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 our understanding of the long-term evolution of fluid flows in crustal environments, with implications for problems as diverse as nuclear waste disposal and crustal evolution. We examine the dynamics of such evolution for several geologically relevant permeability distributions (models for en-echelon cracks, an isolated sloping fractured zone, and two sloping high-permeability zones that are close enough together to interact). Although our focus is 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 between now and chemistry, where dissolution and precipitation is under kinetic control. Examples of thermal convection in a porous medium with spatially variable permeability reveal features of central importance to water-rock interaction. After a transient phase, an accelerated rate of change of porosity may be used with care to decrease computational time, as an alternative to the quasi-stationary state approximation (Lichtner, 1988). Kinetic effects produce features not expected by traditional assumptions made on the basis of equilibrium, for example, that cooling fluids are oversaturated and heating fluids are undersaturated with respect to silicic acid equilibrium. Indeed, we observe regions of downwelling oversaturated fluid experiencing heating and regions of upwelling, yet cooling, deposition along the upper surface of the channel leading to flow which rises less undersaturated fluid. In sloping high-permeability zones, upwelling causes ess vertically with time. In the long term, this change in slope of the now may also lead to the onset of oscillatory behavior near the surface. Downwelling in sloping high-permeability zones tends to become more vertical with time, due to buoyancy effects and dissolution at the core of the downwelling zone. The location of the basal stalk of thermal plumes rising; from the heated lower boundary is inherently unstable. This stalk migrates with time, as the core of the now generally clogs via precipitation, while kinetic effects cause the 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 the locations of saturation state reversals. This is especially true when kinetic rates are "slow." "Fast" kinetics encourages the runaway dissolution regime. 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 environments than the quartz system considered.