人们对两相流动的丰富现象学进行了深入的研究。相反，在地质裂隙中，对这种丰富的现象学的描述还处于初级阶段。这个项目的第一个主要目标是系统地研究它作为流体性质、流动条件以及裂缝几何和闭合的函数。这一目标将通过(I)实验室透明装置实验再现真实的裂缝几何形状，但允许测量两种流体的空间分布和速度，以及(Ii)基于流体力学的基本原理对裂缝空间中的两相流动进行数值模拟来实现。实验室实验将在Géoscience Rennes建立和运行，而数值研究将在汉诺威大学开发和运行，从而建立在两个小组的互补专业知识的基础上。这两个团队还将共同开发一种创新的所谓深度平均二维数值模拟，它直接计算裂缝孔内平均的流体速度。它有望在预测精度和计算效率之间提供一个很好的折衷。这两个小组将根据实验数据和数值数据对粗糙裂缝中的两相流动进行系统的表征。对于涉及数十到数千米长度尺度的实际应用，流体-流体界面的长度尺度是无法解决的。因此，为了在现场实例研究中预测流动，需要使用所谓的连续介质尺度数值模型，这些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流体的空间分布。例如，在注入的流体置换了之前驻留的流体后，地下仍然存在的自然产生的流体的数量，或者流体在给定距离上的旅行时间，往往预测得很差。该项目的第二个主要目标是通过利用新获得的两相流动现象学知识来改进这种大规模预测，以找到适当的大型模型参数，以优化对大型可观测数据的预测，例如地质地层中剩余的驱替流体数量。

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.