Dynamics of convective dissolution from a migrating current of carbon dioxide

doi:10.1016/j.advwatres.2013.06.013

Advances in Water Resources

Volume 62, Part C, December 2013, Pages 511-519

https://doi.org/10.1016/j.advwatres.2013.06.013 Get rights and content

Highlights

•
We study convective dissolution of migrating CO₂ with high-resolution simulations.
•
We define the dissolution flux and study its evolution as the CO₂ migrates.
•
We find that migration-dissolution has three Ra-independent regimes for Ra > 5000.
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We show that an upscaled model can capture the macroscopic dynamics of this process.

Abstract

During geologic storage of carbon dioxide (CO₂), trapping of the buoyant CO₂ after injection is essential in order to minimize the risk of leakage into shallower formations through a fracture or abandoned well. Models for the subsurface behavior of the CO₂ are useful for the design, implementation, and long-term monitoring of injection sites, but traditional reservoir-simulation tools are currently unable to resolve the impact of small-scale trapping processes on fluid flow at the scale of a geologic basin. Here, we study the impact of solubility trapping from convective dissolution on the up-dip migration of a buoyant gravity current in a sloping aquifer. To do so, we conduct high-resolution numerical simulations of the gravity current that forms from a pair of miscible analogue fluids. Our simulations fully resolve the dense, sinking fingers that drive the convective dissolution process. We analyze the dynamics of the dissolution flux along the moving CO₂–brine interface, including its decay as dissolved buoyant fluid accumulates beneath the buoyant current. We show that the dynamics of the dissolution flux and the macroscopic features of the migrating current can be captured with an upscaled sharp-interface model.

Graphical abstract

Introduction

The injection of carbon dioxide (CO₂) into deep saline aquifers is a promising tool for reducing anthropogenic CO₂ emissions [1], [2], [3], [4]. After injection, the buoyant CO₂ will spread and migrate laterally as a gravity current relative to the denser ambient brine, increasing the risk of leakage into shallower formations through fractures, outcrops, or abandoned wells.

One mechanism that acts to arrest and securely trap the migrating CO₂ is dissolution of CO₂ into the brine [5]. Dissolved CO₂ is considered trapped because brine with dissolved CO₂ is denser than the ambient brine, and sinks to the bottom of the aquifer. In addition to providing storage security by hindering the return of the CO₂ to the atmosphere, this sinking fluid triggers a hydrodynamic fingering instability that drives convection in the brine and greatly enhances the rate of CO₂ dissolution [6], [7], [8], [9].

Although this process of convective dissolution is expected to play a major role in limiting CO₂ migration and accelerating CO₂ trapping [4], the interaction of convective dissolution with a migrating gravity current remains poorly understood. This is due primarily to the disparity in scales between the long, thin gravity current and the details of the fingering instability. Resolving these simultaneously has proven challenging for traditional reservoir simulation tools [10]. Upscaled theoretical models [11], [12] and laboratory experiments [13], [14] have recently provided some macroscopic insights, but by design these capture only the averaged dynamics of the dissolution process.

Here, we study the impact of convective dissolution on the migration of a buoyant gravity current in a sloping aquifer by conducting high-resolution numerical simulations of a pair of miscible analogue fluids. Our simulations fully resolve the small-scale features of the convective dissolution process. We define an average dissolution flux and use it to study the dynamic interactions of the fingering instability with the migrating current. We then compare these results with the predictions of an upscaled theoretical model to investigate the degree to which this simple model can capture the macroscopic features of the migrating current.

Section snippets

Analogue fluids

For simplicity, and to focus on the role of convective dissolution, we neglect capillarity and assume that the two fluids are perfectly miscible. We adopt constitutive laws for density and viscosity that are inspired by a pair of miscible analogue fluids that have been used to study this problem experimentally [15], [16], [13], [14]. This system captures three key features of the CO₂-brine system: (1) a density contrast that stratifies the pure fluids and drives the migration of the gravity

Mathematical model

We consider a two-dimensional aquifer in the x–z plane, with dimensional length $L_{x}$ and uniform dimensional thickness $L_{z}$ . The aquifer is tilted by an angle $θ$ relative to horizontal. This can be viewed as a cross-section of a sedimentary basin taken perpendicular to a line-drive array of injection wells [28], [4]. We assume that the aquifer is homogeneous and with isotropic permeability.

We use the classical model for incompressible fluid flow and advective–dispersive mass transport under the

Effect of dissolution on CO₂ migration

We quantify the evolution of the buoyant current with four macroscopic quantities: its mass, its length, the total dissolution rate of CO₂ into the brine, and the average dissolution flux per unit length of the current. These quantities characterize the spreading and migration of the current and the effectiveness of solubility trapping, which have implications for planning and risk assessment [33], [34].

The dissolution flux between two miscible fluids must be defined with care since there is no

Upscaled model

We now consider the extent to which the dynamics of convective dissolution from a migrating gravity current can be captured by a simple upscaled model. Such models have recently been used to develop insight into the physics of CO₂ migration and trapping [12], [18], [19], [31], [37], [39], [40], [41].

We have elsewhere presented an upscaled model for the migration and trapping of a buoyant current of CO₂ in a sloping aquifer [12]. The model adopts the sharp-interface approximation, assumes

Conclusions

Using high-resolution numerical simulations, we have studied the detailed dynamics of convective dissolution from a buoyant current of CO₂ in a sloping aquifer. We have found that, much like for a stationary layer of CO₂ dissolving into brine, the dissolution flux from a buoyant current is characterized by three regimes: an early-time diffusive regime before the onset of convection, an intermediate constant-flux regime, and a late-time decay as convection is suppressed by the accumulation of

Acknowledgments

JJH acknowledges the support from the FP7 Marie Curie Actions of the European Commission, via the CO2-MATE project (PIOF-GA-2009-253678), and the FP7 EU project PANACEA (Grant Agreement No. 282900). CWM gratefully acknowledges the support of a postdoctoral fellowship from the Yale Climate & Energy Institute. RJ acknowledges funding by the US Department of Energy (DE-FE0009738).

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Note that the flatter plume interface that would result from disregarding the CTZ [19,28] would reduce the impact caused by increasing plume thickness.
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Dynamics of convective dissolution from a migrating current of carbon dioxide

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Analogue fluids

Mathematical model

Effect of dissolution on CO2 migration

Upscaled model

Conclusions

Acknowledgments

Energy Convers Manage

Energy Convers Manage

Int J Greenhouse Gas Control

Adv Water Resour

Int J Greenhouse Gas Control

Int J Greenhouse Gas Control

Climate change: a guide to CO2 sequestration

Science

Onshore geologic storage of CO2

Science

Lifetime of carbon capture and storage as a climate-change mitigation technology

Proc Natl Acad Sci

Reservoir storage and containment of greenhouse gases

Transp Porous Media

Onset of convection in anisotropic porous media subject to a rapid change in boundary conditions

Phys Fluids

Onset of convection in a gravitationally unstable diffusive boundary layer in porous media

J Fluid Mech

Numerical simulation studies of the long-term evolution of a CO2 plume in a saline aquifer with a sloping caprock

Transp Porous Media

Vertically-averaged approaches for CO2 migration with solubility trapping

Water Resour Res

CO2 migration in saline aquifers. Part 2: Capillary and solubility trapping

J Fluid Mech

Spreading and convective dissolution of carbon dioxide in vertically confined, horizontal aquifers

Water Resour Res

Convective dissolution of carbon dioxide in saline aquifers

Geophys Res Lett

Convective instability and mass transport of diffusion layers in a Hele-Shaw geometry

Phys Rev Lett

Impact of relative permeability hysteresis on geological CO2 storage

Water Resour Res

Gravity currents with residual trapping

J Fluid Mech

Effect of dissolution on CO₂ migration

Climate change: a guide to CO₂ sequestration

Onshore geologic storage of CO₂

Numerical simulation studies of the long-term evolution of a CO₂ plume in a saline aquifer with a sloping caprock

Vertically-averaged approaches for CO₂ migration with solubility trapping

Impact of relative permeability hysteresis on geological CO₂ storage