Pore network modeling of reactive permeability

Abstract

Geochemical reactions, such as those that occur during storage of carbon dioxide in saline aquifers as part of carbon capture and storage (CCS) mitigation techniques or long-term subsurface mineral weathering, can significantly alter the properties of porous media. Specifically, the mineral dissolution and precipitation reactions that occur may considerably change the porosity and permeability of porous media. In general, porosity increases with dissolution and decreases with precipitation. However, permeability evolution is controlled by the spatial locations of reactions in discrete pores and pore-throats and in the greater pore-network and is less understood. Additionally, reaction fronts may form as reactive fluids moves through porous media, resulting in geochemical reactions that homogenously or heterogeneously propagate through the network. Geochemical reactions have been observed to occur both uniformly and non-uniformly, driven by parameters such as mineral distribution, grain size, and flow rate. Pore network modeling can be employed to simulate the impact of pore scale alterations on permeability, requiring only pore and pore-throat size distributions and pore connectivity. Here, the impact of variations in pore and pore-throat size distributions on reactive permeability for uniform and non-uniform spatial distributions of reactions as well as the impact of reaction front gradients is evaluated. A series of pore network models are created and populated with pore and pore-throat size distributions of varying types (right-skewed, left-skewed, normal, uniform) to represent differences in network topology and characterization methods. The impacts of these distributions on reactive permeability are then simulated for uniform and non-uniform reaction conditions by increasing or decreasing pore and pore-throat sizes in a prescribed manner to reflect dissolution and precipitation, respectively. Overall, simulations reveal that porosity-permeability evolution varies with reaction scenario and is qualitatively consistent for the different pore and pore-throat size distributions. These simulations, however, assume reactions occur to equal extents throughout the pore network. In reality, reactions may propagate with fluid flow and simulation results for propagated dissolution and precipitation reactions are constructed and compared to simulations where the entire pore network was impacted. Simulation results for all scenarios are compared with common macroscopic porosity-permeability relationships. In some cases, these relationships work well but they are unable to reflect porosity-permeability evolution when reactions initiative in small or large pores and pore-throats. In this work, a new modified version of the Verma-Pruess relationship is created that is able to successfully reflect the porosity-permeability evolution for size dependent reactions.