Basic Research Needs for Geosciences - Energetics Meetings and ...

PANEL REPORT: MODELING AND SIMULATION OF GEOLOGIC SYSTEMSCO 2 dissolving into the aqueous phase at early stages migrates away from the phase boundary bymolecular diffusion in the aqueous phase, which is a very slow process. Indeed, effectiveaqueous diffusivities in porous media are of the order of D eff = 10 -10 m 2 /s, so that a time period oft = 10 10 s ( 300 years) is required to achieve a penetration depth of z = (Dt) = 1 m. As hadbeen mentioned, CO 2 dissolution gives rise to a small increase in aqueous phase density, whichafter a certain “incubation time” can induce convective currents in the aqueous phase, whichremove CO 2 from the phase boundary and mix it throughout the permeable thickness of theaquifer at rates that can be orders of magnitude larger than those from molecular diffusion. TheDDC process of CO 2 dissolution, diffusion away from the phase boundary in the aqueous phaseand subsequent convective transport of CO 2 , is a typical example of a multiscale process (Ennis-King et al. 2005). The early onset of convective instability occurs on the scale of a few pores.Convective plumes can continually grow over time until eventually they reach a scale of tens ofmeters or more. The process has been analyzed by applying stability theory to idealizedgeometric shapes subject to highly simplified boundary conditions. High-resolution numericalmodeling for onset and growth of the convective instability has been performed for similarlyidealized systems (see Figure 18; Ennis-King and Paterson 2003; Riaz et al. 2006). We currentlyhave no methods that would allow an accurate modeling of the process for a field-scale domain.“Brute force” approaches using high-resolution space discretization are not expected to befeasible with any foreseeable increase in computing power. Indeed, if 1 cm spatial resolution isrequired, 1 billion grid blocks will provide coverage for only a 101010 m 3 domain, which fallsfar short of the several-kilometer scale required for design and performance assessment of CO 2storage systems.A key concern with geologic storage of CO 2 is secure containment. However, in industrial-scalestorage systems some leakage may be expected (see sidebar on CO 2 leakage), which may giverise to complex, coupled, physical and chemical processes on a broad range of space and timescales. Multiscale, multiphysics problems are also at the heart of understanding solute transportin heterogeneous media, especially when reactive processes are involved, such as sorption,redox, or colloid formation.Focused research efforts are needed to develop modeling capabilities for subsurface processesacross multiple space and time scales to evaluate hazards and risks that may be associated withthe design, operation and monitoring of storage facilities and operations. Among the potentialbenefits of such research is a better understanding of rare events with potentially largeconsequences; i.e., can a large volume of high-pressure CO 2 stored underground give rise to arapid, large volume leak (Pruess 2007)? The modeling capabilities need to accurately representcoupled equilibrium and nonequilibrium processes with nonlinear feedbacks. Thermophysicalproperties of liquid-gas-solid mixtures need to be described accurately, using first-principlesbasedpredictions of chemical speciation, thermodynamics and chemical reactivity (see below).Stochastic effects need to be included in a realistic fashion. Scaleable algorithms are needed toallow efficient simulation on petascale computing platforms.Basic Research Needs for Geosciences: Facilitating 21 st Century Energy Systems 55