 |
Yves Méheust
|
Immiscible two-phase flow in geological fractures
In crystalline rocks of the Earth’s crust, most fluid flows are accommodated by networks of interconnected fractures. Immiscible two-phase flow in such geological fractures is relevant to various industrial contexts, including subsurface fluid storage and hydrocarbon recovery. The fractures are natural objects resulting from thermally- or mechanically-inducted fracturing of a geological formation, followed by mechanical and/or (bio-)chemical weathering over millions of years. Their geometry possesses an inherent stochastic disorder that is well-characterized statistically; the wall roughness is usually Gaussian-distributed while exhibiting a self-affine scale invariance, and the two walls’ topographies are matched with each other at length scales larger than a characteristic ‘correlation’ length.
As in porous media, primary displacement of a resident fluid by an injected one in such geometries is controlled by the joint effect of viscous forces, capillary forces arising from surface tension effects at fluid-fluid interfaces, and gravity. However, capillary forces act in a different manner in fractures as compared to porous media, because in porous media the two principal curvatures of fluid-fluid interfaces are constrained by the medium’s structural heterogeneity, whereas in fractures only the out-of-plane curvature is; the in-plane curvature, in contrast, depends on the history of the displacement.
We use a combination of numerical simulations and analogue experiments to study such displacement in geological fractures, focusing on configurations for which the injected fluid is non-wetting. The numerical simulations adopt a volume-of-fluid approach to either describe the three-dimensional (3D) flow in the fracture’s volume, or directly model the depth-averaged 2D flow along the fracture plane, the latter approach being much more computationally-efficient. The experiments rely on transparent rough walls obtained from realistic synthetic geometries; their position with respect to each other can be adjusted to modify the relative fracture closure. Various morphological features of the fluid phases’ occupation patterns in the fracture plane, as well the pressure drop across the fracture, are analyzed to characterize the flow regimes as a function of three geometric parameters, the viscosity ratio of the fluids, the capillary number and/or Bond numbers, and an additional, novel, non-dimensional number. Phase diagrams are proposed for such primary two-phase flows in geological fractures. Flow configurations which maximize trapping of the displaced fluid are also determined.
About Yves Méheust
Yves Méheust is Associate Professor at the University of Rennes and Senior Member of the Academic Institute of France (IUF). He holds a PhD in hydrogeology (2002) from École Normale Supérieure (ENS) in Paris and an MSc in statistical physics from ENS-Lyon (1998). His work addresses coupled physico-chemical processes occurring at the pore scale/hydrodynamic scale in permeable subsurface environments such as soils, aquifers, fractured crystalline rocks or oil reservoirs. He uses a a combination of laboratory experiments, numerical simulations and theoretical derivations to study the flow of fluids (water, air, CO2, oil) and the transport of heat, chemical species and electrical charges by fluids, as well as how transport and mixing of chemical species impact (bio-)geochemical reactions within the fluids or between the fluids and solid materials. These processes play a fundamental role in the evolution of groundwater quantity and quality, as well as in a number of industrial applications involving underground environments, such as geothermal energy, in situ remediation of soils and aquifers, and underground CO2 storage.
