The impact of porosity waves on crustal reaction progress and CO2 mass transfer

Abstract

Rocks below the brittle-ductile transition can deform viscously and compact while fluid percolates through (two-phase solid–fluid flow). We investigate chemical reaction systematics during two-phase flow using one-dimensional numerical models in which reactive H2O–CO2 fluid ascends down-temperature toward the surface, driving the retrograde reaction: 7CO2+3Calcite+Tremolite=5Dolomite+8Quartz+H2O. The reaction progress is compared to that predicted by fluid–rock reaction during “standard” Darcian flow. A range of layer thicknesses (km-scale) in one- and two-layer systems were investigated at pressure–temperature conditions below the brittle-ductile transition corresponding to ∼13–20 km depth. Model porosity waves of wavelength ∼5 km were generated repeatedly at the base of the flow region using a solitary wave solution with a prescribed initial wave amplitude A = (maximum porosity)/(background porosity of 10−3).Simulation of reaction progress and carbon transfer for the Darcian flow model and for porosity wave transport with A=5, 2.5 and 1.25 yield the following results. First, the overall reaction progress in the two fluid transport models is mainly controlled by the time-integrated fluid flux, and is not strongly dependent on the flow regime. The implication is that the fluid pressure gradient anomalies in the regional-scale porosity waves modeled herein play a negligible role in driving reaction progress. Second, although there are high fluid velocities and thus strong advection in large amplitude porosity waves, the kinetic parameters adopted from experiments predict that the fluid compositions approach local fluid–rock equilibrium in both transport models. Third, regional-scale carbonate-bearing rock layers may be substantial sinks for carbon in ascending fluids if sufficient porosity and permeability can be maintained during reaction. Finally, typical models of retrograde reactions predict that porosity ultimately becomes clogged and, thus, fluid flow is hindered. However, porosity wave transport provides a potential mechanism for creating new porosity to sustain fluid flow during retrogression.