Chemical control of carbon dioxide convective dissolution in porous

media: Enhanced steady-state dissolution flux

Laurence Rongy, Vanessa Loodts, Carelle Thomas, Bernard Knaepen, Anne De WitUniversite libre de Bruxelles (ULB), Nonlinear Physical Chemistry Unit, CP231

1050 Brussels, Belgium. E-mail: lrongy@ulb.ac.be

Key words: CO2 sequestration, chemical reactions, dissolution flux

Dissolution-driven convection in partially miscible systems has regained much interest in the context of CO2

sequestration [1, 2]. A buoyantly unstable density stratification can build upon dissolution of CO2 into brine,thereby driving convection. Dissolution and convection are known to improve the safety of the sequestrationprocess by reducing the risks of leaks of CO2 to the atmosphere. The question remains, however, as to how theefficiency of such process depends on the chemical properties of the storage site.

Chemical reactions can indeed affect convective instabilities as they modify concentrations and thereby thedensity profiles building up in the fluid [3]. We here show that a chemical reaction of CO2 with a dissolvedchemical species can either enhance or decrease the amplitude of convection compared to the non reactive case.We use direct numerical simulations of Darcy’s equations coupled to reaction-diffusion-convection equations toclassify the various possible cases as a function of the contributions to density and diffusion coefficients of thechemical species involved.

We study a general A + B → C reaction in solution where A is the dissolving species (CO2 for instance),B a reactant present in the host phase and C the product. We show that the dynamics vary with the Rayleighnumbers of the chemical species, i.e. with the nature of the chemicals and their influence on the density.Depending on whether the reaction slows down, accelerates or is at the origin of the development of convection,the spatial distributions of species A, B or C, the dissolution flux and the reaction rate are different.

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Fig.1: Calcium carbonate precipitation patterns obtained when an aqueous solution of

carbonate is injected in a horizontal Hele-Shaw cell containing a solution of calcium ions. The amount of precipitate and its spatial distribution vary with the concentrations of the reactants [8].

Width of each image = 15 cm.

Fig.2: Experimental snapshot of a buoyancy-driven instability occurring when gaseous CO2 (top) dissolves in an aqueous solution and forms a locally denser zone, which starts to sink into the

less dense water because of density fingering. Visualisation is here made thanks to a pH sensitive color indicator. The resulting convective mixing favors further dissolution of CO2. Our goal is to understand how chemical reactions can impact these phenomena (Width of field of

view 5 cm) [13].

Fig.3: Numerical comparison of the non reactive (left) and reactive (right) convective dissolution of a species A in a host phase. An A+B->C chemical reaction between the dissolving species A

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Figure 7. Variety of CaCO3 precipitation patterns as a function of the reactant concentrations, obtained when an aqueous

solution of carbonate is injected at an injection speed of 1mL/min into an aqueous solution of calcium ions contained in a

horizontal Hele-Shaw cell. Time=3 min and field of view= 122 mm ⇥ 98 mm. Courtesy G. Schuszter [98].

8. ConclusionsReactive flows are ubiquitously encountered in geological applications ranging from oil recovery,pollution sites remediation or CO2 sequestration and mineralisation to name a few. We havereviewed here some of these applications explaining how chemical reactions can modifyflows due to hydrodynamic instabilities such as viscous fingering, Rayleigh-Taylor, convectivedissolution or double diffusive instabilities by changing the viscosity or density of the reactivesolutions at hand. In particular, reactions can profoundly modify the symmetries of a fingeredpattern by favoring development of the fingers in one direction. They can as well decrease theamplitude of an instability or on the contrary enhance it. More strikingly, reactions are also ableto destabilize interfaces that would be hydrodynamically stable in non reactive conditions.Thisis typically the case when a non monotonic density, viscosity or permeability profile builds up intime thanks to the reaction.

Understanding the related chemo-hydrodynamic patterns paves therefore the way to achemical control of hydrodynamic instabilities that could be of tantamount importance ingeological challenges such as optimizing oil recovery or trapping greenhouse gases in themost efficient and secure way. Developing experimental and theoretical studies on chemo-hydrodynamic geological systems calls for multidisciplinary collaborative efforts of chemists,physicists, engineers and geologists. Open questions and difficulties of linking laboratory-scale experimental results with large-scale in-situ field data remain numerous. In this context,benck-marking and testing of numerical modeling on the basis of experimental exploration ofreaction-diffusion-convection systems in geological context can benefit from studies in patternformation and nonlinear sciences. This multidisciplinary approach of environmental issues opensthus new horizons in which important societal or economical challenges will be addressed.

Competing Interests. The author declares no competing interests.

Funding. Funding from Prodex, the ITN Multiflow network and the FRS-FNRS Forecast programme aregratefully acknowledged.

Physical Chemistry Chemical Physics c7cp01372h

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Enhanced Q1steady-state dissolution flux in reactiveconvective dissolution

V. Loodts, B. Knaepen, L. Rongy and A. De Wit*

Chemical reactions can enhance the dissolution fluxduring the convective dissolution of one phase into a fluidhost phase.

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Figure 1: Numerical comparison of the non reactive (left) and reactive (right) convective dissolution of a species A in ahost phase. An A + B → C chemical reaction between the dissolving species A and a reactant B dissolved in the hostphase can accelerate the convection and the flux of A towards the host phase.

We discuss the impact of chemistry on the interfacial flux of CO2 and the time scales needed for its dissolutionin aqueous solutions [4]. In particular, we show that chemical reactions can enhance the steady-state fluxof dissolving CO2. Therefore chemistry allows for the storage of larger amounts of CO2 during convectivedissolution because the reaction consumes it (reactive effect) and can accelerate the development of convectiveinstabilities (reaction-induced convective effects). Both effects lead to larger dissolving fluxes of CO2 (Fig.1),which contributes to a faster and safer storage.

We illustrate our numerical findings by experimental results showing that reactions accelerate the develop-ment of buoyancy-driven fingering during the convective dissolution of CO2 into aqueous reactive solutions ofalkali hydroxides [5] (see Fig.2).

Figure 2: Fingering patterns recorded at t = 13 min in solutions of LiOH, NaOH, KOH and CsOH at 0.01M and 0.1M.The field of view is 12 cm × 7 cm.

References

[1] A. Firoozabadi and P. Cheng. Prospects for subsurface CO2 sequestration. AIChE Journal, 56, 1398, (2010).

[2] M. R. Soltanian, M. A. Amooie, Z. Dai, D. Cole, and J. Moortgat. Critical dynamics of gravito-convective mixing ingeological carbon sequestration. Scientific Reports, 6, 35921, (2016).

[3] V. Loodts, C. Thomas, L. Rongy, and A. De Wit. Control of convective dissolution by chemical reactions: Generalclassification and application to CO2 dissolution in reactive aqueous solutions. Physical Review Letters, 113, 114501,(2014).

[4] V. Loodts, B. Knaepen, L. Rongy, and A. De Wit. Enhanced steady-state dissolution flux in reactive convectivedissolution. Physical Chemistry Chemical Physics, 19, 18565, (2017).

[5] C. Thomas, V. Loodts, L. Rongy, and A. De Wit. Convective dissolution of CO2 in reactive alkaline solutions: Activerole of spectator ions. International Journal of Greenhouse Gas Control, 53, 230, (2016).

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