Permeability in reactive porous media evolves in complex ways that are hard to predict. Macroscopic empirical equations are often used to estimate permeability evolution but fail to reflect the impact of pore scale reactions on permeability. This work aims to understand the evolution of permeability in systems where mineral dissolution and precipitation occur simultaneously. Pore network models are used for permeability simulation of a Paluxy sandstone using pore and pore‐throat size distributions from the analysis of X‐ray CT images. Pore and pore‐throat radii are increased and decreased to reflect the effects of dissolution and precipitation reactions. Varying spatial distributions of reactions are simulated and the resulting porosity and permeability values compared with commonly used macroscopic equations. It is observed that when dissolution occurs at the inlet and precipitation at the outlet, porosity increases while permeability decreases. Similar results are also observed in the respective opposite scenario where precipitation occurs at the inlet and dissolution at the outlet. When dissolution and precipitation reactions are randomly distributed throughout the network, porosity increases with little change in permeability that is not captured by the empirical equations. When reactions are controlled by pore and pore‐throat sizes, such as dissolution occurring in pores and pore‐throats of small size and precipitation in pores and pore‐throats of larger size, porosity and permeability decrease. When precipitation occurs in pores and pore‐throats of smaller size and dissolution in pores and pore‐throats of larger size, porosity and permeability increase and is described relatively well by the Verma‐Pruess porosity‐permeability equation.
Mineral dissolution and precipitation reactions can significantly alter the porosity and permeability of porous media. While porosity generally increases with dissolution and decreases with precipitation, permeability is controlled by the spatial locations of reactions in discrete pores and pore throats and in the greater pore network. Geochemical reactions have been observed to occur both uniformly and nonuniformly in porous media, driven by parameters such as mineral distribution, grain size, and Peclet and Damkohler numbers. Pore network modeling can be used 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 nonuniform spatial distributions of reactions is evaluated. A series of pore network models are created and populated with pore and pore‐throat size distributions of varying types (skewed, normal, and uniform) to represent differences in network topology and characterization methods. The impacts of these distributions on reactive permeability are then simulated for uniform and nonuniform 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. Common macroscopic porosity‐permeability relationships work well for some reaction scenarios but are unable to reflect size‐dependent reactions. A new modified version of the Verma‐Pruess relationship is created and able to successfully reflect the porosity‐permeability evolution for size‐dependent reactions.
27 November 2019 in ACS Sustainable Chemistry & Engineering