A model for reactive flow in fractured porous media

We consider a model for reactive flow in a fractured formation. The main aim is to study the complex interplay of advection-diffusion flow mechanisms in a fracture-matrix system combined with different chemical reactions taking place. In particular we wish to estimate how a core flooding experiment might behave on larger scale. Brine is injected through a fracture network and ions diffuse into a surrounding low permeable matrix rock where reactions occur. We formulate the interaction between fractures and matrix in the form of transfer functions. The model takes the form of a transport-reaction PDE system for the fracture coupled to an ODE system for the matrix. The chemistry we consider is represented by the interaction of MgCl2 brine with chalk rock. The involved chemistry bears essential features of the interactions between more complex brines such as seawater and carbonate. This is of high interest since chemistry-driven processes can aid to improved oil recovery from fractured carbonate reservoirs. The injected brine dissolves calcite mineral and precipitates magnesite. Cations (Ca2+ and Mg2+) are exchanged on the mineral surface by a substitution mechanism. Reaction kinetic parameters are obtained from experimental data. The numerical code is verified against analytical solutions for the case where reactions do not occur and for linear adsorption of species. First, the flow along a single fracture is analyzed by a parameter study. The behavior can be categorized into regimes determined by the efficiency of species transfer between fracture and matrix and by the efficiency of the dissolution/precipitation reactions. This is further dependent on the magnitude of two characteristic dimensionless numbers alone and the behavior can be translated from one scale to another. The one-fracture model is generalized to study flow through a network of fractures and we relate our previous conclusions to the more complex system. The behavior along individual flow paths is characterized by their dimensionless numbers. The fracture geometry determines the flow distribution (which regions receive the most brine) and the interaction efficiency (whether ions diffuse effectively into the matrix and react). Early injectant breakthrough implies the existence of a zone with low matrix-fracture interaction. (C) 2016 Elsevier Ltd. All rights reserved.