Reactive transport in porous media: Pore-network model approach compared to pore-scale model

摘要

Accurate determination of three macroscopic parameters governing reactive transport in porous media, namely,nthe apparent solute velocity, the dispersion, and the apparent reaction rate, is of key importance for predictingnsolute migration through reservoir aquifers. Two methods are proposed to calculate these parameters as functionsnof the P′eclet and the P′eclet-Dahmk¨ohler numbers. In the first method called the pore-scale model (PSM), thenporous medium is discretized by the level setmethod; the Stokes and convection-diffusion equations with reactionnat the wall are solved by a finite-difference scheme. In the second method, called the pore-network model (PNM),nthe void space of the porous medium is represented by an idealized geometry of pore bodies joined by porenthroats; the flow field is computed by solving Kirchhoff’s laws and transport calculations are performed in thenasymptotic regime where the solute concentration undergoes an exponential evolution with time. Two syntheticngeometries of porous media are addressed by using both numerical codes. The first geometry is constructed innorder to validate the hypotheses implemented in PNM. PSM is also used for a better understanding of the variousnreaction patterns observed in the asymptotic regime. Despite the PNM approximations, a very good agreementnbetween the models is obtained, which shows that PNM is an accurate description of reactive transport. PNM,nwhich can address much larger pore volumes than PSM, is used to evaluate the influence of the concentrationndistribution on macroscopic properties of a large irregular network reconstructed from microtomography images.nThe role of the dimensionless numbers and of the location and size of the largest pore bodies is highlighted.