Long-term flow/chemistry feedback in a porous medium with heterogenous permeability: Kinetic control of dissolution and precipitation

The kinetics of dissolution and precipitation is of central importance to o ur understanding of the long-term evolution of fluid flows in crustal envir onments, with implications for problems as diverse as nuclear waste disposa l and crustal evolution. We examine the dynamics of such evolution for seve ral geologically relevant permeability distributions (models for en-echelon cracks, an isolated sloping fractured zone, and two sloping high-permeabil ity zones that are close enough together to interact). Although our focus i s on a simple quartz matrix system, generic features emerge from this study that can aid in our broader goal of understanding the long-term feedback b etween now and chemistry, where dissolution and precipitation is under kine tic control. Examples of thermal convection in a porous medium with spatial ly variable permeability reveal features of central importance to water-roc k interaction. After a transient phase, an accelerated rate of change of po rosity may be used with care to decrease computational time, as an alternat ive to the quasi-stationary state approximation (Lichtner, 1988). Kinetic e ffects produce features not expected by traditional assumptions made on the basis of equilibrium, for example, that cooling fluids are oversaturated a nd heating fluids are undersaturated with respect to silicic acid equilibri um. Indeed, we observe regions of downwelling oversaturated fluid experienc ing heating and regions of upwelling, yet cooling, deposition along the upp er surface of the channel leading to flow which rises less undersaturated f luid. In sloping high-permeability zones, upwelling causes ess vertically w ith time. In the long term, this change in slope of the now may also lead t o the onset of oscillatory behavior near the surface. Downwelling in slopin g high-permeability zones tends to become more vertical with time, due to b uoyancy effects and dissolution at the core of the downwelling zone. The lo cation of the basal stalk of thermal plumes rising; from the heated lower b oundary is inherently unstable. This stalk migrates with time, as the core of the now generally clogs via precipitation, while kinetic effects cause t he edges of the stalk to dissolve. When oscillatory convection is present, the amplitudes of oscillation generally increase with time in near-surface environments, whereas amplitudes tend to decrease over long times near the heated lower boundary. Runaway dissolution can be moderated by shifts in th e locations of saturation state reversals. This is especially true when kin etic rates are "slow." "Fast" kinetics encourages the runaway dissolution r egime. We examine the scaling behavior of characteristic length scales, of terms in the solute equation, and of the typical deviation from equilibrium , each as a function of the kinetic rate parameters. Many of these features are viewed as generic and of significance for a wider range of geologic en vironments than the quartz system considered.