Nagatsu Lab.｜Tokyo University of Agriculture and Technology

The academic field of study that aims at systematization of ways which deals with not only thinking about chemical reaction processes but also physical reaction process such as flow and mixing of fluid, heat and materials transportation, is referred to as reactive fluid dynamics or reacting flow. Reacting flow can be divided into two categories; one is gas-phase reacting flow and the other is liquid-phase one. The reports of liquid-phase reacting flow is smaller than that of gas-phase reacting flow. The liquid-phase reacting flow is called Chemo-hydrodynamics in Europe and it is a very new field of study that is in 2009 the international meeting was held for the first time on this. Prof. Nagatsu has reported the phenomena called viscous fingering(VF) and controlling flow involving viscosity decrease due to a chemical reaction the first time in the world.

When a more viscous fluid(sticky fluid such as honey) is displaced by a less viscous fluid(free-flowing liquid such as water) in porous media(material containing pores), the boundary of the two fluids becomes hydrodynamically unstable and forms a finger-like pattern. This phenomenon is called "viscous fingering(VF)" or "Saffman-Taylor instability". The below left figure shows the schematic for VF experiment and right one shows the movie during the experiment.

Schematic of VF experiment

Movie during VF experiment

In the oil recovery field, it is said that the petroleum exists as crude oil(which has high viscosity) and the production well is digged for the oil reservoir. Although we can recover the oil using natural pressure, the oil efficiency is said to up to 20%. Then, the another wells (which are called injection wells) are digged to inject water into the reservior to push the oil. The process or method is called "water flooding". During the water flooding, VF occurs due to the less viscous water pushing the more viscous crude oil and the whole oil cannot be recovered. The VF study develops to solve these problems.

Schematic of water flooding
Cited by academist journal (https://academist-cf.com/journal/?p=14770)

We introduce that the VF is caused by the viscosity contrast between two fluids. However, it is reported that the miscibility(solubility) has great influence on the VF patterns. The VF study has been so far forcusing on two types: fully miscible system and immiscible system.

The fully miscible system is the system where the two fluids have infinite mutual solubility such as water-honey and the two fluids finally become one phase. The immiscible system is the system, where the two fluids have no mutual solubility such as oil-water and the two fluids keeps two phase and will be never mixed.

However, we have experimentally succeeded a new type: partially miscible system, and the VF pattern with the partially miscible system is different from the miscible and immiscible ones.

This droplet pattern is new discovery in VF field. We elucidate the mechanism is "phase separation" due to thermodynamic instability and "Korteweg effect", which has spontaneous flow induced during phase separation. The press release on this result is published by Tokyo University of Agriculture and Technology.

The VF study with the partially miscible system contributes to CO₂ sequestration and enhanced oil recovery.

Comparison among fully miscible, immiscible and partially miscible systems

Ryuta X. Suzuki, Yuichiro Nagatsu, Manoranjan Mishra, and Takahiko Ban
Phase separation effects on a partially miscible viscous fingering dynamics
J. Fluid Mech., 898, A11 (2020)

We perform numerical simulations towards the issues, which cannot be understood only by experiments.

One of the examples is the partially miscible VF. We have experimentally suggested the mechanism by using some theoritical studies. Then, we simulate the VF by using the governing equations based on our suggesting mechanism. We can understand whether the suggested mechanism is correct or not by the simulation. On the other hand, we can use simulations to reproduce the situations under conditions being difficult by experiments and to approach unknown problems bacause we can control various parameters.

Governing equations for the partially miscible VF

Partially miscible VF simulation
The less viscous fluid (blue) is injected from left to right direction.

Shoji Seya, Ryuta X. Suzuki, Yuichiro Nagatsu, Takahiko Ban, and Manoranjan Mishra
Numerical study on topological change of viscous fingering induced by a phase separation with Korteweg force
J. Fluid Mech., 938, A18(2022)

As introducing the "Viscous fingering" section, the recovery efficiency of the water flooding is up to 50% due to VF. The third stage, chemical flooding, where chemical solution is injected to improve the oil recovery using chemical reaction, is considered. The chemical flooding has several methods. The one is Alkaline flooding: the addition of one or more chemical compounds to the injected fluid to reduce the interfacial tension between the reservoir oil and the injected fluid. Although the alkaline flooding is good to solve the interfacial tension problem, they cannot solve the VF problem. Thus, VF occurs.

We suggest a novel chemical flooding, calcium flooding, where a viscoelastic material is produced by chemical reaction at the interface between oil and injected solution and the material change the flow inside the reservoir.

The press release on this study is published by TUAT, and we have applied for a patent using this technic. The below figure shows the results of the water flooding, alkaline flooding and calcium flooding. Our suggesting calcium flooding is most effective.

At present, we have studied new methods for enhanced oil recovery to apply for patents.

Comparison among water flooding, alkaline flooding and calcium flooding
Pore volume, PV, is the total volume of very small openings in a bed of adsorbent particles. Here PV is considered flow rate for easy understanding.

Yuichiro Nagatsu, Kizuna Abe, Kaori Konmoto, Keiichiro Omori
Chemical Flooding for Enhanced Heavy Oil Recovery via Chemical-Reaction-Producing Viscoelastic Material
Energ. Fuels, 34, 10655-10665 (2020)

When a less viscous fluid displaces a more viscous one in porous media, VF occurs as introduced in "Viscous fingering" section. Then, what happens when a chemical reaction occurs at the interface between the displcing and displaced fluids during VF experiments?

The answer is still explored because the effect of flow and the property of the product are very complicated. We have report an interesting result occuring when the product by chemical reaction has viscoelasticity.

It is expected that VF patterns are normally changed as the intensity of viscoelasticity of the product increases when the viscosity contrast is the same. In fact, sometimes the viscous fingers become thinner with increase in the viscoelasticity and sometimes thicker (finally circular) with increase in viscoelasticity.

VF patterns under various pH conditions

The above figure shows the VF results, where we discovered the viscoelasticity of the product can be changed maintaining the viscosity contrast by changing pH condition of the displaced fluid. The viscoelasticity at the interface is measured and showed below. The figure shows the intensity of the viscoelasticity increases with the increase in pH. However, VF patterns are changed. For example, the viscous fingers are getting thinner with increase in pH at the range of 10-12 and the fingers become thicker with increase in pH at the range of 12-13.

Interfacial rheological measurement

G' represents strage modulus (corresponding to elasticity) and G'' represents loss modulus (corresponding to viscosity).

We have reported that our suggesting new model can explain the mechanism, which cannot be explained only by the viscoelasticity. We study the unkown phenomena by experiments, numerical simulation and theoretical study because the unxplored phenomena and theories in VF study with chemical reactions exits like this case.

Sae Hirano, Yuichiro Nagatsu, and Ryuta X. Suzuki
Reversal of effects from gel production in a reacting flow dependent on gel strength
Phys. Rev. Fluids, 7, 023201(2022)