Reactive transport under stress: Permeability evolution by chemo-mechanical deformation

The transport of reactive fluids in porous media is important in many natural and engineering processes. Reaction with the solid matrix—e.g. dissolution—changes the transport properties, which in turn affect the rate of reagent transport and hence the reaction. The importance of this highly nonlinear problem has motivated intensive research. Specifically, there have been numerous studies concerning the permeability evolution, especially the process of "wormholing", where preferential dissolution of the most conductive regions leads to a runaway permeability increase. Much less attention, however, has been given to the effect of geomechanics; that is, how the fact that the medium is under stress changes the permeability evolution. Here, we present a novel, mechanistic pore-scale model, simulating the interplay between pore opening by matrix dissolution and pore closure by mechanical compaction, facilitated by weakening caused by the very same process of dissolution. We combine a pore network model of reactive transport with a block-spring model that captures the effect of geomechanics through the update of the network properties. Our simulations show that permeability enhancement is inhibited by stress concentration downstream, in the less dissolved (hence stiffer) regions. Higher stresses lead to stronger inhibition, in agreement with experiments. The effect of stress also depends on the Damkohler number (Da)—the ratio between the flow and the reaction rate. At rapid injection (small Da), where dissolution is relatively uniform, stress has a significant effect on permeability. At slower flow rates (high Da, wormholing regime), stress affects the permeability evolution mostly in early stages, with a much smaller effect on the injected volume required for a significant permeability increase (breakthrough) than at low Da. Interestingly, at higher Da, stress concentration downstream induced by the more heterogeneous dissolution leads to a more homogeneous reagent transport, promoting wormhole competition.