1 Fundamental Investigation of Reactive -Convective Transport Implications for Long -Term Carbon dioxide CO 2 Sequestration

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Fundamental Investigation of Reactive-Convective Transport: Implications for

Long-Term Carbon dioxide (CO2) Sequestration

Md Fahim Shahriar1, Aaditya Khanal1*

1The Jasper Department of Chemical Engineering, The University of Texas at Tyler

Corresponding author E-mail address: aadityakhanal@uttyler.edu

Abstract

The density-driven convection coupled with chemical reaction is the preferred mechanism for permanently storing

CO2 in saline aquifers during its injection into deep saline aquifers. The flow behavior and reaction mechanism of the

injected CO2 depends on the geochemical profile of the formation rock and brine. This study uses a 2D visual Hele-

Shaw cell to evaluate and visualize the density-driven convection formed due to gravitational instabilities, also known

as Rayleigh-Taylor instability. The primary goal of the experiments is to understand the various mechanisms for the

mass transfer of gaseous CO2 into brine with different initial ionic concentrations and flow permeability. Moreover,

the impact of CO2 flow rates, injection locations, reservoir dipping angle, and permeability heterogeneity is also

investigated. We observed that the presence of salts resulted in earlier onset of convection and a larger convective

finger wavelength than the case with no dissolved salts. In addition, experimental data showed a higher lateral mixing

between CO2 fingers when dipping is involved. The visual investigation also revealed that the CO2 dissolution rate,

measured by the rate of the convective fingers advance, depends on the type and concentration of the ions present in

the brine. The CO2 dissolution for solutions with varying salt dissolved, indicated by the area of the pH-depressed

region, is observed to be 0.38-0.77 times compared to when no salt is present. Although convective flow is slowed

down in the presence of salts, the diffusive flux is enhanced, as observed from both qualitative and quantitative results.

Moreover, the reduced formation permeability, introduced by using a flow barrier, resulted in numerous regions not

being swept by the dissolved CO2, indicating an inefficient dissolution. We also investigated the effect of discrete

high conductivity fractures within the flow barriers, which showed an uneven vertical sweep and enhanced flow

channeling. Lastly, the parameters regarding CO2 leakage risk during storage are identified and discussed. Moreover,

the quantitative comparison of the test cases obtained from image analysis elucidated the convection dynamics of the

CO2 storage process. The fundamental insights from this study are applicable for optimizing and improving the geo-

sequestration of CO2 in subsurface formations saturated with brine.

Keywords: CO2 Sequestration; Convective dissolution; Reactive Dissolution; Rayleigh-Taylor instability;

Heterogeniety; Permeability Contrast

1. Introduction

The atmospheric concentration of carbon dioxide (CO2) in May 2021 was recorded as 419.5 parts per million (ppm),

which is approximately 50% higher than at the beginning of the industrial revolution [1]. The growing concerns about

CO2 emissions have led to investigations of possible carbon capture and storage (CCS) methods. CO2 geological

storage in depleted oil reservoirs or saline aquifers is one of the

preferred CCS methods to capture emissions from large point

sources [2]. CO2 geological storage has four primary CO2 trapping

mechanisms: structural, residual, dissolution, and mineral trapping,

as shown in Fig. 1 [1]. Capillary or residual trapping rate is the

highest at the beginning of the CO2 storage period; however,

dissolution trapping becomes more dominant throughout time (Fig.

1). Dissolution trapping captures almost two-thirds of CO2 injected

in the storage volume [3–5]. Even though molecular diffusion of CO2

in brine is slow, the rate of dissolution trapping is accelerated by

other mechanisms, including density-driven convection or Rayleigh-

Taylor instability, dispersion, and advection [6,7].

Several factors can affect the convective-dissolution phenomenon for different geological sites and, therefore,

needs to be considered for accurate CO2 storage prediction. For example, density-driven convection is reduced for

geological storages with lower vertical permeability, resulting in advection becoming the dominant force with

increased transverse mixing [3,8]. Nevertheless, the effect of density-driven convection in CO2 geological storage is

significant, as it prevents CO2 leakage by sinking CO2 rather than rising to shallower formations [3]. Furthermore,

since CO2 dissolution into brine increases the density of brine on the order of 0.1-1%, the density gradient between

the top layer (brine-CO2 solution) and the layer immediately below (pure brine) induces density-driven natural

convection under gravitational instabilities [1–3].

Various studies have identified and extensively studied different aspects of the transport mechanisms during

the density-driven convection or Rayleigh-Taylor instability, including diffusion period, convective period, and

constant flux [9–19]. Hele-Shaw cell, a simple structure usually formed by a narrow gap between two transparent flat

plates, has been used in multiple experimental setups to visualize and mimic the Rayleigh-Taylor instability or

Rayleigh convection (RC) formed during CO2 storage in different geological structures [12,20–23]. Kneafsey and

Pruess [24] conducted laboratory visualization studies and quantitative CO2 absorption tests in transparent Hele-Shaw

cells to investigate the dissolution-induced density-driven convection phenomenon. The quantitative measurements

showed that the density-driven convection initiated faster than predicted. Backhaus et al. [25] studied the density-

driven convection for a lighter fluid (water) placed over a heavier fluid (propylene glycol). The initial instability and

quasi-steady-state were explained by analyzing the convective time and velocity scales along with the finger width

and the rate of mass transport. The test was conducted at standard atmospheric conditions with a Rayleigh number

(Ra) of 6000-90,000 [25]. Meanwhile, a smaller Ra range (100 < Ra < 1700) was adopted in the works of Slim et al.

Figure 1. Fraction of CO2 sequestered by residual

trapping (blue), dissolution (orange), and

mineralization (gray) (Modified from Khanal

and Shahriar [1]). Dissolution is the primary

storage mechanism especially with the passage

of time.

30 120 210 300

Trapping Index

Year

[26]. Potassium permanganate (KMnO4) in water was used as an analog for CO2 in brine at atmospheric conditions,

describing the dissolution-driven convective behavior from the first contact up to 65% average saturation.

Developing scaling relationships, correlations and models can provide important insight into convection

dissolution properties of CO2 for different geological storage [2,27–30]. Motjaba et al. [27] developed two scaling

relationships, one between Rayleigh and Sherwood numbers and the other between Rayleigh numbers and CO2

convective flux. Moreover, density-driven convection behavior was observed using visualization techniques and

quantitative experiments (under 3.45 MPa and 182 < Ra <20860). Robust scaling relations between compensated flux

and transition times between successive regimes in the system for different salt types (NaCl and CaCl2) were examined

by Mahmoodpour et al. [31]. The results showed that different salt types affect both the short and long-term dynamics

of convective dissolution. Faisal et al. [28] obtained correlations between the Rayleigh number and the mass of total

dissolved CO2. The mass of dissolved CO2 was determined under atmospheric conditions (with 3277.88 < Ra <

36,420.87) using a catalytic combustion-based total carbon analyzer (TC-analyzer). Other useful scaling laws,

including the onset of the convection, and wavelength of the initial convective instabilities, were also identified and

discussed [4,32,33]. Tani et al. [34] analyzed the one-phase problem on radial viscous fingering in a Hele-Shaw cell.

The mathematical analysis was performed by modifying Stefan's problem and justifying the time-derivative's

vanishing coefficient in the parabolic equation.

Different visualization techniques, including the Schlieren method, particle image velocimetry (PIV), laser-

induced fluorescence (LIF), and interferometry method, have been adopted to observe the formation and growth of

convective finger structures in Rayleigh convection [2,3,35,36]. Zhang et al. [2] presented a vortex model of CO2

adsorption into the water to characterize the interfacial mass transfer coefficient for the continuous convective period.

The study adopted particle image velocimetry (PIV) and laser-induced fluorescence (LIF) to calculate the solute

concentration distribution and instantaneous liquid velocity in the Hele-Shaw cell. In addition, this study considered

the 3D simulation approach, which provided a critical overview of how the regions inside and outside the convective

fingers have enhanced interfacial mass transfer by reducing the thickness of concentration boundary layers. Moreover,

another recent work by Zhang et al. [37] used a different experimental setup using the UV-induced fluorescence

method to investigate gas-liquid interphase mass transfer in a Hele-Shaw cell. This experimental approach was low

cost and more sensitive to changes than laser devices for pH-sensitive fluorescers.

Mahmoodpour et al. [31] provided critical insight into visualizing the dissolution-driven convection at high-

pressure conditions (up to 535.3 psi). They devised a novel Hele-Shaw apparatus withstanding high pressure and

presented CO2 dissolution-driven convective behavior in a confined brine-saturated porous medium. Tang et al. [3]

designed a Hele-Shaw cell rated to 70 MPa and Ra of 346 to investigate the convection parameters, including critical

onset time of convection, dissolution rate, and gravitational instabilities. This study used the micro-schlieren technique

to conduct the visual inspection. Pressure-volume-temperature (PVT) testing was conducted at 293.15 to 423.15 K

and pressure ranging from 14 to 24 MPa.

The chemical composition of the CO2 storage site also significantly affects the reactive transport of dissolved

CO2, as observed in the works of Thomas et al. [38]. Their work investigated the dissolution of CO2 into an aqueous

solution of bases MOH, where M+ is an alkali metal cation. For bases MOH , the convection is enhanced for counter-

ion M+ sequence of Li+ < Na+ < K+ < Cs+ . The experimental investigation revealed that the concentration of base in

solution strongly impacts the nonlinear finger instability, where higher concentration leads to faster instability and

shorter time for onset of convection. Furthermore, despite M+ ions not actively participating in the geochemical

reactions during the dissolution process, the nature of different M+ ions vary in the instability development.

Loodts et al. [39] observed the effect of pressure, temperature, and NaCl concentration on CO2 dissolution

properties. Their study suggested that increasing CO2 pressure or reducing temperature or salt concentration leads to

higher convective instability. However, temperature has a minimal effect on CO2 dissolution properties, so controlling

the temperature is not essential for the reproducibility of experimental studies [39]. Thomas et al. investigated the

effect of salinity by the dissolution of gaseous CO2 in pure water, Antarctic water, and 0.5-5 M NaCl dissolved in

water [40]. The results showed that higher salt concentration delays the formation of instabilities, resulting in delayed

onset of convection. Moreover, increased convection pattern wavelength and decreased fingers' velocity and the

growth rate increased the salt concentration. Kim and Kim [41] derived and solved linear stability equations for the

effect of chemical reactions in an initially quiescent vertical Hele-Shaw cell. Their nonlinear numerical simulation

showed that chemical reactions enhance the diffusive flux; however, by retarding the onset of buoyancy-driven

convective motion, convective flux is weakened.

Formation dip angle is another key factor of consideration for safely storing CO2 on subsurface geological

sites, as it significantly impacts spatial migration distribution during CO2 dissolution [42–44]. For larger dip angles,

the supercritical CO2 phase could change to a gas phase during upward migration in the reservoir up-dip direction,

where the reservoir formation temperature and hydrostatic pressure are lower [43]. As a result, reservoirs with higher

dip angles have more chance of CO2 leakage during geological storage. Jang et al. [42] simulated the effect of dip

angle and salinity of CO2 storage. For formation dip angles of 0°, 5°, and 10°, the migrated CO2 distances were 60%,

73.3%, and 86.7%, respectively, compared to a 15° dip angle in the 200th year of CO2 migration. Therefore, with a

larger formation dip angle, there is a higher possibility of spatial CO2 migration. They concluded that reservoirs with

higher dip angle and salinity have low CO2 geological storage safety. Wang et al. [43] observed similar effects of

formation dip, where the total CO2 storage amount is inversely proportional to the formation dip angle. The impact of

dip angle is more prominent in storage reservoirs with higher porosity and permeability [43]. As Jing et al. [35]

observed, higher salinity and high dip angle are not conducive to CO2 geological storage. However, the effect of

salinity is observed to be more significant than that of dip angle on the CO2 liquid phase mass fraction.

During the injection of CO2 in deep saline aquifers, the natural fractures present in the formation may

propagate, or new fractures may be induced in the reservoir. The fracture networks in a hydrocarbon reservoir play a

vital role in fluid transport from the pores to the wellbore as they are significantly more conductive than the matrix

[45–48]. The same principle is applicable during the CO2 sequestration operation, which makes it difficult to predict

the movement of plumes during the injection of CO2 in fractured porous media. Hence, natural and induced fracture

networks in the geological storage sites should also be considered to predict CO2 subsurface movement. Due to the

opening and closing of the fractures, the reservoir properties also deviate from the value measured from the core

analysis. The highly permeable conductive fracture networks can act as a pathway for fluid movement, potentially

allowing CO2 migration to neighboring aquifers or the surface through the cap rock [49,50]. Bond et al. [49]

demonstrated improved CO2 migration prediction by incorporating structural geological fractures in the model.

Knowing the spatial distribution of fractures, their orientation, conductivity, and overall contribution to the effective

permeability are desirable for geological sites whose permeability is controlled mainly by faults and fractures [50].

March et al. [51] presented a model for CO2 storage in naturally fractured reservoirs, showing the importance of

selecting an appropriate injection rate to prevent "early spill": the fast flow of CO2 through the highly permeable

fracture storage before any significant CO2 storage.

Despite the considerable investigative analyses on CO2 dissolution-driven convection using the Hele-Shaw

cell, there is still a lack of studies that visualizes CO2 dissolution with a dipping angle involved. Moreover, the effect

of varying CO2 flow rates and injection points needs further insight in terms of quantitative analysis. Furthermore,

despite the considerable theoretical model and experimental investigation on the effect of salinity in CO2 dissolution,

most of the work considers NaCl and ignores the presence of other salts. Lastly, to the best of our knowledge, the

effect of fractures in heterogeneous media is yet to be presented. This experimental study critically investigates the

effect of these factors to address this knowledge gap. Furthermore, this study discusses how the findings of this work

can be translated into improving storage efficiency by providing key insight into CO2 convection dynamics.

The remainder of the paper is organized as follows. Section 2 provides the experimental methods, image

processing sequence, and a brief overview of the experiments considered. Section 3 presents the qualitative

visualization of CO2 dissolution in different homogeneous and heterogeneous media. Additionally, the effect of

fractures on CO2 dissolution is also discussed. Section 4 presents our findings in terms of quantitative data. Lastly,

Section 5 is devoted to the discussion, and Section 6 presents the main conclusion of this study.

2. Experimental Method

2.1. Experimental Setup and Materials

Experiments are performed in a Hele-Shaw cell composed of two transparent 0.5 in (12.9 mm) thick plexiglass

separated by precision silicone shims with a thickness of 1 mm along the sidewalls. The front plexiglass panel was

drilled at the bottom of the cell and was fitted with a ball valve with a diameter of 0.25 in (6.35 mm). The internal cell

dimensions were a length and height of 259 mm and 284 mm, respectively. The height (H) of the water column is 243

mm, as shown in Fig. 2a. This port was used to fill and drain the reactor of the experimental fluid. At the top, three

holes at the center of the front plexiglass were drilled so that the CO2 could be securely injected into the cell using an

18-gauge dispensing needle, as shown in Fig. 2a. A digital camera (Nikon D7000 with 50 mm lens) was focused on

the cell to take pictures at 20 seconds intervals for 2.5 hours. We maintain a 4 cm distance from the injection point to

the top of the water to avoid disturbing and introducing shear stress at the interface. The experiments were conducted

using the same protocol to ensure a similar controlled environment. The schematic of the experimental setup is

presented in Fig. 2b.

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1FundamentalInvestigationofReactive-ConvectiveTransport:ImplicationsforLong-TermCarbondioxide(CO2)SequestrationMdFahimShahriar1,AadityaKhanal1*1TheJasperDepartmentofChemicalEngineering,TheUniversityofTexasatTylerCorrespondingauthorE-mailaddress:aadityakhanal@uttyler.eduAbstractThedensity-drivenconve...

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1 Fundamental Investigation of Reactive -Convective Transport Implications for Long -Term Carbon dioxide CO 2 Sequestration

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