Dynamics of convective dissolution from a migrating current of carbon dioxide
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 and uniform dimensional thickness . The aquifer is tilted by an angle 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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2015, Advances in Water ResourcesCitation Excerpt :Note that the flatter plume interface that would result from disregarding the CTZ [19,28] would reduce the impact caused by increasing plume thickness.