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.