Multiphase Flow Dynamics: Insights from Single Microchannels to Porous Media with Uniform and Hierarchical Microchannel Networks

Abstract

Multiphase
flow in porous media are widespread in emerging subsurface application including geological
carbon sequestration and underground hydrogen storage. Multiple fluid interactions introduce
more complexity compared to single phase flow. On the gas-liquid interface,
mass transfer and interfacial instability may arise. The study of multi-phase
interaction behaviour allows the insights gained to be applied to underground
storage and recovery applications. In
this thesis, a microfluidics
platform with high-speed imaging system was built
to investigate gas-liquid flow in single microchannel as a simple model and interfacial
instability in porous media with microchannel network:

At the beginning, to simply the model, mass transfer of deformed bubble flow in the single rectangular and square microchannels was experimentally studied by using water
as liquid phase and CO₂ as gas phase. Depending on flow rates, flow patterns including slug
flow, bubbly flow, and annular flow were observed in rectangular and square
microchannels. Flow pattern map was proposed and compared with the maps in the
literatures. By using digital image processing, the bubble volume especially that
of deformed bubbles in rectangular and square microchannels was calculated
based on 2D projection and 3D slicing, correspondingly. Scaling laws including important
parameters of bubbles were derived to provide the guidance of microreactor
design. Mass transfer coefficients were calculated based on bubble volume. The empirical
correlations involving dimensionless numbers were fitted to precisely predict mass
transfer coefficients. Further, to be universality, a semi-theoretical model considering
length ratio of liquid and gas phases was developed to predict measured mass transfer coefficients in
square microchannel precisely.

Next, I began to switch our perspective from single microchannel
to porous media with microchannel networks. In the first step, the gas-liquid two-phase displacement in porous
media with microchannel network was first investigated. By varying capillary
numbers Ca and viscosity ratios M in a wide range, flow
pattern involving viscous fingering (VF), capillary
fingering (CF) and crossover zone (CZ) can be observed. Finger morphologies at
breakthrough moment and steady state in three different flow regions was visualized. The main difference between VF and CF is that the gas stops invading in CF region after
breakthrough, whereas in VF region gas can continue to expand
until almost all the liquid phase is displaced. Invasion velocity, phase
saturation and fingering complexity were quantified based on digital image
processing. Fingering dynamical behaviors in different flow pattern before and after breakthrough was investigated. Time evolution of
fingering displacement after breakthrough demonstrated an unobserved circle, consisting of new finger generation, cap invasion,
breakthrough and finger disappearance. The circle repeats until steady state. Finally,
local dynamical invasion behavior was studied and a stepwise way of gas
invasion was exposed.

In numerous geological gas storage procedures, the injected gas
infiltrates aquifers containing multiple fluids, such as depleted oil
reservoirs nearing the completion of extraction following water or brine
flooding. Therefore,
in the next step, I expanded the two-phase displacement to viscous-dependent three-phase displacement. By varying gas (G)
invasion scenarios of a high viscous defending liquid (HL), low viscous liquid
(LL), and their co-existing multi-fluid system, the influence of fluid
viscosity was investigated. The residual saturation of the initial phase
suggests that the displacement efficiency follows the order G→(L→L) > L→L > G→L, regardless
of the injection flow rate. The introduction of a third gas phase improves the
displacement efficiency and potential energy savings without incurring higher
pumping power costs. The finger patterns of gas invasion in G→(LL→HL) and G→(HL→LL) displacement are very sensitive
to the order of occupation of HL and LL within the pore space. Notably, a novel
yarn-like gas pattern was observed during G→(LL→HL) displacement, in which gas
invading speech is ultra-fast. Analysis of the local invasive behaviour
revealed the main mechanism for the formation of yarn-like fingers, i.e., the
tendency of gas to invade the interconnected LL channels, whereas the dispersed
HL prevented the bypass expansion of the gas. Two types of ganglia movement and
connection in G→(LL→HL) displacement were discovered, i.e. ¨catch up to
connect¨ and ¨expand to connect¨. Finally, the time evolution of finger
topological connectivity confirms that disconnected ganglia that appear before
the breakthrough will expand and reconnect again after breakthrough.

In the finally step, I further extend our research to cyclic
gas-liquid invasion. To model the actual underground porous structure, a hierarchical
porous media featuring multiple-level of pore sizes was designed and fabricated
by 3D printing. The impact of hierarchical structure on invasion behavior was
investigated during gas-liquid cyclic injection in uniform and hierarchical structures.
By analyzing fingering morphology and quantifying phase
saturation at each order structure,
it was found that gas prefers to invade in the 1st-order structure and is
trapped by capillary force in the 2nd-order structure. Then, connectivity and
permeability were quantified by using the Euler number and Lattice Boltzmann
method (LBM), respectively. Compared to uniform structure, the hierarchical
structure exhibits higher connectivity and relative permeability. The hysteresis effect occurs during gas-liquid cycle
invasion. The Land model confirms that the saturation hysteresis effect is
weaker in hierarchical structures compared
to uniform structures. To investigate the
underlying cause, I examined ganglia mobilization behavior and analyzed local
invasion behavior. In contrast to homogeneous structures, ganglion movement is
limited in hierarchical structures. I found the connection-jumping invasion
method is the main mechanism behind this suppression.

The discoveries in this thesis contribute to
an improved comprehension of the interaction dynamics of multiphase flow at the
microscale, particularly concerning the optimization of strategies for
subsurface resource storage and extraction applications.

• author: Yang, Shuo
• supervisor: Johan Revstedt; Lei Wang
• opponent: Prof. Berg, Carl Fredrik, Norwegian University of Science and Technology, Norway.
• organization: LTH Profile Area: The Energy Transition; Heat Transfer; NanoLund: Centre for Nanoscience
• publishing date: 2024-08-16
• type: Thesis
• publication status: published
• subject: Fluid Mechanics and Acoustics; Geophysics; Oceanography, Hydrology, Water Resources; Energy Engineering
• keywords: mass transfer, bubble flow, porous media, invading dynamics, hierarchical structure, underground gas storage
• pages: 93 pages
• defense location: Lecture Hall M:B, building M, Klas Anshelms väg 4, Faculty of Engineering LTH, Lund University, Lund.
• defense date: 2024-09-11 09:00:00
• ISBN: 978-91-8104-097-5
• ISBN: 978-91-8104-096-8
• language: English
• LU publication?: yes
• id: b43b3d7f-f615-4751-a267-bb409c21fbbd
• date added to LUP: 2024-08-14 15:23:44
• date last changed: 2024-08-16 13:21:06