E-mail: edward.bolton@yale.edu

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

by Edward W. Bolton, Antonio C. Lasaga, and Danny M. Rye

American Journal of Science, v. 299, pp. 1-68, 1999.

The long-term feedback between flow and chemistry, where dissolution and precipitation is under kinetic control, is examined for the quartz matrix system. Examples of thermal convection in a porous medium with spatially variable permeability reveal features of central importance to water-rock interaction. Kinetic effects produce features not expected by traditional assumptions made on the basis of equilibrium, e.g. 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 upwelling, yet cooling, undersaturated fluid. In sloping high permeability zones, upwelling leads to increasing slopes of the flow with time, due to the deposition along the upper surface of the channel. In the long term, this 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. 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 flow generally clogs via precipitation, whereas the edges of the stalk are dissolving, via kinetic effects. 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 is moderated by shifts in the locations of saturation state reversals.