Evolution of Porosity-Permeability Relationships in Biogeochemically Altered Porous Media: A Pore-Scale Study

Abstract:

Biogeochemical processes in subsurface including biomineral, biofilm, and biogas formation can dramatically alter the behavior of fluid flow and the hydrodynamics of porous media. There exit analytical solutions such as Kozeny-Carman equation which can be used to predict the permeability-porosity relationships in porous media. However, due to the complexity of various biogeochemical products (e.g. calcium carbonates, biofilms, biogenic gas), their pore-scale behavior and their interplay with pore structure, such analytical solutions would not provide accurate predictions. Thus, numerical modeling plays a critical role in understanding the evolution of porous media during these biogeochemical processes. In this study, a wide range of 2D pore-networks were generated by adjusting the statistical and spatial pore and tube size distribution of the networks. Various scenarios resembling different biogeochemical products were numerically assigned into the networks to reform the pore structure morphology. Numerical simulations including the evolution of pore structure, porosity-permeability relationships and pressure distribution along networks were investigated. The numerical models were also validated via a three-dimensional pore network extracted from micro-CT images of a soil sediment. The numerical simulations verified that local pore-clogging by biogeochemical processes leads to the development of isolated pore clusters and impermeable zones. The evolution of impermeable zones results in the formation of preferential flow paths towards the mobile zones. The results suggest that simple porosity-permeability reduction relationships dramatically overestimate the permeability in biogeochemically altered sediments. The distinction between porosity and mobile porosity must take into the consideration in prediction of hydraulic fields . The results of this study can be used to validate numerical models, which aim to predict the impacts of biomineralization, biofilm formation and biogenic gas formation on the permeability and fluid flow regimes in sediments.