Abstract
When CO2 is injected in saline aquifers, dissolution causes a local increase in brine density that can cause Rayleigh-Taylor-type gravitational instabilities. Depending on the Rayleigh number, density-driven flow may mix dissolved CO2 throughout the aquifer at fast advective time-scales through convective mixing. Heterogeneity can impact density-driven flow to different degrees. Zones with low effective vertical permeability may suppress fingering and reduce vertical spreading, while potentially increasing transverse mixing. In more complex heterogeneity, arising from the spatial organization of sedimentary facies, finger propagation is reduced in low permeability facies, but may be enhanced through more permeable facies. The connectivity of facies is critical in determining the large-scale transport of CO2-rich brine. We perform high-resolution finite element simulations of advection-diffusion transport of CO2 with a focus on facies-based bimodal heterogeneity. Permeability fields are generated by a Markov Chain approach, which represent facies architecture by commonly observed characteristics such as volume fractions. CO2 dissolution and phase behavior are modeled with the cubic-plus-association equation-of-state. Our results show that the organization of high-permeability facies and their connectivity control the dynamics of gravitationally unstable flow. We discover new flow regimes in both homogeneous and heterogeneous media and present quantitative scaling relations for their temporal evolution.
Highlights
Geological sequestration of carbon dioxide (CO2) has been proposed as a technology to reduce the amount of CO2 emitted into the atmosphere[1,2,3,4,5,6]
CO2 is injected into a confined domain and phase behavior is described by the cubic-plus-association equation of state (EOS)
To quantify the spreading of dissolved CO2 during the complex gravito-convective mixing, we evaluate global measures for the spatial variance and dispersion-width of CO2 as well as the evolution of a dispersion coefficient, the maximum density contrast Δρmax, and the velocity of the fastest growing unstable finger
Summary
Geological sequestration of carbon dioxide (CO2) has been proposed as a technology to reduce the amount of CO2 emitted into the atmosphere[1,2,3,4,5,6]. In order to quantify the effects of vertical heterogeneity, Green and Ennis-King[28] developed a simple model consisting of a random and uncorrelated distribution of horizontal, impermeable barriers with a given overall volume fraction (0.04% and 0.15%) They found that in a homogeneous medium with equivalent effective vertical permeability, compared to heterogeneous medium, convection begins at later onset times. The complex patterns in such fluvial architectures[30] exist in many candidate aquifers for CO2 sequestration[9,31,32] and result in variable connectivity and tortuous flow pathways[33] Representing such discontinuous, correlated heterogeneity in reservoir simulations is non-trivial[34], and its influence on the characteristics of density-driven flow has not been studied in detail. We perform high-resolution two-dimensional simulations of gravitational fingering in both homogeneous and heterogeneous media to investigate the influence of facies-based heterogeneity and connectivity on the transport of dissolved CO2
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