Immiscible three-phase flow in porous media: dynamics and wettability effects at the pore scale

Abstract

The simultaneous flow of three phases in porous media is ubiquitous in natural and engineered processes such as carbon dioxide storage, oil recovery, contaminant removal, food and drug manufacturing, and chemical reactors. However, only few studies have been found in the literature on the subject. The recent advent of non-invasive three-dimensional imaging provided by X-ray microtomography has yielded a breakthrough in the study of multiphase flow in porous media. We use these techniques and advanced image analysis methods to provide insights on the physics of three-phase flow in porous media. We investigate the mutual arrangement of the phases in the pore space – pore occupancy –, the types of displacements arising by the simultaneous flow of three phases, the formation of layers and the mechansims with which one of the fluids can be trapped inside the pores. We observe that, in general, the wettability of the system has major effects on these phenomena and hence we perform experiments at different wettability conditions. We perform a series of displacement experiments in small rock samples, where water first displaces oil, followed by gas injection and then secondary waterflooding to displace gas and oil. We study the amount of trapping of oil and gas, as well as oil recovery. We perform experiments using laboratory-based X-ray scanners to image the fluid configurations at the end of each displacement sequence. We also use time-resolved imaging at a synchrotron to examine the dynamics of displacements, acquiring pore-scale images approximately every minute. The results show that pore occupancy is strictly linked to the wettability of the system, as the water-oil-gas wettability order from the most to the least wetting, observed in water-wet media, changed as a function of wettability. Mixed-wet systems are characterised by variable wettability in space and hence lead to nonconstant wettability orders. Multiple displacements – a chain of displacments where one phase displaces a second which in turn displaces a third and so on – are frequently observed in water-wet media but appear to be inhibited in mixed-wet systems. The formation of oil spreading layers sandwiched between water and gas, and water wetting layers on the surface of the grains, is favoured in water-wet systems, while oil spreads in wetting layers in mixed-wet media. The formation of spreading water or gas layers is not allowed by thermodynamic constraints. Gas trapping frequently happens in both water- and mixed-wet media, with fundamental differences caused by the different configurations of the phases in the pore space. However, in general we measure enhanced trapping of gas in three-phase flow with respect to two-phase flow and higher amount of trapped gas in mixed-wet systems with respect to water-wet media, providing for secure gas storage.Using time-resolved imaging we study invasion patterns for two and three-phase flow in a mixed-wet rock. A distinct flow invasion pattern is observed for two-phase flow in mixed-wet media, controlled by the thermodynamic contact angle, which stems from an energy balance at the pore scale. The four Minkowski functionals – volume, area, total and Gaussian curvatures – are used to provide complete topological description of the dynamics of three-phase flow in water- and mixed-wet media. Specifically, the Gaussian curvatures are used to interpret the shape of the interfaces and the connectivity of the phases, which ultimately strongly influence the flow and the permeability of each fluid. This study provides in general an effective and universal methodology to study three-phase flow at the pore scale, and the results have implications for many applications, both to storage and recovery in the subsurface, but also for engineered materials. The findings of this thesis suggest favourable oil recovery with gas injection in oil fields and augmented trapping of gas with a chase water injection. This is critical, for example, for safe storage of CO2 underground to mitigate the climate change.