Fracture alteration driven by geochemical reactions has been investigated extensively via experimental observations and numerical simulations, given its importance to multiple subsurface energy systems. It is increasingly evident that, large contrast of properties between the fracture and the rock matrix give rise to morphological and mineralogical heterogeneities across scales and thus complex temporal evolution. For example, in multi-reactivity systems, preferential dissolution of the fast-reacting minerals results in the development of an altered porous layer bordering the fracture, which plays an important role in the subsequent reactions. To reveal the controlling process in fractured porous media, we performed three-dimensional steady state simulations based on the micro-continuum approach from a multi-scale perspective. In particular, modeling is enabled by a framework that couples a multiphysics code (COMSOL Multiphysics, COMSOL) with a reactive transport code (CrunchFlow). In terms of a full depiction of advective and diffusive transport in the altered layer in addition to the fracture, the chemical-physical parameters of the altered layer, i.e. porosity, permeability, effective diffusion coefficient and reactive surface area, were varied systematically to examine their impacts on the overall behaviors. Our simulation results showed that calcite dissolution, which controls the alteration of the matrix, is most sensitive to the diffusivity of the altered layer, indicating diffusion across the altered layer being the rate limiting process. Dolomite dissolution, which controls the development of the altered layer and fracture enlargement, has a more complicated dependence on different rate-limiting processes. In particular, even though the consideration of advection within the altered layer did not affect the bulk chemistry significantly, noticeable spatial variations were observed, increasing reaction rate close to the fracture interface. In this regard, advective flow provides an additional transport mechanism and tends to enhance dolomite dissolution within the altered layer, especially in regions where the fracture apertures are small and when the porosity and thus permeability of the altered layer is high.

Investigation of reactive transport processes in a fractured dolostone via multi-scale simulations

ACS Spring Meeting 2021

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