ONE-DIMENSIONAL REACTIVE TRANSPORT MODEL FOR GEOLOGICAL CARBON SEQUESTRATION

The capture of carbon dioxide from large point sources and injection into geological formations such as deep saline aquifers is a potential option for green house gas mitigation that has received a great deal of attention in recent years. However, there are several issues that need to be studied before this option can become economically and socially acceptable. A few of these key issues include the underground storage time, the changes in a formation’s porosity and permeability induced by CO₂ injection, and the ultimate fate of the injected CO₂. These issues are all affected by the chemical interaction between the brine, the injected CO₂, and the mineral content of the aquifer. In the current study a one-dimensional reactive transport model was developed using the PHREEQC geochemical modeling code to predict the long-term chemical behavior of a deep saline aquifer after CO₂ injection. The model predicts the emergence of four well-defined reaction fronts, each characterized by the total dissolution of a mineral species and the sudden drop in pH level of the solution. These fronts emerge as a result of stepwise mineral buffering reactions, which stabilize the pH within the regions between reaction fronts. The results also indicate that the transport of carbon is significantly retarded with respect to the flow of the brine itself. This is due to the precipitation of carbonate minerals (mainly siderite). This precipitation of carbonates is of particular interest since it may lead to permanent storage of carbon in a stable, solid form.